Global warming refers to 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 last 100 years. 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] 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.[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. However, a few individual scientists disagree with some of the main conclusions of the IPCC.[4]
Climate models referenced by the IPCC 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 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.[1] This reflects the large heat capacity of the oceans.
An increase in global temperatures is expected to cause other changes, including sea level rise, increased intensity of extreme weather events,[5] and changes in the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, 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.[6] The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.[7] The term "anthropogenic global warming" is sometimes used when focusing on human-induced changes.
Causes
-
Earth's climate changes in response to external forcing, including variations in its orbit around the sun (orbital forcing),[8][9][10] volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus[11] 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. In contrast to the scientific consensus that recent warming is mainly attributable to elevated levels of greenhouse gases, other hypotheses 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.[1][2][3][4]
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 0.5 °C (0.9 °F) would still occur.[12]
Greenhouse gases in the atmosphere
-
Recent increases in atmospheric carbon dioxide (CO
2). The monthly CO
2 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 CO
2 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.
Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 30 °C (54 °F), without which Earth would be uninhabitable.[13] The debate centers on 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%.[14][15] 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 methane 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] 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.[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, 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.[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 (reinforcing) 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[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. 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.)[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. 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. 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 (about 125 to 500 km for models used in the IPCC Fourth Assessment Report). 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.[22]
Another important feedback process is ice-albedo feedback.[23] 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 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.
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 favour of smaller phytoplankton that are poorer biological pumps of carbon.[24]
Solar variation
Solar variation over the last 30 years.
-
Variations in solar output, possibly amplified by cloud feedbacks, may have contributed to recent warming.[25] A difference between this mechanism and greenhouse warming is that an increase in solar activity should warm the stratosphere while greenhouse warming should cool the stratosphere. Cooling in the lower stratosphere has been observed since at least 1960,[26] which would not be expected if solar activity were the main contributor to recent warming. (Reduction of stratospheric ozone also has a cooling influence but substantial ozone depletion did not occur until the late 1970s.) 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]
A few papers suggest 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 warming over the period 1900–2000, and about 25–35% between 1980 and 2000.[27] 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.[28] Nevertheless, 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.
In 2006, a team of scientists from the United States, Germany, and Switzerland found no net increase of solar brightness over the last thousand years. Solar cycles lead to a small increase of 0.07% in brightness over the last 30 years. This effect is far too minute to contribute significantly to global warming.[29][30] A 2007 paper by Lockwood and Fröhlich further confirms the lack of a correlation between solar output and global warming for the time since 1985.[31]
Temperature changes
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.
-
Recent
Global temperatures on both land and sea have increased by 0.75 °C (1.35 °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).[32] 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 [5]. Since the northern hemisphere has more land mass than the southern 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.[33] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[34][35]
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,[36] 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.[37] Ruddiman's interpretation of the historical record, with respect to the methane data, has been disputed.[38]
Pre-human climate variations
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.
- 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.[39]
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.[40][41]
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
The projected temperature increase for a range of stabilization scenarios (the coloured 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, 2007.
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 21
st 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 some simplifications being necessary because of limitations in computer power. These models predict that the effect of adding greenhouse gases is to produce a warmer climate.[42] However, even when the same assumptions of future GHG levels are used, there still remains a considerable range of climate sensitivity.
Including uncertainties in future greenhouse gas concentrations and climate modelling, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100.[1] 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.[43] 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.[44][45][46]
The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[47] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.
Attributed and expected effects
-
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.
Though it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause other changes, including glacial retreat 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, 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 the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[48] 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.[49]
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.[50] 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.[48] 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.[1]
Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[51] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency 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.[52] Two populations of Bay checkerspot butterfly are being threatened by changes in precipitation, though few mechanistic studies have documented extinctions due to recent climate change.[53]
Economics
-
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).[50] 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%.[54] 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,[55] while others have supported the general attempt to quantify economic risk, even if not the specific numbers.[56][57]
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.[58]
Adaptation and mitigation
-
The broad agreement among climate scientists that global temperatures will continue to increase has led 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. There has also been business action on climate change, including efforts at increased energy efficiency and (still limited) moves to 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.[59].
The United States (historically the world's largest greenhouse gas emitter), Australia, and Kazakhstan have not ratified the treaty.
China (which is expected to soon overtake the US in greenhouse gas emissions) and India have ratified the treaty, but as developing countries, are exempt from its provisions. Chinese Premier Wen Jiabao has called on the nation to redouble its efforts to tackle pollution and global warming.[60]
This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.[61]
The world's primary body for crafting a response is the Intergovernmental Panel on Climate Change (IPCC), a UN-sponsored activity which holds periodic meetings between national delegations on the problems of global warming, and issues working papers and assessments on the current status of the science of climate change, impacts, and mitigation. It convenes four different working groups examining various specific issues. For example, in May 2007, the IPCC held conferences in Bonn, Germany,[62] and in Bangkok, Thailand.[63]
In the absence of clear concerted action by the US Federal government, various state, local, and regional governments 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 (RGGI),[64] is a state-level emissions capping and trading program, which was founded on January 18, 2007 and is comprised of eight Northeastern US states.
Issue debate, political processes and laws
-
- See also: Climate change denial and Scientific opinion on climate change
Over the past several years, public perceptions and attitudes concerning the causes and importance of global warming have changed.[65] 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 negligible compared to the developed world.[66] 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 their rationale for continued non-ratification.[67] In the Western world, the idea of human influence on climate and efforts to combat it has gained wider acceptance in Europe than in the United States.[68][69]
Fossil fuel organizations and companies such as American Petroleum Institute and ExxonMobil, represented by individuals such as Philip Cooney and some think tanks such as the Competitive Enterprise Institute and the Cato Institute, have campaigned to downplay the risks of climate change,[70][71][72][73] described by some as climate change denial. Environmental groups and public figures have launched campaigns emphasizing the risks. Recently, some fossil fuel companies have scaled back such efforts[74] or called for policies to reduce global warming.[75]
This issue has sparked debate regarding the benefits of limiting industrial emissions of greenhouse gases versus the effects on economic activity. In the U.S., the political manipulation of scientific testimonies and reports has also become an issue.[76][77] There has also been discussion in several countries about the cost of adopting alternate, cleaner energy sources in order to reduce emissions.[78]
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 CO2 emissions (mainly from coal power plants and cars), are expected to exceed those of the U.S. within the next few years (and according to one report may have already done so[79]). China has contended that it has less obligation to reduce emissions, since its emissions per capita are about one-fifth those of the U.S.; the U.S. contends that if they must bear the costs of reducing emissions, so should China.[80] India will also soon be one of the biggest sources of industrial emissions, and has made assertions similar to China's on this issue.[81]
In an interview with the BBC, President Bush´s advisor and director of the Office of Science and Technology Policy, John Marburger said that climate change is taking place and there is more than 90 percent certainty that it is due to man-made greenhouse gas (GHG) emissions. [82]
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.[83] CO2 dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era,[84] and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[1][85] 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.[86]
Another related issue that may have partially mitigated global warming in the late twentieth century is global dimming, the gradual reduction in the amount of global direct irradiance at the Earth's surface. 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 global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[1]
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.
