thumb|280px|right|Global mean surface temperatures 1856 to 2005. thumb|280px|right|Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980. Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades. The Earth's average near-surface atmospheric temperature rose 0.6 ± 0.2 ° Celsius (1.1 ± 0.4 ° Fahrenheit) in the 20th century.Climate Change 2001: Working Group I: The Scientific Basis, Part 5
The current scientific consensus is that "most of the observed warming over the last 50 years is likely to have been attributable to human activities".Climate Change 2001: Working Group I: The Scientific Basis, Part 7 The main cause of the human-induced component of warming is the increase in atmospheric greenhouse gases (GHGs), especially carbon dioxide (CO2), due to activities such as the burning of fossil fuels, land clearing, and agriculture. Greenhouse gases are gases that contribute to the greenhouse effect. This effect was first described by Joseph Fourier in 1824, and was first investigated quantitatively in 1896 by the Swedish chemist Svante Arrhenius.Philosophical Magazine 41, 237-276 (1896)
An increase in global temperatures can in turn cause other changes, such as a rising sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacial retreat, reduced summer stream flows, species extinctions and increases in the ranges of disease vectors. Warming is expected to affect the number and magnitude of these events; however, it is difficult to connect specific events to global warming. Although most studies focus on the period up to 2100, warming (and sea level rise due to thermal expansion) is expected to continue past then, since CO2 has an estimated atmospheric lifetime of 50 to 200 years.U.S. Environmental Protection Agency, Non-CO2 Gases Economic Analysis and Inventory, Global Warming Potentials and Atmospheric Lifetimes (Years)
Only a small minority of climate scientists disagree that humanity's actions have played a major role in recent warming. There is more significant uncertainty regarding how much climate change should be expected in the future. There is a hotly contested political and public debate over what, if anything, should be done to reduce or reverse future warming, and how to deal with the predicted consequences. Terminology
The term "global warming" is a specific case of the more general term "climate change" (which can also refer to "global cooling," such as occurs during ice ages). In principle, "global warming" is neutral as to the causes, but in common usage, "global warming" generally implies a human influence. However, the UNFCCC uses "climate change" for human-caused change, and "climate variability" for other changes.United Nations Framework Convention on Climate Change, Article I Some organizations use the term "anthropogenic climate change" for human-induced changes. Also, "global warming" may refer to both the observed and the predicted warming. Historical warming of the Earth
thumb|280px|right|Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. thumb|280px|right|Global mean surface temperature anomaly 1850 to 2006 relative to 1961–1990
thumb|280px|right|Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980
Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. The global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the 100 year period ending in 2005, which is higher than the earlier estimate of 0.6 ± 0.2 °C for the period ending in 2000. The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations" via the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward. These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. While individual scientists have voiced disagreement with the conclusions of the IPCC, the overwhelming majority of scientists working on climate change are in agreement with the conclusions. Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further during the 21st century. The range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. The delay in reaching equilibrium is a result of the large heat capacity of the oceans. Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors. Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. There is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol, aimed at reducing greenhouse gas emissions. Terminology
The term "global warming" is a specific example of the broader term climate change, which can also refer to global cooling. In common usage, the term refers to recent warming and implies a human influence. The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes. Causes
thumb|280px|right|Carbon dioxide during the last 400,000 years and (inset above) the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle.thumb|280px|right|Recent increases in atmospheric carbon dioxide (CO2). The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the Northern Hemisphere's late spring, and declines during the Northern Hemisphere growing season as plants remove some CO2 from the atmosphere. The Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing), volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus identifies elevated levels of greenhouse gases due to human activity as the main influence. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain the observed increase in mean global temperature. One such hypothesis proposes that warming may be the result of variations in solar activity. None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about would still occur. Greenhouse gases in the atmosphere
The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warm a planet's atmosphere and surface. Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable.Note that the Greenhouse Effect produces a temperature increase of about
33 °C (59 °F) with respect to black body predictions and not a surface temperature of 33 °C (91 °F) which is 32 °F higher. The average surface temperature is about 14 °C (57 °F). Also note that both the
Celsius and Fahrenheit temperatures are expressed to 2 significant figures even though the conversion formula produces 3. Rather, the issue is how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases. On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%. Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago. Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation. The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume. Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used. Feedbacks
The effects of forcing agents on the climate are complicated by various feedback processes. One of the most pronounced feedback effects relates to the evaporation of water. In the case of warming by the addition of long-lived greenhouse gases such as CO2, the initial warming will cause more water to be evaporated into the atmosphere. Since water vapor itself acts as a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so forth until a new dynamic equilibrium concentration of water vapor is reached with a much larger greenhouse effect than that due to CO2 alone. Although this feedback process involves an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer. This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime. Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report. Another important feedback process is ice-albedo feedback. When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism contributing to warming. A massive release of CH4 from methane clathrates could cause rapid warming, according to the clathrate gun hypothesis. The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon. Solar variation
thumb|280px|right|Solar variation over the last 30 years. A few papers suggest that the Sun's contribution may have been underestimated. Two researchers at Duke University, Bruce West and Nicola Scafetta, have estimated that the Sun may have contributed about 45–50% of the increase in the average global surface temperature over the period 1900–2000, and about 25–35% between 1980 and 2000. A paper by Peter Stott and other researchers suggests that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated. They nevertheless conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming since the mid-20th century is likely attributable to the increases in greenhouse gases. A different hypothesis is that variations in solar output, possibly amplified by cloud seeding via galactic cosmic rays, may have contributed to recent warming. It suggests magnetic activity of the sun is a crucial factor which deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. One predicted effect of an increase in solar activity would be a warming of the stratosphere; however, the observed effect since at least 1960 has been a cooling of the lower stratosphere, which is one of the predicted results of greenhouse gas warming. Reduction of stratospheric ozone also has a cooling influence, although substantial ozone depletion did not occur until the late 1970s. Solar variation combined with changes in volcanic activity probably did have a warming effect from pre-industrial times to 1950, but a cooling effect since. In 2006, Peter Foukal and other researchers from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles led to a small increase of 0.07% in brightness over the last 30 years. This effect is far too small to contribute significantly to global warming. A paper by Mike Lockwood and Claus Fröhlich found no relation between global warming and solar radiation since 1985, whether through variations in solar output or variations in cosmic rays. Henrik Svensmark and Eigil Friis-Christensen, the main proponents of cloud seeding by galactic cosmic rays, disputed the findings of Lockwood and Fröhlich.http://www.spacecenter.dk/publications/scientific-report-series/Scient_No._3.pdf/view Another recent study speculates that other bodies within the Solar System may influence the climate in a complex manner.http://www.griffith.edu.au/conference/ics2007/pdf/ICS176.pdf
Temperature changes
thumb|280px|right|Two millennia of mean surface temperatures according to different reconstructions, each smoothed on a decadal scale. The unsmoothed, annual value for 2004 is also plotted for reference. Relative to the period 1860–1900, global temperatures on both land and sea have increased by 0.75 °C (1.4 °F), according to the instrumental temperature record. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C/decade against 0.13 °C/decade (Smith, 2005). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979, according to satellite temperature measurements. Over the one or two thousand years before 1850, world temperature is believed to have been relatively stable, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age. Recent
Global temperatures on both land and sea have increased by relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect.Working group I, section 3.2.2.2 of the 2007 IPPC page 243 Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age. Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land. Since the Northern Hemisphere has more land mass than the Southern Hemisphere it warms faster; also there are extensive areas of seasonal snow cover subject to the snow-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the asymmetry of warming as the major gases are essentially well-mixed between hemispheres. Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree. Estimates prepared by the World Meteorological Organization and the UK Climatic Research Unit concluded that 2005 was still only the second warmest year, behind 1998. Goddard Institute for Space Studies, GISS Surface Temperature Analysis Real Climate, 2005 temperatures
Depending on the time frame, a number of temperature records are available based on different data sets. The longest perspective is available from various proxy records for recent millennia; see temperature record of the past 1000 years for a discussion of these records and their differences. An approximately global instrumental record of temperature near the earth's surface begins in about 1860. Global observations of the atmosphere well above the earth's surface using data from radiosondes began shortly after World War II. Satellite temperature measurements of the tropospheric temperature date from 1979. The attribution of recent climate change is clearest for the most recent period of the last 50 years, for which the most detailed data are available. Causes
thumb|280px|right|Carbon dioxide during the last 400,000 years and the rapid rise since the Industrial Revolution; changes in the Earth's orbit around the Sun, known as Milankovitch cycles, are believed to be the pacemaker of the 100,000 year ice age cycle. The climate system varies both through natural, "internal" processes as well as in response to variations in external "forcing" from both human and non-human causes, including solar activity, volcanic emissions, and greenhouse gases. Climatologists agree that the earth has warmed recently. The detailed causes of this change remain an active field of research, but the scientific consensus identifies greenhouse gases as the primary cause of the recent warming. Outside of the scientific community, however, this conclusion can be controversial. Adding carbon dioxide (CO2) or methane (CH4) to Earth's atmosphere, with no other changes, will make the planet's surface warmer; greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be an estimated 30 °C (54 °F) lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding carbon dioxide or methane to the Earth's atmosphere will, absent any mitigating actions or effects, result in warmer surface temperatures on Earth. Rather, the debate is about what the net effect of the addition of carbon dioxide and methane will be, when allowing for compounding or mitigating factors. One example of an important feedback process is ice-albedo feedback. The increased CO2 in the atmosphere warms the Earth's surface and leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are less reflective than ice, and so absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues. Due to the thermal inertia of the earth's oceans and slow responses of other indirect effects, the Earth's current climate is not in equilibrium with the forcing imposed by increased greenhouse gases. Climate commitment studies indicate that, even if greenhouse gases were stabilized at present day levels, a further warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur. Greenhouse gases in the atmosphere
thumb|280px|right|Plots of atmospheric Carbon dioxide and global temperature during the last 650,000 years. Greenhouse gases are transparent to shortwave radiation from the sun, the main source of heat on the Earth. However, they absorb some of the longer infrared radiation emitted by the Earth, thereby reducing radiational cooling and hence raising the temperature of the Earth. How much they warm the world by is shown in their global warming potential. The atmospheric concentrations of carbon dioxide and methane have increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that carbon dioxide values this high were last attained 40 million years ago. About three-quarters of the anthropogenic (man-made) emissions of carbon dioxide to the atmosphere during the past 20 years are due to fossil fuel burning. The rest of the anthropogenic emissions are predominantly due to land-use change, especially deforestation.Climate Change 2001: Working Group I: The Scientific Basis, Part 6
The longest continuous instrumental measurement of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically by approximately 21% from the initial reading of 315 ppmv, as shown by the Keeling curve, to over 380 ppmv in 2006.Earth System Research Laboratory, Trends in Atmospheric Carbon Dioxide Earth System Research Laboratory, NOAA/ESRL Global Monitoring Division The monthly CO2 measurements display small seasonal oscillations in an overall yearly uptrend; each year's maximum is reached during the northern hemisphere's late spring and declines during the northern hemisphere growing season as plants remove some CO2 from the atmosphere. Methane, the primary constituent of natural gas, enters the atmosphere both from biological production and leaks from natural gas pipelines and other infrastructure. Some biological sources are natural, such as termites or forests,Global warming - the blame is not with the plants RealClimate, "Scientists baffled!"
BBC News, "Plants revealed as methane source" but others have been increased or created by agricultural activities such as the cultivation of rice paddies.Climate Change 2001: Working Group I: The Scientific Basis, "Estimates of the global methane budget (in Tg(CH4)/yr) from different sources compared with the values adopted for this report (TAR)." Recent evidence indicates that methane concentrations have begun to stabilize, perhaps due to reductions in leakage from fuel transmission and storage facilities.Physorg.com, "Level of important greenhouse gas has stopped growing"
Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future carbon dioxide scenarios,Climate Change 2001: Working Group I: The Scientific Basis, 3.7.3.3 SRES scenarios and their implications for future CO2 concentration ranging from 541 to 970 parts per million by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal and tar sands are extensively used. Carbon sink ecosystems (forests and oceansOceanOutfall Community Website, Information) are being degraded by pollutants.OceanOutfall Community Website, Los Angeles Times: Ocean Report Degradation of major carbon sinks results in higher atmospheric carbon dioxide levels. 280px|right|thumb|Anthropogenic emission of greenhouse gases broken down by sector for the year 2000. Globally, the majority of anthropogenic greenhouse gas emissions arise from fuel combustion. The remainder is accounted for largely by "fugitive fuel" (fuel consumed in the production and transport of fuel), emissions from industrial processes (excluding fuel combustion), and agriculture: these contributed 5.8%, 5.2% and 3.3% respectively in 1990. Current figures are broadly comparable.UNFCC, Greenhouse Gas Inventory Data Around 17% of emissions are accounted for by the combustion of fuel for the generation of electricity. A small percentage of emissions come from natural and anthropogenic biological sources, with approximately 6.3% derived from agriculturally produced methane and nitrous oxide. Climate sensitivity is a measure of the equilibrium response to increased GHGs and other anthropogenic and natural climate forcings. It is found by observationalJournal of Climate, Vol. 15, No. 22 and model studies. This sensitivity is usually expressed in terms of the temperature response expected from a doubling of CO2 in the atmosphere, which, according to the 2001 IPCC report, is estimated to be between 1.5 and 4.5 °C (2.7–8.1 °F) (with a statistical likelihood of 66-90%).The Scientific Basis, Footnotes for Summary for Policymakers This should not be confused with the expected temperature change by a given date, which also includes a dependence on the future GHG emissions and a delayed response due to thermal lag, principally from the oceans. Models referenced by the Intergovernmental Panel on Climate Change (IPCC), using a range of SRES scenarios, project that global temperatures will increase between 1.4 and 5.8 °C (2.5 to 10.5 °F) between 1990 and 2100. Positive feedback effects, such as the expected release of methane from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes), may lead to significant additional sources of greenhouse gas emissions.ZNet, Warming Hits 'Tipping Point' Note that the anthropogenic emissions of other pollutants—notably sulfate aerosols—exert a cooling effect; this partially accounts for the plateau/cooling seen in the temperature record in the middle of the twentieth century,Climate Change 2001: Working Group I: The Scientific Basis, Chapter 12 though this may also be due to intervening natural cycles. Other hypotheses
The extent of the scientific consensus on global warming—that "most of the observed warming over the last 50 years is likely to have been attributable to human activities"—has been investigated: In the journal Science in December 2004, Dr Naomi Oreskes published a study of the abstracts of 928 refereed scientific articles in the ISI citation database identified with the keywords "global climate change". This study concluded that 75% of the 928 articles either explicitly or implicitly accepted the consensus view — the remainder of the articles covered methods or paleoclimate and did not take any stance on recent climate change. The study did not report how many of the 928 abstracts explicitly accepted the hypothesis of human-induced warming, but none of the 928 articles surveyed accepted any other hypothesis. Contrasting with the consensus view, other hypotheses have been proposed to explain all or most of the observed increase in global temperatures. Some of these hypotheses (listed here without comment on their validity or lack thereof) include:
The warming is within the range of natural variation. The warming is a consequence of coming out of a prior cool period, namely the Little Ice Age. The warming is primarily a result of variances in solar irradiance, possibly via modulation of cloud cover . It is similar in concept to the operating principles of the Wilson cloud chamber, but on a global scale where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of cloud condensation nuclei. The observed warming actually reflects the Urban Heat Island, as most readings are done in heavily populated areas which are expanding with growing population . The solar variation theory
thumb|280px|right|30 years of solar variability. Modeling studies reported in the IPCC Third Assessment Report (TAR) did not find that changes in solar forcing were needed in order to explain the climate record for the last four or five decades . These studies found that volcanic and solar forcings may account for half of the temperature variations prior to 1950, but the net effect of such natural forcings has been roughly neutral since then . In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be eight times larger than the change in forcing due to increasing solar activity over the same period . Since the TAR, some studies (Lean et al., 2002, Wang et al., 2005) have suggested that changes in irradiance since pre-industrial times are less by a factor of 3 to 4 than in the reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean, 2000.). Other researchers (e.g. Stott et al. 2003 ) believe that the effect of solar forcing is being underestimated and propose that solar forcing accounts for 16% or 36% of recent greenhouse warming. Others (e.g. Marsh and Svensmark 2000 ) have proposed that feedback from clouds or other processes enhance the direct effect of solar variation, which if true would also suggest that the effect of solar variability was being underestimated. In general the level of scientific understanding of the contribution of variations in solar irradiance to historical climate changes is "very low" . The present level of solar activity is historically high. Solanki et al. (2004) suggest that solar activity for the last 60 to 70 years may be at its highest level in 8,000 years; Muscheler et al. disagree, suggesting that other comparably high levels of activity have occurred several times in the last few thousand years . Solanki concluded based on their analysis that there is a 92% probability that solar activity will decrease over the next 50 years. In addition, researchers at Duke University (2005) have found that 10–30% of the warming over the last two decades may be due to increased solar output . In a review of existing literature, Foukal et al. (2006) determined both that the variations in solar output were too small to have contributed appreciably to global warming since the mid-1970s and that there was no evidence of a net increase in brightness during this period. Expected effects
The expected effects of global warming are many and various, both for the environment and for human life. These effects include sea level rise, repercussions to agriculture, reductions in the ozone layer, increased intensity and frequency of extreme weather events, and the spread of disease. In some cases, the effects may already be manifest, although it is difficult to attribute specific incidents of natural phenomena to long-term global warming. Since the mid-1970s, the total annual power of hurricanes has increased markedly because their average intensity and duration have increased; in addition, there has been a high correlation of hurricane power with tropical sea-surface temperatureNature: 2005 Aug 4;436(7051):686-8.. In spite of such strong evidence, the relationship between global warming and hurricanes is still being debated. A draft statement by the World Meteorological Organization acknowledges the differing viewpoints on this issue . The extent and probability of these consequences is a matter of considerable uncertainty. A summary of probable effects and recent understanding can be found in the report of the IPCC Working Group II . Some scientists have concluded global warming is already causing death and disease across the world through flooding, environmental destruction, heat waves and other extreme weather events. (Reuters, February 9 2006; archived)
Effects on ecosystems
Both primary and secondary effects of global warming — such as higher temperatures, lessened snow cover, rising sea levels and weather changes — may influence not only human activities, but also ecosystems. Some species may be forced out of their habitats (possibly to extinction) because of changing conditions, while others may flourish. Similarly, changes in timing of life patterns, such as annual migration dates, may alter regional predator-prey balance. The effect of advanced spring arrival dates in Scandinavia on birds that over winter in sub-Saharan Africa has been ascribed to evolutionary adaptation of the species to climatic warming . Ocean pH is lowering as a result of increased carbon dioxide levels. Lowering of ocean pH, along with changing water temperature and ocean depth will have a damaging effect on coral reefs. Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release significant amounts of the potent greenhouse gas, methane, which is trapped in permafrost and ice clathrate compounds . There are also ecological effects of melting polar ice: for example, polar bears use sea ice to reach their prey and they must swim to another ice floe when one breaks up. Ice is now becoming further separated and dead polar bears have been found in the water, believed to have drowned. More recently, some scientists have suggested that the observed cannibalistic behavior in polar bears may be the result of food shortages brought on by global warming (Amstrup et al. 2006). Effect on glaciers
280px|right|thumb|Global glacial mass balance in the last fifty years, reported to the WGMS and the NSIDC. The increased downward trend in the late 1980s is symptomatic of the increased rate and number of retreating glaciers. Global warming has led to negative glacier mass balance, causing glacier retreat around the world. Oerlemans (2005) showed a net decline in 142 of the 144 mountain glaciers with records from 1900 to 1980. Since 1980 global glacier retreat has increased significantly. Similarly, Dyurgerov and Meier (2005) averaged glacier data across large-scale regions (e.g. Europe) and found that every region had a net decline from 1960 to 2002, though a few local regions (e.g. Scandinavia) have shown increases. Some glaciers that are in disequilibrium with present climate have already disappeared and increasing temperatures are expected to cause continued retreat in the majority of alpine glaciers around the world. Upwards of 90% of glaciers reported to the World Glacier Monitoring Service have retreated since 1995 . Of particular concern is the potential for failure of the Hindu Kush and Himalayan glacial melts. The melt of these glaciers is a large and reliable source of water for China, India, and much of Asia, and these waters form a principal dry-season water source. Increased melting would cause greater flow for several decades, after which "some areas of the most populated region on Earth are likely to 'run out of water'" (T. P. Barnett, J. C. Adam and D. P. Lettenmaier 2005)
Miniature rock glaciers
Rock glaciers — caches of ice under boulders — are among other water signs such as drying meadows and warming lakes that scientists are studying in the Sierras in the western United States . Connie Millar searches for the rock glaciers in the Yosemite area of the Sierra crest. She hypothesizes that rock glaciers will be predictors of how ecosystems change with rising temperatures. Millar is leading an effort (the Consortium for Integrated Climate Research in Western Mountains ) to co-ordinate the work of many scientists to see how the pieces of the Global Warming puzzle may fit. Destabilization of ocean currents
There is also some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift. Sea level rise and environmental refugees
thumb|right|The termini of the glaciers in the Bhutan-Himalaya. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades. According to USGS researchers, glaciers in the Himalaya are wasting at alarming and accelerating rates, as indicated by comparisons of satellite and historic data, and as shown by the widespread, rapid growth of lakes on the glacier surfaces. The researchers have found a strong correlation between increasing temperatures and glacier retreat. Rising global temperatures will melt glaciers and expand the water of the seas through the mechanism of thermal expansion, leading to sea level rise. Even a relatively small rise in sea level would make some densely settled coastal plains uninhabitable and create a significant refugee problem. If the sea level were to rise in excess of 4 meters (13 ft) almost every coastal city in the world would be severely affected, with the potential for major damage to world-wide trade and economy. Presently, the IPCC predicts sea level rise is most probable to be just short of half a metre, and at least between 9 and 88 cm through 2100 - but they also warn that global warming during that time may lead to irreversible changes in the Earth's glacial system and ultimately melt enough ice to raise sea level many meters over the next millennia. It is estimated that around 200 million people could be affected by sea level rise, especially in Vietnam, Bangladesh, China, India, Thailand, Philippines, Indonesia, Nigeria and Egypt. An example of the ambiguity of the concept of environmental refugees is the emigration from the island nation of Tuvalu, which has an average elevation of approximately one meter above sea level. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation and many residents have been leaving the islands. However, it is far from clear that rising sea levels from global warming are a substantial factor - best estimates are that sea level has been rising there at approximately 1–2 millimeters per year (~1/16th in/yr), but that shorter timescale factors—ENSO, or tides—have far larger temporary effects . Spread of disease
One of the largest known outbreaks of Vibrio parahaemolyticus gastroenteritis has been attributed to generally rising ocean temperature where infected oysters were harvested in Prince William Sound, Alaska in 2005. Before this, the northernmost reported risk of such infection was in British Columbia, 1000 km to the south (McLaughlin JB, et al.). Global warming may extend the range of vectors conveying infectious diseases such as malaria. A warmer environment boosts the reproduction rate of mosquitoes and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse . Global warming has been implicated in the recent spread to the north Mediterranean region of bluetongue disease in domesticated ruminants associated with mite bites (Purse, 2005). Hantavirus infection, Crimean-Congo hemorrhagic fever, tularemia and rabies increased in wide areas of Russia during 2004–2005. This was associated with a population explosion of rodents and their predators but may be partially blamed on breakdowns in governmental vaccination and rodent control programs. Similarly, despite the disappearance of malaria in most temperate regions, the indigenous mosquitoes that transmitted it were never eliminated and remain common in some areas. Thus, although temperature is important in the transmission dynamics of malaria, many other factors are influential . Financial effects
Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study (UNEP summary) that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$150 billion each year in the next decade. These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike. According to the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%. The United Nations' Environmental Program recently announced that severe weather around the world has made 2005 the most costly year on record . Although there is "no way to prove that [a given hurricane] either was, or was not, affected by global warming" , global warming is thought to increase the probability of hurricanes emerging. Preliminary estimates presented by the German insurance foundation Munich Re put the economic losses at more than US$200 billion, with insured losses running at more than US$70 billion. Nicholas Stern in the Stern Review has warned that one percent of global GDP is required to be invested in order to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GDP . Stern’s report suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions. Tony Blair said the Stern Review showed that scientific evidence of global warming was "overwhelming" and its consequences "disastrous". Biomass production
The creation of biomass by plants is influenced by the availability of water, nutrients, and carbon dioxide. Part of this biomass is used (directly or indirectly) as the energy source for nearly all other life forms, including feed-stock for domestic animals, and fruits and grains for human consumption. It also includes timber for construction purposes. While it's thought that an increase in carbon dioxide levels should speed up plant growth, which would slow down the effects of global warming, a new study has found the opposite to be true. Scientists at Stanford have found that "elevated atmospheric carbon dioxide actually reduces plant growth when combined with other likely consequences of climate change -- namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil." . A rising temperature can also increase the growing season in colder regions. It is sometimes argued that these effects can create a greener, richer planet, with more available biomass. However, there are many other factors involved, and it is currently unclear if plants really benefit from global warming. Plant growth can be limited by a number of factors, including soil fertility, water, temperature, and carbon dioxide concentration. Ocean plants (phytoplankton) are actually harmed by global warming, presumably with negative impact on ocean ecosystems . IPCC models currently predict a possible modest increase in plant productivity. However, there are several negative ramifications: decreases in productivity may occur at above-optimal temperatures; greater variation in temperature is likely to decrease wheat yields; in experiments, grain and forage quality declines if CO2 and temperature are increased; and the reductions in soil moisture in summer, which are likely to occur, would have a negative effect on productivity. Satellite data show that the productivity of the northern hemisphere
did indeed increase from 1982 to 1991
. However, more recent studies
found that from 1991 to 2002, widespread droughts had actually caused
a decrease in summer photosynthesis in the mid and high latitudes of
the northern hemisphere. right|thumb|NOAA projects that by the 2050s, there will only be 54% of the volume of sea ice there was in the 1950s. Opening up of the Northwest Passage in summer
Melting Arctic ice may open the Northwest Passage in summer in approximately ten years, which would cut 5,000 nautical miles (9,300 km) from shipping routes between Europe and Asia. This would be of particular relevance for supertankers that are too big to fit through the Suez Canal and currently have to go around the southern tip of Africa. According to the Canadian Ice Service, the amount of ice in Canada's eastern Arctic Archipelago decreased by 15% between 1969 and 2004 . A similar opening is possible in the Arctic north of Siberia, allowing much faster East Asian to Europe transport. Adverse effects of the melting of ice include a potential increase in the rate of global warming, since ice reflects 90% of solar heat, while open water absorbs 90% . thumb|150px|right|The Greenlandic ice-cap has shrunk noticeably since 1978. Further global warming (positive feedback)
Some effects of global warming themselves contribute directly to further global warming, in a vicious circle, the nature of which may be difficult to predict in advance. The melting of permafrost and ice caps appears to be causing the release of large amounts of additional carbon dioxide or methane from decaying vegetation trapped beneath . In addition, methane clathrate deposits on the ocean floor might release more methane (the clathrate gun hypothesis). There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO2 levels
Melting could also lead to increased heat absorption because ice reflects more solar radiation (i.e., it has higher albedo) than land or water. Because sea ice and seasonal snow cover are more reflective than the underlying sea, any meltback may lead to further warming. Warmer temperatures in the oceans reduce the productivity (growth) of ocean phytoplankton (algae). This is expected to reduce the amount of carbon dioxide taken up by photosynthesis in the ocean , which would again increase the effects of anthropogenic CO2 releases on the overall amount of CO2 in the atmosphere, and hence increase the greenhouse effect. This is a concern because ocean photosynthesis is as large a part of the planet's overall carbon balance as land photosynthesis. Mitigation
thumb|280px|right|The Energy Information Administration predicts world energy and fossil fuel usage will rise in the next decades. The likelihood that global temperatures will continue to significantly increase has led to proposals to mitigate global warming. Mitigation covers all actions aimed at reducing the negative effects or the likelihood of global warming. There are five categories of actions that can be taken to mitigate global warming:
Reduction of energy use (conservation)
Shifting from carbon-based fossil fuels to alternative energy sources
Carbon capture and storage
Carbon sequestration
Planetary engineering to cool the earth, including screening out sunlight or increasing albedo
Strategies for mitigation of global warming include development of new technologies; carbon offsets; renewable energy such as biodiesel, wind power, and solar power; nuclear power; electric or hybrid automobiles; fuel cells; energy conservation; carbon taxes; enhancing natural carbon dioxide sinks; population control; and carbon capture and storage. Many environmental groups encourage individual action against global warming, often aimed at the consumer, and there has been business action on climate change. The world's primary international agreement on combating climate change is the Kyoto Protocol. The Kyoto Protocol is an amendment to the United Nations Framework Convention on Climate Change (UNFCCC). Countries that ratify this protocol commit to reduce their emissions of carbon dioxide and five other greenhouse gases, or engage in emissions trading if they maintain or increase emissions of these gases. Although the governments of 163 countries ratified the Kyoto Protocol, (notably excluding the United States and Australia), there is a growing debate about how effective the Kyoto protocol has been. Some politicians, including President of the United States George W. Bush , Prime Minister of Australia John Howard had argued that the cost of mitigating global warming via the Kyoto protocol is too large to be practical. This view may be proving correct, as the signatories of the Kyoto protocol are currently struggling to meet their targets , including Europe and Japan. After only five years, Canada has given up entirely. Also, of the 163 countries that have signed and ratified Kyoto, only 31 are actually required to lower greenhouse emissions. Some segments of the business community have accepted global warming and its attribution to anthropogenic causes as valid, as well as a need for actions such as carbon emissions trading and carbon taxes. Adaptation strategies accept some warming as a foregone conclusion and focus on preventing or reducing undesirable consequences. Examples of such strategies include defense against rising sea levels or ensuring food security. Climate models
thumb|280px|Calculations of global warming from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions. thumb|280px|The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F)
Scientists have studied global warming with computer models of the climate (see below). Before the scientific community accepts a climate model, it has to be validated against observed climate variations. As of 2006, sufficiently high-resolution models successfully simulate summer/winter differences, the North Atlantic Oscillation, and El Niño . All validated current models predict that the net effect of adding greenhouse gases will be a warmer climate in the future. However, the amount of predicted warming varies by model, and there still remains a considerable range of climate sensitivity predicted by the models which survive these tests; one of the most important sources of this uncertainty is believed to be different ways of handling clouds. Part of the technical summary of the IPCC TAR includes a recognition of the need to quantify this uncertainty: "In climate research and modeling, we should recognize that we are dealing with a coupled non-linear system, and therefore that the prediction of a specific future climate is not possible. Rather the focus must be on the probability distribution of the system's possible future states by the generation of ensembles of model solutions." (See , page 78.) An example of a study which aims to do this is the Climateprediction.net project; their methodology is to investigate the range of climate sensitivities predicted for the 21st century by those models which are first shown to give a reasonable simulation of late 20th century climate change. As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C (2.5 °F–10.4 °F) between 1990 and 2100 . They have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors. In addition to having their own characteristic climate sensitivity, models have also been used to derive independent assessments of climate sensitivity. Climate models can produce a good match to observations of global temperature changes over the last century . Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,
though the cooling may also be due in part to natural variability. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another, so that warming in recent decades has been driven mainly by non-CO2 greenhouse gases.http://www.pnas.org/cgi/content/full/97/18/9875
Paleoclimatologist William Ruddiman has argued that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation. Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed. Pre-human climate variations
thumb|280px|right|Curves of reconstructed temperature at two locations in Antarctica and a global record of variations in glacial ice volume. Today's date is on the left side of the graph. Earth has experienced warming and cooling many times in the past. The recent Antarctic EPICA ice core spans 800,000 years, including eight glacial cycles timed by orbital variations with interglacial warm periods comparable to present temperatures. A rapid buildup of greenhouse gases caused warming in the early Jurassic period (about 180 million years ago), with average temperatures rising by 5 °C (9 °F). Research by the Open University indicates that the warming caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, CO2 levels dropped back to normal over roughly the next 150,000 years. Sudden releases of methane from clathrate compounds (the clathrate gun hypothesis) have been hypothesized as a cause for other warming events in the distant past, including the Permian-Triassic extinction event (about 251 million years ago) and the Paleocene-Eocene Thermal Maximum (about 55 million years ago). Climate models
thumb|280px|Calculations of global warming prepared in or before 2001 from a range of climate models under the SRES A2 emissions scenario, which assumes no action is taken to reduce emissions. thumb|280px|The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F). Scientists have studied global warming with computer models of the climate. These models are based on physical principles of fluid dynamics, radiative transfer, and other processes, with simplifications being necessary because of limitations in computer power and the complexity of the underlying system (climate). These models predict that the effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of future greenhouse gas levels are used, there still remains a considerable range of climate sensitivity. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC anticipates a warming of by the end of the 21st century, relative to 1980–1999. Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human-derived causes. Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the carbon cycle to the models generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback . Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds . Most global climate models, when run to project future climate, are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may be run by adding a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback. The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small-scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considered climate models "to be suitable tools to provide useful projections of future climates" . In December, 2005 Bellouin et al. suggested in Nature that the reflectivity effect of airborne pollutants was about double that previously expected, and that therefore some global warming was being masked. If supported by further studies, this would imply that existing models under-predict future global warming. Defining dangerous global warming
Although global warming has been seen as potentially dangerous for some time, the first international attempt to define what constitutes a 'dangerous' level occurred at the Avoiding Dangerous Climate Change scientific conference in February 2005. This took place in Exeter, United Kingdom under the UK presidency of the G8 . At the conference it was said that increasing damage was forecast if the globe warms to about 1 to 3 °Celsius (1.8 to 5.4 °Fahrenheit) above pre-industrial levels. It was concluded that the stabilization of greenhouse gases at the equivalent of 450 ppmv CO2 would provide a 50% likelihood of limiting global warming to the average figure of 2 °C (3.6 °F). Stabilization below 400 ppm would give a relatively high certainty of not exceeding 2 °C, while stabilization at 550 ppm would mean it was likely that 2 °C would be exceeded. It was stated that unless 'urgent and strenuous mitigation actions' were taken in the next 20 years, it was almost certain that by 2050 global temperatures will have risen to between 0.5 and 2 °C (0.9 and 3.6°F) above current levels. With carbon dioxide levels currently around 381 ppm and rising by 2ppm per year, without such action greenhouse gasses are likely to reach 400ppm by 2016, 450ppm by 2041, and 550ppm by around 2091. Other related issues
Ocean acidification
Increased atmospheric carbon dioxide increases the amount of CO2 dissolved in the oceans. This ameliorates the greenhouse effect by removing the greenhouse gas from the atmosphere. Unfortunately, carbon dioxide gas dissolved in the ocean reacts with water to form carbonic acid. The net effect, even accounting for warming of the oceans, is an observed and accelerating ocean acidification. Since biosystems are adapted to a narrow range of pH this is a very serious concern directly driven by increased atmospheric CO2 and not global warming. Ocean acidification would not be created by any other postulated climate change mechanisms. In fact, the result of these other forcings would be to decrease the acidity of the oceans as the sea temperature warmed. This is exactly contrary to what has been observed. Roger Revelle and Hans Suess first described the chemistry of the process. Relationship to ozone depletion
Although they are often interlinked in the mass media, the connection between global warming and ozone depletion is not strong. There are five areas of linkage:
The same carbon dioxide radiative forcing that produces near-surface global warming is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes. thumb|280px|right|Radiative forcing from various greenhouse gases and other sources
Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: 1) reduced ozone allows more solar radiation to penetrate, thus warming the troposphere instead of the stratosphere. 2) The resulting colder stratosphere emits less long-wave radiation down to the troposphere, thus having a cooling effect. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system of about −0.15 ± 0.10 W/m² . One of the strongest predictions of the greenhouse effect theory is that the stratosphere will cool. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the NOAA Geophysical Fluid Dynamics Laboratory show that above 20 km, the greenhouse gases dominate the cooling. Ozone depleting chemicals are also greenhouse gases, representing 0.34 ±0.03 W/m², or about 14% of the total radiative forcing from well-mixed greenhouse gases . Decreased ozone leads to an increase in ultraviolet levels. Ultraviolet radiation may be responsible for the death of ocean algae, which operate as a carbon dioxide sink in the ocean. Increased UV, therefore, may lead to a decrease in carbon dioxide uptake, thereby raising global carbon dioxide levels. Relationship to global dimming
Some scientists now consider that the effects of global dimming (the reduction in sunlight reaching the surface of the planet, possibly due to aerosols) may have masked some of the effect of global warming. If this is so, the indirect aerosol effect is stronger than previously believed, which would imply that the climate sensitivity to greenhouse gases is also stronger. Concerns about the effect of aerosol on the global climate were first researched as part of concerns over global cooling in the 1970s. Pre-human global warming
The Earth has experienced natural global warming and cooling many times in the past, and can offer useful insights into present processes. It is thought by some geologists that a rapid buildup of greenhouse gases caused the Earth to experience global warming in the early Jurassic period, with average temperatures rising by 5 °C (9.0 °F). Research by the Open University published in Geology (32: 157–160, 2004 ) indicates that this caused the rate of rock weathering to increase by 400%. As such weathering locks away carbon in calcite and dolomite, carbon dioxide levels dropped back to normal over roughly the next 150,000 years. Sudden releases of methane from clathrate compounds (the Clathrate Gun Hypothesis) have been hypothesized as a cause for other past global warming events, including the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum. However, warming at the end of the last glacial period is thought not to be due to methane release . Instead, natural variations in the Earth's orbit (Milankovitch cycles) are believed to have triggered the retreat of ice sheets by changing the amount of solar radiation received at high latitude and led to deglaciation. The greenhouse effect is also invoked to explain how the Earth made it out of the Snowball Earth period 600 million years ago. During this period all silicate rocks were covered by ice, thereby preventing them from combining with atmospheric carbon dioxide. The atmospheric carbon dioxide level gradually increased until it reached a level that could have been as much as 350 times the current level. At this point temperatures were raised enough to melt the ice, even though the reflective ice surfaces had been reflecting most sunlight back into space. Increased amounts of rainfall would quickly wash the carbon dioxide out of the atmosphere, and thick layers of abiotic carbonate sediment have been found on top of the glacial rocks from this period. Using paleoclimate data for the last 500 million years Veizer et al. (2000, Nature 408, pp. 698–701) concluded that long-term temperature variations are only weakly related to carbon dioxide variations. Most paleoclimatologists believe this is because other factors, such as continental drift and mountain building have larger effects in determining very long term climate. However, Shaviv and Veizer (2003, ) proposed that the biggest long-term influence on temperature is actually the solar system's motion around the galaxy, and the ways in which this influences the atmosphere by altering the flux of cosmic rays received by the Earth. Afterwards, they argued that over geologic times a change in carbon dioxide concentrations comparable to doubling pre-industrial levels, only results in about 0.75 °C (1.3 °F) warming rather than the usual 1.5–4.5 °C (2.7–8.1 °F) reported by climate models . They acknowledge (Shaviv and Veizer 2004) however that this conclusion may only be valid on multi-million year time scales when glacial and geological feedback have had a chance to establish themselves. Rahmstorf et al. 2004 argue that S+V have highly and arbitrarily tuned their data, and that their conclusions are unreliable. Pre-industrial global warming
Paleoclimatologist William Ruddiman has argued (e.g., Scientific American, March 2005) that human influence on the global climate began around 8,000 years ago with the start of forest clearing to provide land for agriculture and 5,000 years ago with the start of Asian rice irrigation. He contends that forest clearing explains the rise in carbon dioxide levels in the current interglacial that started 8,000 years ago, contrasting with the decline in carbon dioxide levels seen in the previous three interglacials. He further contends that the spread of rice irrigation explains the breakdown in the last 5,000 years of the correlation between the Northern Hemisphere solar radiation and global methane levels, which had been maintained over at least the last 11 22,000-year cycles. Ruddiman argues that without these effects, the Earth would be nearly 2 °C cooler and "well on the way" to a new ice age. Ruddimann's interpretation of the historical record, with respect to the methane data, has been disputed. References
Association of British Insurers Financial Risks of Climate Change, June 2005, (PDF) Accessed 7 January 2006
Choi, O. and A. Fisher (2003) "The Impacts of Socioeconomic Development and Climate Change on Severe Weather Catastrophe Losses: Mid-Atlantic Region (MAR) and the U.S." Climate Change, vol. 58 pp. 149
Ealert Global warming - the blame is not with the plants
Emanuel, K.A. (2005) "Increasing destructiveness of tropical cyclones over the past 30 years." Nature 436, pp. 686–688. ftp://texmex.mit.edu/pub/emanuel/PAPERS/NATURE03906.pdf
Kennett J. P., Cannariato K. G., Hendy I. L. & Behl R. J.American Geophysical Union, Special Publication, Methane Hydrates in Quaternary Climate Change: The Clathrate Gun Hypothesis. 54, (2003). Questions about Clathrate Gun Hypothesis (source of information)
(online version requires registration)
(online version requires registration)
Naomi Oreskes, 2004 Beyond the Ivory Tower: The Scientific Consensus on Climate Change - The author discussed her survey of 928 peer-reviewed scientific abstracts on climate change. Retrieved December 8, 2004. Also available as a 1 page PDF file
RealClimate Scientists Baffled
"Shafts of ancient ice pulled from Antarctica's frozen depths show that for at least 650,000 years three important heat-trapping greenhouse gases never reached recent atmospheric levels caused by human activities, scientists are reporting today." (November 25 2005)
Smith, T.M. and R.W. Reynolds, 2005: A global merged land and sea surface temperature reconstruction based on historical observations (1880–1997). J. Climate, 18, 2021–2036. (online version requires registration)
UNEP summary (2002) Climate risk to global economy, Climate Change and the Financial Services Industry, United Nations Environment Programme Finance Initiatives Executive Briefing Paper (UNEP FI) (PDF) Accessed 7 January 2006
Wired Careful Where You Put That Tree
See also
Wikinews
Wikinews:Ice shelf breaks free in Canadian Arctic
Scientific assessment
Climate Change Science Program
Global Atmosphere Watch
List of scientists opposing global warming consensus
National Assessment on Climate Change
Scientific opinion on climate change
Climate science
Climate change
Global cooling
Global dimming
Iris hypothesis
Timeline of environmental events
Atlantic Multidecadal Oscillation
Maunder Minimum (1645-1715)
Opinion and controversy
An Inconvenient Truth
Are We Changing Planet Earth? Environmental skepticism
Global warming controversy
:Category:Global warming skeptics
Wise use
Remediation and regulation
Carbon offset
Economics of global warming
Energy conservation
Iron fertilization
Massachusetts v. Environmental Protection Agency
United Kingdom Climate Change Programme
United Nations Framework Convention on Climate Change
Miscellaneous
Global warming in popular culture
Glossary of climate change
Phenology
Tragedy of the commons
External links
Scientific
Global Warming and Ozone Depletion Freeview video interview with F.Sherwood Rowland (Nobel Prize for work on Ozone), by the Vega Science Trust. Global Warming Information from the Ocean & Climate Change Institute, Woods Hole Oceanographic Institution
Intergovernmental Panel on Climate Change (IPCC)
IPCC Third Assessment Report published in 2001
Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability
A summary of the above IPCC report - by GreenFacts
NASA's Global Hydrology and Climate Center
Mauna Loa Observatory, Hawaii - Latest CO2 Measurements and Data
Maps & Graphics on Global Warming from the UN Environment Programme GRID-Arendal
NOAA's Global Warming FAQ
RealClimate - A group blog of climate scientists
National Center for Atmospheric Research - Overview of NCAR research on climate change
Potsdam Institute for Climate Impact Research
Discovery of Global Warming — An extensive introduction to the topic and the history of its discovery
Introduction to climate change: Lecture notes for meteorologists (World Meteorological Organization) (PDF)
Pew Center on Global Climate Change — basic science
NOAA ESRL Global Monitoring Division
Global Warming Site, U.S. Environmental Protection Agency
Final Report of U.S. Climate Change Science Program
Melting lakes in Siberia emit greenhouse gas
Danish National Space Centre: SKY Experiment
GlobalWarming.org
Polar ice-related links
How rapidly is permafrost changing? What are the impacts of these changes? from *NOAA
Melting Russian Permafrost Could Accelerate Global Warming - ENS (7 September 2006)
Experts warn North Pole will be 'ice free' by 2040
Other
Ad hoc committee report on the ‘hockey stick’ global climate reconstruction - The Wegman Report, discussing statistical errors in Global Warming studies. Climate Ark - climate change and global warming portal providing news, search, links and analysis
Science and Technology Librarianship: Global Warming and Climate Change Science — Extensive commented list of Internet resources — Science and Technology Sources on the Internet. Union of Concerned Scientists Global Warming page
BBC: Global warming risk 'much higher'
Watch and read 'Tipping Point', Australian science documentary about effects of global warming on rare, common, and endangered wildlife
Summary by "Physicians and Scientists for Responsible Application of Science and Technology"
A report by the Competitive Enterprise Institute, a pro-business group of global warming skeptics
Newest reports on US EPA website
An optimistic outlook on Global Warming from PBNV.com Apr 25, 2006
The Discovery of Global Warming from historian Spencer Weart, Director of the Center for History of Physics of the American Institute of Physics (AIP). IPS Inter Press Service - Independent news on global warming and its consequences. Category:Climate change feedbacks and causes
Category:History of climate
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zh-min-nan:Choân-kiû sio-lo̍h-hoà
zh-yue:全球變暖 Attributed and expected effects
left|thumb|280px|Sparse records indicate that glaciers have been retreating since the early 1800s. In the 1950s measurements began that allow the monitoring of glacial mass balance, reported to the WGMS and the NSIDC. Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes, reduced summer streamflows, species extinctions, and increases in the range of disease vectors. Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures. Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure. A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones. Additional anticipated effects include sea level rise of between 1990 and 2100, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections. However, few mechanistic studies have documented extinctions due to recent climate change and one study suggests that projected rates of extinction are uncertain. Economic
thumb|right|280px|The projected temperature increase for a range of stabilization scenarios (the colored bands). The black line in middle of the shaded area indicates 'best estimates'; the red and the blue lines the likely limits. From the work of IPCC AR4. Some economists have tried to estimate the aggregate net economic costs of damages from climate change across the globe. Such estimates have so far failed to reach conclusive findings; in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (tC) (US$-3 per tonne of carbon dioxide) up to US$350/tC (US$95 per tonne of carbon dioxide), with a mean of US$43 per tonne of carbon (US$12 per tonne of carbon dioxide). One widely publicized report on potential economic impact is the Stern Review; it suggests that extreme weather might reduce global gross domestic product by up to 1%, and that in a worst-case scenario global per capita consumption could fall 20%. The report's methodology, advocacy and conclusions have been criticized by many economists, primarily around the Review's assumptions of discounting and its choices of scenarios,Tol and Yohe (2006) "A Review of the Stern Review" World Economics 7(4): 233-50. See also other critiques in World Economics 7(4). while others have supported the general attempt to quantify economic risk, even if not the specific numbers. In a summary of economic cost associated with climate change, the United Nations Environment Programme emphasizes the risks to insurers, reinsurers, and banks of increasingly traumatic and costly weather events. Other economic sectors likely to face difficulties related to climate change include agriculture and transport. Developing countries, rather than the developed world, are at greatest economic risk. Security
In November 2007, the Center for Strategic and International Studies and the Center for a New American Security published a report highlighting the national security effects of climate change. These security effects include increased competition for resources between countries, mass migration from the worst affected areas, challenges to the cohesion of major states threatened by the rise in sea levels, and, as a consequence of these factors, an increased risk of armed conflict, including even nuclear conflicts. Adaptation and mitigation
The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO2 emissions. There has also been business action on climate change, including efforts at increased energy efficiency and limited moves towards use of alternative fuels. One important innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances. The world's primary international agreement on combating global warming is the Kyoto Protocol, an amendment to the United Nations Framework Convention on Climate Change (UNFCCC) negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions. Only the United States and Kazakhstan have not ratified the treaty, with the United States historically being the world's largest emitter of greenhouse gas. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.Climate talks face international hurdles, by Arthur Max, Associated press, 5/14/07. Claiming "serious harm" to the United States economy and the exemption of "80 percent of the world, including major population centers" like China and India, from the treaty, George W. Bush contends that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns. Still, various state, local, and regional governments within the United States have begun their own initiatives to indicate support and compliance with the Kyoto Protocol on a local basis. For example, the Regional Greenhouse Gas Initiative is a state-level emissions capping and trading program involving eight Northeastern U.S. states, which was founded on December 20, 2005. China and India, though exempt from its provisions as developing countries, have ratified the Kyoto Protocol. China may have passed the U.S. in total annual greenhouse gas emissions according to some recent studies. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming. The IPCC's Working Group III is responsible for crafting reports that deal with the mitigation of global warming and analyzing the costs and benefits of different approaches. In the 2007 IPCC Fourth Assessment Report, they conclude that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6% increase and 3% decrease in global gross domestic product. Social and political debate
thumb|280px|Per capita greenhouse gas emissions in 2000
thumb|280px|Per country greenhouse gas emissions in 2000
Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed. Increased awareness of the scientific findings surrounding global warming has resulted in political and economic debate. Poor regions, particularly Africa, appear at greatest risk from the suggested effects of global warming, while their actual emissions have been small compared to the developed world. At the same time, developing country exemptions from provisions of the Kyoto Protocol have been criticized by the United States and Australia, and have been used as part of the rationale for continued non-ratification by the U.S. In the Western world, the idea of human influence on climate has gained wider acceptance in Europe than in the United States. The issue of climate change has sparked debate weighing the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail. There has been discussion in several countries about the cost and benefits of adopting alternative energy sources in order to reduce carbon emissions. Organizations and companies such as the Competitive Enterprise Institute and ExxonMobil have emphasized more conservative climate change scenarios while highlighting the potential economic cost of stricter controls. Likewise, various environmental lobbies and a number of public figures have launched campaigns to emphasize the potential risks of climate change and promote the implementation of stricter controls. Some fossil fuel companies have scaled back their efforts in recent years, or called for policies to reduce global warming. Another point of debate is the degree to which newly developed economies such as India and China should be expected to constrain their emissions. China's gross national CO2 emissions are expected to exceed those of the U.S. within the next few years, and may have already done so according to a 2006 report. ; China has contended that it has less of an obligation to reduce emissions since its per capita emissions are roughly one-fifth that of the United States.China: US should take lead on climate, by Michael Casey, Associated Press, 12/7/07. India, also exempt from Kyoto restrictions and another of the biggest sources of industrial emissions, has made similar assertions.India's glaciers give grim message on warming, by Somni Sengupta, 7/17/07, New York Times via oregonlive.com. However, the U.S. contends that if they must bear the cost of reducing emissions, then China should do the same. Chinese object to climate draft, BBC, 5/1/07; In Battle for U.S. Carbon Caps, Eyes and Efforts Focus on China,by Steven Mufson, Washington Post, 6/6/07. Related climatic issues
A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. CO2 dissolved in the ocean reacts with water to form carbonic acid, resulting in acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,
and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2. Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services. Global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface, may have partially mitigated global warming in the late twentieth century. From 1960 to 1990 human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of the global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents. Ozone depletion, the steady decline in the total amount of ozone in Earth's stratosphere, is frequently cited in relation to global warming. Although there are areas of linkage, the relationship between the two is not strong. See also
Glossary of climate change
References
Further reading
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External links
Scientific
Intergovernmental Panel on Climate Change and UN Climate Panel Report's Key Findings. Nature Reports Climate Change
The UK Met Office Hadley Centre site
NOAA's Global Warming FAQ
Outgoing Longwave Radiation pentad mean – NOAA Climate Prediction Center
Discovery of Global Warming – An extensive introduction to the topic and the history of its discovery, written by Spencer R. Weart
Caution urged on climate 'risks'
NASA Finds Sun-Climate Connection in Old Nile Records
News in Science - Night flights are worse for global warming
Educational
What Is Global Warming? – Simulation from National Geographic
The EdGCM (Educational Global Climate Modelling) Project – A free research-quality simulation for students, educators, and scientists alike, with a user-friendly interface that runs on desktop computers
Daily global temperatures and trends from satellites – Interactive graphics from NASA
The Pew Center on global climate change
Governments
European Union Web site about global warming. Newest reports on U.S. EPA Web site
Other
Science and Technology Sources on the Internet – Extensive commented list of Internet resources
Union of Concerned Scientists Global Warming page
Watch and read 'Tipping Point', Australian science documentary about effects of global warming on rare, common, and endangered wildlife
IPS Inter Press Service – Independent news on global warming and its consequences. Gateway to the UN System's Work on Climate Change
Category:Climate change feedbacks and causes
Category:History of climate
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Category:Economic problems
Category:Carbon Finance
af:Aardverwarming
ang:Woruldlicu wearmung
ar:احترار عالمي
zh-min-nan:Choân-kiû sio-lo̍h-hoà
bs:Globalno zatopljenje
bg:Глобално затопляне
ca:Escalfament global
cs:Globální oteplování
da:Global opvarmning
de:Globale Erwärmung
et:Globaalne soojenemine
el:Παγκόσμια θέρμανση
es:Calentamiento global
eo:Tutmonda varmiĝo
eu:Berotze globala
fa:گرمایش زمین
fr:Réchauffement climatique
fur:Riscjaldament globâl
gd:Blàthachadh na cruinne
gl:Quentamento global
zh-classical:全球暖化
ko:지구 온난화
hy:Գլոբալ տաքացում
hi:भूमंडलीय ऊष्मीकरण
hr:Globalno zatopljenje
id:Pemanasan global
it:Riscaldamento globale
he:התחממות עולמית
lt:Visuotinis atšilimas
hu:Globális felmelegedés
mk:Глобално затоплување
mn:Дэлхийн дулаарал
nl:Opwarming van de Aarde
ja:地球温暖化
no:Global oppvarming
nn:Global oppvarming
oc:Escalfament global
pl:Globalne ocieplenie
pt:Aquecimento global
ro:Încălzire globală
ru:Глобальное потепление
simple:Global warming
sk:Globálne otepľovanie
sr:Глобално загревање
sh:Globalno zatopljenje
fi:Ilmaston lämpeneminen
sv:Global uppvärmning
ta:புவி வெப்பநிலை அதிகரிப்பு
th:ปรากฏการณ์โลกร้อน
vi:Sự nóng lên của khí hậu toàn cầu
tr:Küresel ısınma
uk:Глобальне потепління
yi:גלאבאלע אנווארימונג
zh-yue:全球變暖
zh:全球变暖
Highlight colors show which account likely added that new text: UBeR William M. Connolley