Saturday, May 26, 2007
GLOBAL WARMING
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.
Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the past century. 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,"[1] which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a small cooling effect since 1950.[2][3] 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. The American Association of Petroleum Geologists is the only scientific society that rejects these conclusions,[4][5] and a few individual scientists also disagree with parts of them.[6]
Climate models referenced by the IPCC reflect too much solar energy into space when compared to the actual climate, due to a positive surface albedo bias, [7] and an ice-albedo feedback to global warming that is too low.[8][9] These models project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100.[1] The range of projections reflects the use of differing scenarios of future greenhouse gas emissions and results of models with differences in 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.[1] This reflects the large heat capacity of the oceans.
An increase in global temperatures can in turn cause other changes, including sea level rise, and changes in the amount and pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. Other effects may include changes in agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate 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 combating greenhouse gas emissions.
Contents[hide]
1 Terminology
2 Causes
2.1 Greenhouse gases in the atmosphere
2.2 Feedbacks
2.3 Solar variation
3 History
3.1 From the present to the dawn of human settlement
3.2 Pre-human climate variations
4 Climate models
5 Attributed and expected effects
5.1 Economics
6 Mitigation and adaptation
7 Issue debate and political processes
8 Related climatic issues
9 See also
10 References
11 Further reading
12 External links
12.1 Scientific
12.2 Educational
12.3 Other
//
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.[10] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[11] The term "anthropogenic climate change" is sometimes used when focusing on human-induced changes.
Causes
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.
Main articles: Attribution of recent climate change and scientific opinion on climate change
The climate system varies through natural, internal processes and in response to variations in external forcing factors including solar activity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[12] identifies increased 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. Contrasting with the scientific consensus, other hypotheses have been proposed to explain most of the observed increase in global temperatures. Among these hypotheses are that the warming is caused by natural fluctuations in the climate or that warming is mainly a result of variations in solar radiation.[13]
None of the effects of forcing are instantaneous. 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. Climate commitment studies indicate that even if greenhouse gases were stabilized at present day levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[14]
Greenhouse gases in the atmosphere
Main article: Greenhouse effect
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 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 warms a planet's atmosphere and surface.
Greenhouse gases create a natural greenhouse effect, without which mean temperatures on Earth would be an estimated 30 °C (54 °F) lower so that Earth would be uninhabitable.[15] Thus scientists do not "believe in" or "oppose" the greenhouse effect as such; rather, the debate concerns the net effect of the addition of greenhouse gases, while allowing for associated positive and negative feedback mechanisms.
On Earth, the major natural 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%. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. 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.[16] "About three-quarters of the anthropogenic [man-made] emissions of CO2 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."[17]
The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[18] 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, 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.[19] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[20]
Positive feedback effects such as the expected release of CH4 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[21] not included in climate models cited by the IPCC.[1]
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. CO2 injected into the atmosphere causes a warming of the atmosphere and the earth's surface. The warming causes 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 at a slight increase in humidity and with a much larger greenhouse effect than that due to CO2 alone.[22] 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 and debate. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect. Seen from above, the same clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Increased global water vapor concentration may or may not cause an increase in global average cloud cover. The net effect of clouds thus has not been well modeled, however, cloud feedback is second only to water vapor feedback and is positive in all the models that contributed to the IPCC Fourth Assessment Report.[22]
Another important feedback process is ice-albedo feedback.[23] 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 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 is an additional mechanism contributing to warming. Possible positive feedback due to CH4 release from melting seabed ices is a further mechanism to be considered.
Solar variation
Solar variation over the last 30 years.
Main article: Solar variation
Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming.[24] A difference between this mechanism and greenhouse warming is that an increase in solar activity should produce a warming of the stratosphere while greenhouse warming should produce a cooling of the stratosphere. Reduction of stratospheric ozone also has a cooling influence but substantial ozone depletion did not occur until the late 1970's. Cooling in the lower stratosphere has been observed since at least 1960.[25]
Other phenomena such as solar variation combined with volcanoes have probably had a warming effect from pre-industrial times to 1950, but a cooling effect since 1950.[1] However, some research has suggested that the Sun's contribution may have been underestimated. Two researchers at Duke University have estimated that the Sun may have contributed about 40–50% of the global surface temperature warming over the period 1900–2000, and about 25–35% between 1980 and 2000.[26] Stott and coauthors suggest 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.[27] They conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.
History
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.
Main article: Temperature record
From the present to the dawn of human settlement
Global temperatures on both land and sea have increased by 0.75 °C (1.4 °F) 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. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[28] 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.
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.[29] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[30][31]
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,[32] though the cooling may also be due in part to natural variability.
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.[33] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[34]
Pre-human climate variations
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.
Further information: Paleoclimatology
See also: Snowball Earth
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.[35]
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 (9.0 °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.[36][37]
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
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.
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).
Main article: Global climate model
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 some simplifications being necessary because of limitations in computer power. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of fossil fuel consumption and CO2 emission are used, the amount of projected warming varies between models and there still remains a considerable range of climate sensitivity.
Including uncertainties in the models and in future greenhouse gas concentrations, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100. 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.
Climate models can produce a good match to observations of global temperature changes over the last century, but "cannot yet simulate all aspects of climate."[38] 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.
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.[39][40][41]
The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[42] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.
Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the past century. 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,"[1] which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect. Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a small cooling effect since 1950.[2][3] 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. The American Association of Petroleum Geologists is the only scientific society that rejects these conclusions,[4][5] and a few individual scientists also disagree with parts of them.[6]
Climate models referenced by the IPCC reflect too much solar energy into space when compared to the actual climate, due to a positive surface albedo bias, [7] and an ice-albedo feedback to global warming that is too low.[8][9] These models project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100.[1] The range of projections reflects the use of differing scenarios of future greenhouse gas emissions and results of models with differences in 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.[1] This reflects the large heat capacity of the oceans.
An increase in global temperatures can in turn cause other changes, including sea level rise, and changes in the amount and pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. Other effects may include changes in agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate 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 combating greenhouse gas emissions.
Contents[hide]
1 Terminology
2 Causes
2.1 Greenhouse gases in the atmosphere
2.2 Feedbacks
2.3 Solar variation
3 History
3.1 From the present to the dawn of human settlement
3.2 Pre-human climate variations
4 Climate models
5 Attributed and expected effects
5.1 Economics
6 Mitigation and adaptation
7 Issue debate and political processes
8 Related climatic issues
9 See also
10 References
11 Further reading
12 External links
12.1 Scientific
12.2 Educational
12.3 Other
//
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.[10] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[11] The term "anthropogenic climate change" is sometimes used when focusing on human-induced changes.
Causes
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.
Main articles: Attribution of recent climate change and scientific opinion on climate change
The climate system varies through natural, internal processes and in response to variations in external forcing factors including solar activity, volcanic emissions, variations in the earth's orbit (orbital forcing) and greenhouse gases. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[12] identifies increased 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. Contrasting with the scientific consensus, other hypotheses have been proposed to explain most of the observed increase in global temperatures. Among these hypotheses are that the warming is caused by natural fluctuations in the climate or that warming is mainly a result of variations in solar radiation.[13]
None of the effects of forcing are instantaneous. 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. Climate commitment studies indicate that even if greenhouse gases were stabilized at present day levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[14]
Greenhouse gases in the atmosphere
Main article: Greenhouse effect
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 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 warms a planet's atmosphere and surface.
Greenhouse gases create a natural greenhouse effect, without which mean temperatures on Earth would be an estimated 30 °C (54 °F) lower so that Earth would be uninhabitable.[15] Thus scientists do not "believe in" or "oppose" the greenhouse effect as such; rather, the debate concerns the net effect of the addition of greenhouse gases, while allowing for associated positive and negative feedback mechanisms.
On Earth, the major natural 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%. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. 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.[16] "About three-quarters of the anthropogenic [man-made] emissions of CO2 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."[17]
The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[18] 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, 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.[19] Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.[20]
Positive feedback effects such as the expected release of CH4 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[21] not included in climate models cited by the IPCC.[1]
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. CO2 injected into the atmosphere causes a warming of the atmosphere and the earth's surface. The warming causes 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 at a slight increase in humidity and with a much larger greenhouse effect than that due to CO2 alone.[22] 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 and debate. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect. Seen from above, the same clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Increased global water vapor concentration may or may not cause an increase in global average cloud cover. The net effect of clouds thus has not been well modeled, however, cloud feedback is second only to water vapor feedback and is positive in all the models that contributed to the IPCC Fourth Assessment Report.[22]
Another important feedback process is ice-albedo feedback.[23] 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 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 is an additional mechanism contributing to warming. Possible positive feedback due to CH4 release from melting seabed ices is a further mechanism to be considered.
Solar variation
Solar variation over the last 30 years.
Main article: Solar variation
Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming.[24] A difference between this mechanism and greenhouse warming is that an increase in solar activity should produce a warming of the stratosphere while greenhouse warming should produce a cooling of the stratosphere. Reduction of stratospheric ozone also has a cooling influence but substantial ozone depletion did not occur until the late 1970's. Cooling in the lower stratosphere has been observed since at least 1960.[25]
Other phenomena such as solar variation combined with volcanoes have probably had a warming effect from pre-industrial times to 1950, but a cooling effect since 1950.[1] However, some research has suggested that the Sun's contribution may have been underestimated. Two researchers at Duke University have estimated that the Sun may have contributed about 40–50% of the global surface temperature warming over the period 1900–2000, and about 25–35% between 1980 and 2000.[26] Stott and coauthors suggest 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.[27] They conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.
History
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.
Main article: Temperature record
From the present to the dawn of human settlement
Global temperatures on both land and sea have increased by 0.75 °C (1.4 °F) 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. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[28] 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.
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.[29] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[30][31]
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,[32] though the cooling may also be due in part to natural variability.
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.[33] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[34]
Pre-human climate variations
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.
Further information: Paleoclimatology
See also: Snowball Earth
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.[35]
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 (9.0 °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.[36][37]
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
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.
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).
Main article: Global climate model
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 some simplifications being necessary because of limitations in computer power. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of fossil fuel consumption and CO2 emission are used, the amount of projected warming varies between models and there still remains a considerable range of climate sensitivity.
Including uncertainties in the models and in future greenhouse gas concentrations, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100. 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.
Climate models can produce a good match to observations of global temperature changes over the last century, but "cannot yet simulate all aspects of climate."[38] 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.
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.[39][40][41]
The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[42] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.
warmer and sicker? Global warming and human health
Global warming will have many impacts on human health
In August 2003, Europe suffered its worst heatwave in recent memory. In France, temperatures peaked at about 40°C; unprepared for that kind of heat, many people – mostly the sick and elderly – succumbed. In all, nearly 15,000 deaths in France that summer were attributed to the high temperatures; across Europe, the scorching weather may have claimed as many as 35,000 lives.
According to the bulk of scientific opinion, the world is getting warmer. It is difficult, if not impossible, to prove the causes of this warming, but many scientists are convinced that increasing concentrations of greenhouse gases in the atmosphere are at least partly to blame.
How might the climate change?
Scientists use computer-based models to predict the effects on global climate of different levels of greenhouse gases in the atmosphere. According to the most recent projections of the Intergovernmental Panel on Climate Change (IPCC), the global mean temperature could increase by 1.4°C to 5.8°C between 1990 and 2100. The climatic effects of such a temperature increase might include:
Related site: Climate modelsDescribes a number of different computer climate models and how they are used.(Hadley Centre for Climate Prediction and Research, UK)
more frequent extreme high maximum temperatures and less frequent extreme low minimum temperatures;
an increase in the variability of climate, with changes to both the frequency and severity of extreme weather events;
alterations to the natural biological range of certain infectious diseases;
rising sea levels.
In Australia the climate is expected to become significantly warmer: by 2070 the annual average temperature is predicted to increase by 1°C to 6°C over most of Australia. The number of extreme rainfall events – such as those leading to flooding – is also expected to increase, even though most of the country is anticipated to become drier overall in the 21st century.
Heatwaves and cold snaps
Perhaps the most obvious impact of global warming will be the direct effects: a warmer planet will experience more extreme heatwaves. As seen in Europe in 2003, heatwaves often lead to an increase in the number of human deaths, particularly in temperate countries where people are often not accustomed to very hot weather and where houses and other infrastructure are not designed to cope with it. The sick and elderly are most vulnerable because their bodies are less able to increase cardiac output and sweat function for cooling purposes; they are often less able to afford cooling technologies.
It is difficult to predict the future effect on mortality levels, because as heatwaves become more frequent we can expect societies to adjust – technologically, behaviourally and physiologically. Technological adaptations such as the installation of effective air-conditioners and the construction of heat-minimising houses will happen more quickly among the rich, so heatwaves are likely to have a disproportionate effect in less-developed countries and in the poorer segments of rich societies.
In countries that currently experience extremely cold weather in winter, an increase in the mortality rate in summer might be offset by a decrease in winter mortality. Northern countries with severe winters have a higher mortality rate in winter because more sick and elderly people succumb in cold weather and because blizzards and extreme cold create dangerous conditions in which accidental deaths are more likely. Winters will tend to be milder under global warming, with the likely effect that winter death rates will decline.
Without measures to mitigate the effects of extreme heat, and with an increase in the proportion of older people in the population, we might therefore expect higher death tolls in Australia's future heatwaves. It is estimated that there are currently about 1100 heat-related deaths per year.
Extreme events and disasters
Most computer models generated by scientists indicate that the future climate will be more variable than in the past, and that droughts and floods will be more severe. Some of the health effects of weather-related disasters, in addition to the immediate death and injury to people and damage to property, include:
increases in psychological stress, depression, and feelings of isolation amongst people affected by natural disasters;
decreases in nutrition due to poorer agricultural yields caused, for example, by prolonged drought and problems of food distribution;
increases in disease transmission due to a breakdown in sewerage and garbage services. For example, cholera is one disease that thrives in such situations, particularly when flooding causes the contamination of drinking water by sewerage systems.
Australia's climate is naturally variable, although generally arid. In such a country, the implications of an even greater variation in rainfall is likely to be profound. Apart from the ecological and agricultural impacts, the availability of water may be reduced, with implications for human health. More frequent drought conditions would increase the risk of bushfires, which can kill people, release large quantities of particulate matter that can cause respiratory problems, and degrade water catchments.
Infectious diseases
Many infectious diseases are dependent on vector organisms, which are sensitive to environmental factors and therefore will be affected by global warming. Biological modelling under various climate scenarios suggests a widening of the potential transmission zone of some disease-causing pathogens and their vectors, such as mosquitoes (Box 1: Mosquito-borne disease).
Food- and water-borne diseases are also susceptible to climate change. Already, Australians suffer an estimated 4.6 million cases of diarrhoea or gastroenteritis each year, often caused by food contaminated with bacteria, parasites and (to a lesser extent) fungi and viruses. Food-poisoning bacteria grow best when the ambient temperature is in the range 35-37°C. Scientists speculate that if temperatures rise under global warming, the incidence of diseases caused by food-poisoning and by the contamination of drinking (and swimming) water could increase dramatically.
Rising sea levels
Scientists predict that sea levels will rise as the global temperature rises, due to the melting of land-based ice in the polar regions and glaciers, and the thermal expansion of the oceans. According to the most recent projections, sea levels could rise between 9 and 88 centimetres by the year 2100. A rise of this magnitude would have disastrous consequences for people living on low-lying islands, such as the Maldives group in the Indian Ocean and many South Pacific islands. Higher sea levels lead to coastal flooding and an increase in the frequency of extreme high water levels from storm surges. Related problems are the contamination of coastal freshwater supplies with encroaching sea water, and the degradation of fishing and agricultural areas.
The number of Australian fatalities from coastal flooding and storm surges has historically been low. It is currently estimated that 250 people each year experience coastal flooding due to storm surges, but this number could double by 2050. For the Pacific region as a whole, however, the number of people exposed to coastal flooding could be between 60,000 and 90,000 in an average year, a 50-fold increase on today's estimates.
Warmer and sicker?
Considerable uncertainty remains about how the climate may change and how such changes might affect human health. It seems likely, however, that people living in tropical and sub-tropical areas will be most affected. Affluent countries and social groups will be best able to adapt to climate change by reducing the impacts of natural disasters such as flooding, fire and drought, by maintaining high-quality health and emergency infrastructures, and by installing technologies that help ward off the worst climatic effects.
Neither uncertainty nor complacency should be allowed to prevent action to reduce greenhouse gas emissions. The risk to human (and ecological) well-being is too great, and prevention will be far better – and easier – than cure.
Box
In August 2003, Europe suffered its worst heatwave in recent memory. In France, temperatures peaked at about 40°C; unprepared for that kind of heat, many people – mostly the sick and elderly – succumbed. In all, nearly 15,000 deaths in France that summer were attributed to the high temperatures; across Europe, the scorching weather may have claimed as many as 35,000 lives.
According to the bulk of scientific opinion, the world is getting warmer. It is difficult, if not impossible, to prove the causes of this warming, but many scientists are convinced that increasing concentrations of greenhouse gases in the atmosphere are at least partly to blame.
How might the climate change?
Scientists use computer-based models to predict the effects on global climate of different levels of greenhouse gases in the atmosphere. According to the most recent projections of the Intergovernmental Panel on Climate Change (IPCC), the global mean temperature could increase by 1.4°C to 5.8°C between 1990 and 2100. The climatic effects of such a temperature increase might include:
Related site: Climate modelsDescribes a number of different computer climate models and how they are used.(Hadley Centre for Climate Prediction and Research, UK)
more frequent extreme high maximum temperatures and less frequent extreme low minimum temperatures;
an increase in the variability of climate, with changes to both the frequency and severity of extreme weather events;
alterations to the natural biological range of certain infectious diseases;
rising sea levels.
In Australia the climate is expected to become significantly warmer: by 2070 the annual average temperature is predicted to increase by 1°C to 6°C over most of Australia. The number of extreme rainfall events – such as those leading to flooding – is also expected to increase, even though most of the country is anticipated to become drier overall in the 21st century.
Heatwaves and cold snaps
Perhaps the most obvious impact of global warming will be the direct effects: a warmer planet will experience more extreme heatwaves. As seen in Europe in 2003, heatwaves often lead to an increase in the number of human deaths, particularly in temperate countries where people are often not accustomed to very hot weather and where houses and other infrastructure are not designed to cope with it. The sick and elderly are most vulnerable because their bodies are less able to increase cardiac output and sweat function for cooling purposes; they are often less able to afford cooling technologies.
It is difficult to predict the future effect on mortality levels, because as heatwaves become more frequent we can expect societies to adjust – technologically, behaviourally and physiologically. Technological adaptations such as the installation of effective air-conditioners and the construction of heat-minimising houses will happen more quickly among the rich, so heatwaves are likely to have a disproportionate effect in less-developed countries and in the poorer segments of rich societies.
In countries that currently experience extremely cold weather in winter, an increase in the mortality rate in summer might be offset by a decrease in winter mortality. Northern countries with severe winters have a higher mortality rate in winter because more sick and elderly people succumb in cold weather and because blizzards and extreme cold create dangerous conditions in which accidental deaths are more likely. Winters will tend to be milder under global warming, with the likely effect that winter death rates will decline.
Without measures to mitigate the effects of extreme heat, and with an increase in the proportion of older people in the population, we might therefore expect higher death tolls in Australia's future heatwaves. It is estimated that there are currently about 1100 heat-related deaths per year.
Extreme events and disasters
Most computer models generated by scientists indicate that the future climate will be more variable than in the past, and that droughts and floods will be more severe. Some of the health effects of weather-related disasters, in addition to the immediate death and injury to people and damage to property, include:
increases in psychological stress, depression, and feelings of isolation amongst people affected by natural disasters;
decreases in nutrition due to poorer agricultural yields caused, for example, by prolonged drought and problems of food distribution;
increases in disease transmission due to a breakdown in sewerage and garbage services. For example, cholera is one disease that thrives in such situations, particularly when flooding causes the contamination of drinking water by sewerage systems.
Australia's climate is naturally variable, although generally arid. In such a country, the implications of an even greater variation in rainfall is likely to be profound. Apart from the ecological and agricultural impacts, the availability of water may be reduced, with implications for human health. More frequent drought conditions would increase the risk of bushfires, which can kill people, release large quantities of particulate matter that can cause respiratory problems, and degrade water catchments.
Infectious diseases
Many infectious diseases are dependent on vector organisms, which are sensitive to environmental factors and therefore will be affected by global warming. Biological modelling under various climate scenarios suggests a widening of the potential transmission zone of some disease-causing pathogens and their vectors, such as mosquitoes (Box 1: Mosquito-borne disease).
Food- and water-borne diseases are also susceptible to climate change. Already, Australians suffer an estimated 4.6 million cases of diarrhoea or gastroenteritis each year, often caused by food contaminated with bacteria, parasites and (to a lesser extent) fungi and viruses. Food-poisoning bacteria grow best when the ambient temperature is in the range 35-37°C. Scientists speculate that if temperatures rise under global warming, the incidence of diseases caused by food-poisoning and by the contamination of drinking (and swimming) water could increase dramatically.
Rising sea levels
Scientists predict that sea levels will rise as the global temperature rises, due to the melting of land-based ice in the polar regions and glaciers, and the thermal expansion of the oceans. According to the most recent projections, sea levels could rise between 9 and 88 centimetres by the year 2100. A rise of this magnitude would have disastrous consequences for people living on low-lying islands, such as the Maldives group in the Indian Ocean and many South Pacific islands. Higher sea levels lead to coastal flooding and an increase in the frequency of extreme high water levels from storm surges. Related problems are the contamination of coastal freshwater supplies with encroaching sea water, and the degradation of fishing and agricultural areas.
The number of Australian fatalities from coastal flooding and storm surges has historically been low. It is currently estimated that 250 people each year experience coastal flooding due to storm surges, but this number could double by 2050. For the Pacific region as a whole, however, the number of people exposed to coastal flooding could be between 60,000 and 90,000 in an average year, a 50-fold increase on today's estimates.
Warmer and sicker?
Considerable uncertainty remains about how the climate may change and how such changes might affect human health. It seems likely, however, that people living in tropical and sub-tropical areas will be most affected. Affluent countries and social groups will be best able to adapt to climate change by reducing the impacts of natural disasters such as flooding, fire and drought, by maintaining high-quality health and emergency infrastructures, and by installing technologies that help ward off the worst climatic effects.
Neither uncertainty nor complacency should be allowed to prevent action to reduce greenhouse gas emissions. The risk to human (and ecological) well-being is too great, and prevention will be far better – and easier – than cure.
Box
Subscribe to:
Posts (Atom)