Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about 0.8 °C (1.4 °F) with about two thirds of the increase occurring over just the last three decades.[2] Warming of the climate system is unequivocal, and scientists are more than 90% certain most of it is caused by increasing concentrations of greenhouse gases produced by human activities such as deforestation and burning fossil fuels.[3][4][5][6] These findings are recognized by the national science academies of all the major industrialized countries.[7][A]
Climate model projections are summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicate that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest.[8] The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.[9][10]
An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, and a probable expansion of subtropical deserts.[11] Warming is expected to bestrongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent occurrence of extreme weatherevents including heatwaves, droughts and heavy rainfall events, species extinctions due to shifting temperature regimes, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others.[12] In a 4 °C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved.[13]
Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC),[14] whose ultimate objective is to prevent "dangerous" anthropogenic (i.e., human-induced) climate change.[15] Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions[16]:10[17][18][19]:9 and to assist in adaptation to global warming.[16]:13[19]:10[20][21] Parties to the UNFCCC have agreed that deep cuts in emissions are required,[22] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.[22][B] Analyses by the United Nations Environment Programme (published in 2011)[23] and International Energy Agency (2011)[24] suggest that current efforts to reduce emissions may be inadequately stringent to meet the UNFCCC's 2 °C target.
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.[30][31][32] Current estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, “the error estimate for individual years ... is at least ten times larger than the differences between these three years.”[33] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, “The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant...”[34]
Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.[35] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.[36][37]
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[38] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.[39] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[40]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[41]
Climate model projections are summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicate that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest.[8] The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.[9][10]
An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, and a probable expansion of subtropical deserts.[11] Warming is expected to bestrongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent occurrence of extreme weatherevents including heatwaves, droughts and heavy rainfall events, species extinctions due to shifting temperature regimes, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others.[12] In a 4 °C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved.[13]
Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC),[14] whose ultimate objective is to prevent "dangerous" anthropogenic (i.e., human-induced) climate change.[15] Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions[16]:10[17][18][19]:9 and to assist in adaptation to global warming.[16]:13[19]:10[20][21] Parties to the UNFCCC have agreed that deep cuts in emissions are required,[22] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.[22][B] Analyses by the United Nations Environment Programme (published in 2011)[23] and International Energy Agency (2011)[24] suggest that current efforts to reduce emissions may be inadequately stringent to meet the UNFCCC's 2 °C target.
Observed temperaturchangesEvidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.[25][26][27] The Earth's average surface temperature, expressed as a linear trend, rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade). The urban heat island effect is very small, estimated to account for less than 0.002 °C of warming per decade since 1900.[28] Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Climate proxies show the temperature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.[29]
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.[30][31][32] Current estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, “the error estimate for individual years ... is at least ten times larger than the differences between these three years.”[33] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, “The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant...”[34]
Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.[35] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.[36][37]
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[38] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.[39] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[40]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[41]