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Global warming is the observed and projected increase in the average temperature of Earth's atmosphere and oceans that became apparent by the latter half of the 20th century. The Earth's average near-surface atmospheric temperature rose 0.6 ? 0.2 ? Celsius (1.1 ? 0.4 ? Fahrenheit) in the 20th century.[1]
The scientific consensus on global warming has been summarized by the IPCC: "In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations".[2] Only a small minority of climate scientists disagree that humanity's actions have played a major role in recent warming. [3]
Greenhouse gases, such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone occur naturally in Earth's atmosphere. These gases absorb infrared radiation causing the greenhouse effect which heats our planet, Mars, Venus and other celestial bodies with atmospheres. It is believed that without greenhouse gases the Earth's surface would be up to 30? C cooler. Recently, the amount of greenhouse gas in the atmosphere has increased due to the burning of fossil fuels by humans.
Increasing global temperatures are expected to cause a broad range of changes. Sea levels are expected to rise by about 0.5m by 2100. Changes in temperature and precipitation patterns are likely to increase the frequency, duration, and intensity of other extreme weather events, such as floods, droughts, heat waves, and tornadoes. Other consequences may include altered agricultural yields, further glacial retreat and reduced summer stream flows. Although warming is expected to affect the number and magnitude of these events, it is difficult to connect specific events to global warming.
World net carbon-emission rates would need to be reduced approximately 60%?80% by 2050 to keep global temperatures within 1? C (1.8? F) above present[4]. A 1? C rise would likely raise sea levels by no more than approximately 5 meters (16 ft) over the next 200 to 1000 years[5]. A projection of current trends?as represented by IPCC scenarios A1B or A2?gives temperatures 3? C above present by the year 2100 or soon afterwards[6]. A 3? C rise would likely raise sea levels by 25 ? 10 meters (82 ? 33 ft) [7] Although most studies focus on the period up to 2100, warming is expected to continue past then because CO2 has an estimated atmospheric lifetime of 50 to 200 years.[8]
Some degree of uncertainty remains regarding exactly how much climate change should be expected in the future. A hotly contested political and public debate has yet to be resolved, regarding whether anything should be done, and what could be cost-effectively done to reduce or reverse future warming, or to deal with the expected consequences.
Causes
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
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.[12]
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.[13][14] 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,[15][16][17] but others have been increased or created by agricultural activities such as the cultivation of rice paddies.[18] Recent evidence indicates that methane concentrations have begun to stabilize, perhaps due to reductions in leakage from fuel transmission and storage facilities.[19]
Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage. The rate of rise 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,[20] 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 oceans[21]) are being degraded by pollutants.[22] Degradation of major carbon sinks results in higher atmospheric carbon dioxide levels.
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.[23] 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 observational[24] 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%).[25] 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.[26] 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,[27] though this may also be due to intervening natural cycles.
The scientific consensus on global warming has been summarized by the IPCC: "In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations".[2] Only a small minority of climate scientists disagree that humanity's actions have played a major role in recent warming. [3]
Greenhouse gases, such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone occur naturally in Earth's atmosphere. These gases absorb infrared radiation causing the greenhouse effect which heats our planet, Mars, Venus and other celestial bodies with atmospheres. It is believed that without greenhouse gases the Earth's surface would be up to 30? C cooler. Recently, the amount of greenhouse gas in the atmosphere has increased due to the burning of fossil fuels by humans.
Increasing global temperatures are expected to cause a broad range of changes. Sea levels are expected to rise by about 0.5m by 2100. Changes in temperature and precipitation patterns are likely to increase the frequency, duration, and intensity of other extreme weather events, such as floods, droughts, heat waves, and tornadoes. Other consequences may include altered agricultural yields, further glacial retreat and reduced summer stream flows. Although warming is expected to affect the number and magnitude of these events, it is difficult to connect specific events to global warming.
World net carbon-emission rates would need to be reduced approximately 60%?80% by 2050 to keep global temperatures within 1? C (1.8? F) above present[4]. A 1? C rise would likely raise sea levels by no more than approximately 5 meters (16 ft) over the next 200 to 1000 years[5]. A projection of current trends?as represented by IPCC scenarios A1B or A2?gives temperatures 3? C above present by the year 2100 or soon afterwards[6]. A 3? C rise would likely raise sea levels by 25 ? 10 meters (82 ? 33 ft) [7] Although most studies focus on the period up to 2100, warming is expected to continue past then because CO2 has an estimated atmospheric lifetime of 50 to 200 years.[8]
Some degree of uncertainty remains regarding exactly how much climate change should be expected in the future. A hotly contested political and public debate has yet to be resolved, regarding whether anything should be done, and what could be cost-effectively done to reduce or reverse future warming, or to deal with the expected consequences.
Causes
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
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.[12]
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.[13][14] 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,[15][16][17] but others have been increased or created by agricultural activities such as the cultivation of rice paddies.[18] Recent evidence indicates that methane concentrations have begun to stabilize, perhaps due to reductions in leakage from fuel transmission and storage facilities.[19]
Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage. The rate of rise 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,[20] 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 oceans[21]) are being degraded by pollutants.[22] Degradation of major carbon sinks results in higher atmospheric carbon dioxide levels.
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.[23] 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 observational[24] 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%).[25] 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.[26] 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,[27] though this may also be due to intervening natural cycles.
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