Climate model projections summarized in the 2007 IPCC report indicate that the global surface temperature is likely to rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.[2] The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts.[5] Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent and intense precipitation events, extreme weather events, species extinctions due to shifting isotherms, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional changes is uncertain.[6] As a result of contemporary increases in atmospheric carbon dioxide, the oceans have become more acidic, a result that is predicted to continue.[7][8]
The scientific consensus is that anthropogenic global warming is occurring. This finding is recognized by the national science academies of all the major industrialized countries and is not rejected by any scientific body of national or international standing.[9][10][11][B] According to a recent Gallup poll, “People nearly everywhere, including majorities in developed Asia and Latin America, are more likely to attribute global warming to human activities rather than natural causes. The U.S. is the exception, with nearly half (47%) -- and the largest percentage in the world -- attributing global warming to natural causes.”[12]
The Kyoto Protocol is aimed at stabilizing greenhouse gas concentration to prevent a "dangerous anthropogenic interference".[13] As of November 2009, 187 states had signed and ratified the protocol.[14] Proposed responses to global warming include mitigation to reduce emissions, adaptation to the effects of global warming, and geoengineering to remove greenhouse gases from the atmosphere.
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Temperature changes
Main article: Temperature record
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.[21][22][23] Current estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, “the error estimate for individual years … is at least ten times larger than the differences between these three years.”[24] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, “The 2010 nominal value of +0.53°C ranks just ahead of those of 2005 (+0.52°C) and 1998 (+0.51°C), although the differences between the three years are not statistically significant…”[25]
Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.[26] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.[27][28]
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[29] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.[30] The Northern Hemisphere warms faster than the Southern Hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[31]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[32]
External forcings
External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radiative forcing due to changes in atmospheric composition (mainly greenhouse gas concentrations), changes in solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.[33] Attribution of recent climate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens of thousands of years and thus are too gradual to have caused the temperature changes observed in the past century.Greenhouse gases
Greenhouse effect schematic showing energy flows between space, the atmosphere, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m2).
This graph is known as the "Keeling Curve" and it shows the long-term increase of atmospheric carbon dioxide (CO2) concentrations from 1958-2008. Monthly CO2 measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO2.
Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).[35][C] The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO2), which causes 9–26 percent; methane (CH4), which causes 4–9 percent; and ozone (O3), which causes 3–7 percent.[36][37][38] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have different effects on radiation from water vapor.
Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs and nitrous oxide. The concentrations of CO2 and methane have increased by 36% and 148% respectively since 1750.[39] These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores.[40][41][42][43] Less direct geological evidence indicates that CO2 values higher than this were last seen about 20 million years ago.[44] Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.[45]
Over the last three decades of the 20th century, GDP per capita and population growth were the main drivers of increases in greenhouse gas emissions.[46] CO2 emissions are continuing to rise due to the burning of fossil fuels and land-use change.[47][48]:71 Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological, technological, and natural developments.[49] In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced.[50][51] These emission scenarios, combined with carbon cycle modelling, have been used to produce estimates of how atmospheric concentrations of greenhouse gases will change in the future. Using the six IPCC SRES "marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO2 could range between 541 and 970 ppm.[52] This is an increase of 90-250% above the concentration in the year 1750. Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, oil sands or methane clathrates are extensively exploited.[53]
The popular media and the public often confuse global warming with the ozone hole, i.e., the destruction of stratospheric ozone by chlorofluorocarbons.[54][55] Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect.[56]
Particulates and soot
In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the radiation budget.[60] Sulfates act as cloud condensation nuclei and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, known as the Twomey effect.[61] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect.[62] Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.[63]
Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.[64] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.[65] The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.[66]
Solar variation
Main article: Solar variation
Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere, an increase in solar activity should warm the stratosphere while an increase in greenhouse gases should cool the stratosphere.[33] Observations show that temperatures in the stratosphere have been cooling since 1979, when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.[72]
A related hypothesis, proposed by Henrik Svensmark, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate.[73] Other research has found no relation between warming in recent decades and cosmic rays.[74][75] The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.[76]
Feedback
Main article: Climate change feedback
Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback increases the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, a significant greenhouse gas. The main negative feedback is radiative cooling, which increases as the fourth power of temperature; the amount of heat radiated from the Earth into space increases with the temperature of Earth's surface and atmosphere. Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming. A wide range of potential feedback process exist, such as Arctic methane release and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change.[77]Climate models
Main article: Global climate model
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 and regionally divided economic development.
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).
Global climate model projections of future climate most often have used estimates of greenhouse gas emissions from the IPCC Special Report on Emissions Scenarios (SRES). In addition to human-caused emissions, some models also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain. Some observational studies also show a positive feedback.[81][82][83] Including uncertainties in future greenhouse gas concentrations and climate sensitivity, the IPCC anticipates a warming of 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) by the end of the 21st century, relative to 1980–1999.[2]
Models are also 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. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.[33]
The physical realism of models is tested by examining their ability to simulate current or past climates.[84] 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.[45] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. Observed Arctic shrinkage has been faster than that predicted.[85] Precipitation increased proportional to atmospheric humidity, and hence significantly faster than current global climate models predict.[86][87]
Attributed and expected effects
Main articles: Effects of global warming and Regional effects of global warming
Global warming may be detected in natural, ecological or social systems as a change having statistical significance.[88] Attribution of these changes e.g., to natural or human activities, is the next step following detection.[89]Natural systems
Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades.[91] Over the course of the 21st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20th century.
In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090-2099, relative to 1980-1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.[92]
Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean and parts of the North Atlantic Ocean.[91] Snow cover area and sea ice extent are expected to decrease, with the Arctic expected to be largely ice-free in September by the 2037.[93] The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.
Ecological systems
In terrestrial ecosystems, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.[18] Future climate change is expected to particularly affect certain ecosystems, including tundra, mangroves, and coral reefs.[91] It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures.[94] Overall, it is expected that climate change will result in the extinction of many species and reduced diversity of ecosystems.[95]Social systems
Vulnerability of human societies to climate change mainly lies in the effects of extreme weather events rather than gradual climate change.[96] Impacts of climate change so far include adverse effects on small islands,[97] adverse effects on indigenous populations in high-latitude areas,[98] and small but discernable effects on human health.[99] Over the 21st century, climate change is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition and increased health impacts.[100]Future warming of around 3 ºC (by 2100, relative to 1990-2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition.[97] A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects.[99] Warming above 3 ºC could result in crop yields falling in temperate regions, leading to a reduction in global food production.[101] Most economic studies suggest losses of world gross domestic product (GDP) for this magnitude of warming.[102][103]
Responses to global warming
Mitigation
Main article: Climate change mitigation
See also: Fee and dividend
Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere.[104] Many countries, both developing and developed, are aiming to use cleaner, less polluting, technologies.[48]:192 Use of these technologies aids mitigation and could result in substantial reductions in CO2 emissions. Policies include targets for emissions reductions, increased use of renewable energy, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions.[105] Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO2 produced by factories and gas or coal power stations and then stores it, usually underground.[106]Adaptation
Main article: Adaptation to global warming
Other policy responses include adaptation to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention.[107] The ability to adapt is closely linked to social and economic development.[105] Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.Geoengineering
Another policy response is engineering of the climate (geoengineering). This policy response is sometimes grouped together with mitigation.[108] Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published.[109] Geoengineering encompasses a range of techniques to remove CO2 from the atmosphere or to block incoming sunlight. As most geoengineering techniques would affect the entire globe, the use of effective techniques, if they can be developed, would require global public acceptance and an adequate global legal and regulatory framework.[110]UNFCCC
Most countries are Parties to the United Nations Framework Convention on Climate Change (UNFCCC).[111] The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system.[112] As is stated in the Convention, this requires that GHGs are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can proceed in a sustainable fashion.The UNFCCC recognizes differences among countries in their responsibility to act on climate change.[113] In the Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the treaty) took on legally binding commitments to reduce their emissions.[114] Policy measures taken in response to these commitments have reduced emissions.[115] For many developing (non-Annex I) countries, reducing poverty is their overriding aim.[116]
At the 15th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord.[117] Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2 °C.[118] The 16th Conference of the Parties (COP16) was held at Cancún in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet the 2 °C goal. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.[119]
Views on global warming
Main articles: Global warming controversy and Politics of global warming
Per capita greenhouse gas emissions in 2005, including land-use change.
Total greenhouse gas emissions in 2005, including land-use change.
Politics
Developed and developing countries have made different arguments over who should bear the burden of economic costs for cutting emissions. Developing countries often concentrate on per capita emissions, that is, the total emissions of a country divided by its population.[123] Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.[124] This is used to make the argument that the real problem of climate change is due to the profligate and unsustainable lifestyles of those living in rich countries.[123]On the other hand, Banuri et al. point out that total carbon emissions,[123] carrying capacity, efficient energy use and civil and political rights are very important issues. Land is not the same everywhere. Not only the quantity of fossil fuel use but also the quality of energy use is a key debate point.[citation needed] Efficient energy use supporting technological change might[vague] help reduce excess carbon dioxide in Earth's atmosphere.[citation needed] The use of fossil fuels for conspicuous consumption and excessive entertainment are issues that can conflict with civil and political rights. People[who?] in developed countries argue that history has proven the difficulty of implementing fair rationing programs in different countries because there is no global system of checks and balances or civil liberties.
The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries.[114] Developing countries are not subject to limitations. This exemption led the U.S. and Australia to decide not to ratify the treaty,[125] [126][127] although Australia did finally ratify the treaty in December 2007.[128] Debate continued at the Copenhagen climate summit and the Cancún climate summit.
Public opinion
In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in developed, and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite belief.[129] In the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of Cardiff University said that "results show the different stages of engagement about global warming on each side of the Atlantic", adding, "The debate in Europe is about what action needs to be taken, while many in the U.S. still debate whether climate change is happening."[130][131] A 2010 poll by the Office of National Statistics found that 75% of UK respondents were at least "fairly convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006.[132] A January 2011 ICM poll in the UK found 83% of respondents viewed climate change as a current or imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll asking the same question, though there had been a slight polarisation of opposing views.[133]A survey in October, 2009 by the Pew Research Center for the People & the Press showed decreasing public perception in the United States that global warming was a serious problem. All political persuasions showed reduced concern with lowest concern among Republicans, only 35% of whom considered there to be solid evidence of global warming.[134] The cause of this marked difference in public opinion between the United States and the global public is uncertain but the hypothesis has been advanced that clearer communication by scientists both directly and through the media would be helpful in adequately informing the American public of the scientific consensus and the basis for it.[135] The U.S. public appears to be unaware of the extent of scientific consensus regarding the issue, with 59% believing that scientists disagree "significantly" on global warming.[136]
By 2010, with 111 countries surveyed, Gallup determined that there was a substantial decrease in the number of Americans and Europeans who viewed Global Warming as a serious threat. In the United States, a little over half the population (53%) now viewed it as a serious concern for either themselves or their families; a number 10 percentage points below the 2008 poll (63%). Latin America had the biggest rise in concern, with 73% saying global warming was a serious threat to their families.[137]
Other views
Most scientists accept that humans are contributing to observed climate change.[47][138] National science academies have called on world leaders for policies to cut global emissions.[139] However, some scientists and non-scientists question aspects of climate-change science.[140][141]Organizations such as the libertarian Competitive Enterprise Institute, conservative commentators, and some companies such as ExxonMobil have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.[142][143][144][145] In the finance industry, Deutsche Bank has set up an institutional climate change investment division (DBCCA),[146] which has commissioned and published research[147] on the issues and debate surrounding global warming.[148] Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions.[149] Some fossil fuel companies have scaled back their efforts in recent years,[150] or called for policies to reduce global warming.[151]
Etymology
The term global warming was probably first used in its modern sense on 8 August 1975 in a science paper by Wally Broecker in the journal Science called "Are we on the brink of a pronounced global warming?".[152][153][154] Broecker's choice of words was new and represented a significant recognition that the climate was warming; previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going.[155] The National Academy of Sciences first used global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible."[156] The report made a distinction between referring to surface temperature changes as global warming, while referring to other changes caused by increased CO2 as climate change.[155]Global warming became more widely popular after 1988 when NASA climate scientist James Hansen used the term in a testimony to Congress.[155] He said: "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming."[157] His testimony was widely reported and afterward global warming was commonly used by the press and in public discourse.[155]
See also
 | Book: Global warming |
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Notes
- ^ Increase is for years 1905 to 2005. Global surface temperature is defined in the IPCC Fourth Assessment Report as the average of near-surface air temperature over land and sea surface temperature. These error bounds are constructed with a 90% confidence interval.
- ^ The 2001 joint statement was signed by the national academies of science of Australia, Belgium, Brazil, Canada, the Caribbean, the People's Republic of China, France, Germany, India, Indonesia, Ireland, Italy, Malaysia, New Zealand, Sweden, and the UK. The 2005 statement added Japan, Russia, and the U.S. The 2007 statement added Mexico and South Africa. The Network of African Science Academies, and the Polish Academy of Sciences have issued separate statements. Professional scientific societies include American Astronomical Society, American Chemical Society, American Geophysical Union, American Institute of Physics, American Meteorological Society, American Physical Society, American Quaternary Association, Australian Meteorological and Oceanographic Society, Canadian Foundation for Climate and Atmospheric Sciences, Canadian Meteorological and Oceanographic Society, European Academy of Sciences and Arts, European Geosciences Union, European Science Foundation, Geological Society of America, Geological Society of Australia, Geological Society of London-Stratigraphy Commission, InterAcademy Council, International Union of Geodesy and Geophysics, International Union for Quaternary Research, National Association of Geoscience Teachers, National Research Council (US), Royal Meteorological Society, and World Meteorological Organization.
- ^ Note that the greenhouse effect produces an average worldwide temperature increase of about 33 °C (59 °F) compared to black body predictions without the greenhouse effect, not an average surface temperature of 33 °C (91 °F). The average worldwide surface temperature is about 14 °C (57 °F).
- ^ In the IPCC Fourth Assessment Report, published in 2007, this attribution is given a probability of greater than 90%, based on expert judgement.[158] According to the US National Research Council Report – Understanding and Responding to Climate Change - published in 2008, "[most] scientists agree that the warming in recent decades has been caused primarily by human activities that have increased the amount of greenhouse gases in the atmosphere."[47]
