Global Warming

Global warming has a big impact on the Earth, having both negative and positive effects. Global warming occurs because of an increase in the emission of greenhouse gases. The burning of fossil fuels only adds to these emissions. Temperatures around the globe are slowly warming, and this affects every continent and area on Earth. This is also known as the greenhouse effect. This has become a problem because too much solar radiation is getting trapped in the Earth’s atmosphere, which has caused the Earth to become warmer. Greenhouse gases are responsible for retaining this energy. The more greenhouse gases in the atmosphere, the more solar radiation becomes trapped and the warmer the planet gets.

Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the last century. The Intergovernmental Panel on Climate Change (IPCC) concludes that increasing greenhouse gas concentrations resulting from human activity such as fossil fuel burning and deforestation caused most of the observed temperature increase since the middle of the 20th century.The IPCC also concludes that variations in natural phenomena such as solar radiation and volcanoes produced most of the warming from pre-industrial times to 1950 and had a small cooling effect afterward.These basic conclusions have been endorsed by more than 45 scientific societies and academies of science,^[B] including all of the national academies of science of the major industrialized countries.A small number of scientists dispute the consensus view.

Climate model projections summarized in the latest IPCC report indicate that the global surface temperature will probably rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century. 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. Some other uncertainties include how warming and related changes will vary from region to region around the globe. Most studies focus on the period up to the year 2100. However, warming is expected to continue beyond 2100 even if emissions stop, because of the large heat capacity of the oceans and the long lifetime of carbon dioxide in the atmosphere.

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. The continuing retreat of glaciers, permafrost and sea ice is expected, with warming being strongest in the Arctic. Other likely effects include increases in the intensity of extreme weather events, species extinctions, and changes in agricultural yields.

Political and public debate continues regarding climate change, and what actions (if any) to take in response. The available options are mitigation to reduce further emissions; adaptation to reduce the damage caused by warming; and, more speculatively, geoengineering to reverse global warming. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions.

Temperature changes

The most commonly discussed measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a linear trend, this temperature rose by 0.74°C ±0.18°C over the period 1906-2005. The rate of warming over the last 50 years of that period was almost double that for the period as a whole (0.13°C ±0.03°C per decade, versus 0.07°C ± 0.02°C per decade). The urban heat island effect is estimated to account for about 0.002 °C of warming per decade since 1900.^[8] 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 regionally-varying 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. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998. Temperatures in 1998 were unusually warm because the strongest El Niño in the past century occurred during that year.

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). 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. 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 the 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.

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.

Greenhouse gases

Greenhouse effect schematic showing energy flows between the atmosphere, space, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m²).

The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface. It was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.^[17] Existence of the greenhouse effect as such is not disputed, even by those who do not agree that the recent temperature increase is attributable to human activity. The question is instead how the strength of the greenhouse effect changes when human activity increases the concentrations of greenhouse gases in the atmosphere.

Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F).^[18][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^{[not
in
citation given]}; and ozone (O₃), which causes 3–7 percent.^[19][20] Clouds also affect the radiation balance, but they are composed of liquid water or ice and so are considered separately from water vapor and other gases.

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 the mid-1700s.^[21] 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.^[22] Less direct geological evidence indicates that CO₂ values this high were last seen approximately 20 million years ago.^[23] 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, particularly deforestation.^[24]

CO₂ concentrations are continuing to rise due to burning of fossil fuels and land-use change. The future rate of rise will depend on uncertain economic, sociological, technological, and natural developments. Accordingly, 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.^[25] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100 if coal, tar sands or methane clathrates are extensively exploited.^[26]

The destruction of stratospheric ozone by chlorofluorocarbons is sometimes mentioned in relation to global warming. Although there are a few areas of linkage, the relationship between the two is not strong. Reduction of stratospheric ozone has a cooling influence, but substantial ozone depletion did not occur until the late 1970s.^[27] Tropospheric ozone contributes to surface warming.^[28]

Aerosols and soot

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.^[29] The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert a cooling effect by increasing the reflection of incoming sunlight. James Hansen and colleagues have proposed that the effects of the products of fossil fuel combustion—CO₂ and aerosols—have largely offset one another in recent decades, so that net warming has been driven mainly by non-CO₂ greenhouse gases.^[30]

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget.^[31] Sulfate aerosols 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.^[32] This effect also causes droplets to be of more uniform size, which reduces growth of raindrops by collision-coalescence. Clouds modified by pollution have been shown to produce less drizzle, making the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.^[33]

Soot may cool or warm, depending on whether it is airborne or deposited. Atmospheric soot aerosols directly absorb solar radiation, which heats the atmosphere and cools the surface. Regionally (but not globally), as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds.^[34] When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo can also directly heat the surface.^[35] The influences of aerosols, 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.^[36]

Solar variation

Variations in solar output have been the cause of past climate changes.^[37] Although solar forcing is generally thought to be too small to account for a significant part of global warming in recent decades,^[38][39] a few studies disagree, such as a recent phenomenological analysis that indicates the contribution of solar forcing may be underestimated.^[40]

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.^[2] Observations show that temperatures in the stratosphere have been steady or 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.^[41]

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.^[42] Other research has found no relation between warming in recent decades and cosmic rays.^[43][44] A recent study concluded that 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.^[45]

Feedback

A positive feedback is a process that amplifies some change. Thus, when a warming trend results in effects that induce further warming, the result is a positive feedback; when the warming results in effects that reduce the original warming, the result is a negative feedback. The main positive feedback in global warming involves the tendency of warming to increase the amount of water vapor in the atmosphere. The main negative feedback in global warming is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.

Water vapor feedback

If the atmosphere is warmed, the saturation vapor pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO₂ alone. Although this feedback process causes 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.^[46]

Cloud feedback

Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, 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, details that are difficult to represent in climate models.^[46]

Lapse rate

The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.^[47]

Ice-albedo feedback

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water, both have a lower albedo than the white sea ice. The melting ice contributes to the ice-albedo feedback.

When 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.^[48]

Arctic methane release

Warming is also the triggering variable for the release of methane from sources both on land and on the deep ocean floor, making both of these possible feedback effects. Thawing permafrost, such as the frozen peat bogs in Siberia, creates a positive feedback due to the release of CO₂ and methane.^[49]

Reduced absorption of CO₂ by the oceans

Ocean ecosystems' ability to sequester carbon is expected to decline as the oceans warm. This is because warming reduces the nutrient levels of the mesopelagic zone (about 200 to 1000 m deep), which limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.^[50]

Gas release

Release of miscellaneous gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Such releases may have direct climate effects, such as Nitrous oxide^[51] released from peat and indirect effects, such as Dimethyl sulfide^[52] released from oceans.

Climate models

The main tools for projecting future climate changes are computer models of the climate. These models are based on physical principles including fluid dynamics and radiative transfer. Although they attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. All modern climate models are in fact combinations of models for different parts of the Earth that are coupled to one another. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes.^[53] Climate models project a warmer climate due to increasing levels of greenhouse gases.^[54] Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.^[55]

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.^[56][57][58]

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.^[1] A 2008 paper predicts that the global temperature may not increase during the next decade because short-term natural fluctuations may temporarily outweigh greenhouse gas-induced warming.^[59]

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 1975 is dominated by man-made greenhouse gas emissions.

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.^[24] The physical realism of models is tested by examining their ability to simulate current or past climates.^[60] While a 2007 study by David Douglass and colleagues found that the models did not accurately predict observed changes in the tropical troposphere,^[61] a 2008 paper published by a 17-member team led by Ben Santer noted errors in the Douglass study, and found instead that the models and observations were not statistically different.^[62] Not all effects of global warming are accurately predicted by the climate models used by the IPCC. For example, observed Arctic

Attributed and expected effects

Environmental

It usually is impossible to connect specific weather events to global warming. Instead, global warming is expected to cause changes in the overall distribution and intensity of events, such as changes to the frequency and intensity of heavy precipitation. Broader effects are expected to include glacial retreat, Arctic shrinkage, and worldwide sea level rise. 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 that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events are attributable in part to global warming.^[64] Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and some adverse health effects from warmer temperatures.^[65]

Social and economic effects of global warming may be exacerbated by growing population densities in affected areas. Temperate regions are projected to experience some benefits, such as fewer cold-related deaths.^[66] 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.^[64] 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 (see Atlantic Multidecadal Oscillation), 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 0.18 to 0.59 meters (0.59 to 1.9 ft) in 2090-2100 relative to 1980-1999,^[1] new trade routes resulting from arctic shrinkage,^[67] possible thermohaline circulation slowing, increasingly intense (but less frequent) hurricanes and extreme weather events,^[68] reductions in the ozone layer, changes in agriculture yields, changes in the range of climate-dependent disease vectors,^[69] which has been linked to increases in the prevalence of malaria and dengue fever,^[70] and ocean oxygen depletion.^[71] Increased atmospheric CO₂ increases the amount of CO₂ dissolved in the oceans.^[72] CO₂ dissolved in the ocean reacts with water to form carbonic acid, resulting in ocean acidification. Ocean surface pH is estimated to have decreased from 8.25 near the beginning of the industrial era to 8.14 by 2004,^[73] and is projected to decrease by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO₂.^[1][74] Heat and carbon dioxide trapped in the oceans may still take hundreds years to be re-emitted, even after greenhouse gas emissions are eventually reduced.^[6] Since organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns and disruptions in food webs.^[75] 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.^[76] However, few mechanistic studies have documented extinctions due to recent climate change,^[77] and one study suggests that projected rates of extinction are uncertain.^[78]

Economic

"Stabilization scenario categories as reported in Figure SPM.7 (coloured bands) and their relationship to equilibrium global mean temperature change above preindustrial, using (i) “best estimate” climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). Coloured shading shows the concentration bands for stabilization of greenhouse gases in the atmosphere corresponding to the stabilization scenario categories I to VI as indicated in Figure SPM.7. The data are drawn from AR4 WGI, Chapter 10.8. [i.e. from the IPCC Fourth Assessment Report, Working Group I]"

The IPCC reports the aggregate net economic costs of damages from climate change globally (discounted to the specified year). In 2005, the average social cost of carbon from 100 peer-reviewed estimates is US$12 per tonne of CO₂, but range -$3 to $95/tCO₂. The IPCC's gives these cost estimates with the caveats, "Aggregate estimates of costs mask significant differences in impacts across sectors, regions and populations and very likely underestimate damage costs because they cannot include many non-quantifiable impacts."^[79]

One widely publicized report on potential economic impact is the Stern Review, written by Sir Nicholas Stern. It suggests that extreme weather might reduce global gross domestic product by up to one percent, and that in a worst-case scenario global per capita consumption could fall by the equivalent of 20 percent.^[80] The response to the Stern Review was mixed. The Review's methodology, advocacy and conclusions were criticized by several economists, including Richard Tol, Gary Yohe,^[81] Robert Mendelsohn^[82] and William Nordhaus.^[83] Economists that have generally supported the Review include Terry Barker,^[84] William Cline,^[85] and Frank Ackerman.^[86] According to Barker, the costs of mitigating climate change are 'insignificant' relative to the risks of unmitigated climate change.^[87]

According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others.^[88] Developing countries dependent upon agriculture will be particularly harmed by global warming.^[

Responses to global warming

The broad agreement among climate scientists that global temperatures will continue to increase has led some nations, states, corporations and individuals to implement responses. These responses to global warming can be divided into mitigation of the causes and effects of global warming, adaptation to the changing global environment, and geoengineering to reverse global warming.

Mitigation

Based on future projections of climate change, the IPCC report makes a number of predictions.^[9]It is predicted that over most land areas, the frequency of warm spells/heat waves will very likely increase. It is likely that:

Mitigation of global warming is accomplished through reductions in the rate of anthropogenic greenhouse gas release. Models suggest that mitigation can quickly begin to slow global warming, but that temperatures will appreciably decrease only after several centuries.^[90] The world's primary international agreement on reducing greenhouse gas emissions is the Kyoto Protocol, an amendment to the UNFCCC negotiated in 1997. The Protocol now covers more than 160 countries and over 55 percent of global greenhouse gas emissions.^[91] As of June 2009, only the United States, historically the world's largest emitter of greenhouse gases, has refused to ratify the treaty. The treaty expires in 2012. International talks began in May 2007 on a future treaty to succeed the current one.^[92] UN negotiations are now gathering pace in advance of a meeting in Copenhagen in December 2009.^[93]

Many environmental groups encourage individual action against global warming, as well as community and regional actions. Others have suggested a quota on worldwide fossil fuel production, citing a direct link between fossil fuel production and CO₂ emissions.^[94][95]

There has also been business action on climate change, including efforts to improve energy efficiency and limited moves towards use of alternative fuels. In January 2005 the European Union introduced its European Union Emission Trading Scheme, through which companies in conjunction with government agree to cap their emissions or to purchase credits from those below their allowances. Australia announced its Carbon Pollution Reduction Scheme in 2008. United States President Barack Obama has announced plans to introduce an economy-wide cap and trade scheme.^[96]

The IPCC's Working Group III is responsible for crafting reports on mitigation of global warming and the costs and benefits of different approaches. The 2007 IPCC Fourth Assessment Report concludes that no one technology or sector can be completely responsible for mitigating future warming. They find there are key practices and technologies in various sectors, such as energy supply, transportation, industry, and agriculture, that should be implemented to reduced global emissions. They estimate that stabilization of carbon dioxide equivalent between 445 and 710 ppm by 2030 will result in between a 0.6 percent increase and three percent decrease in global gross domestic product.^[97]

Adaptation:

A wide variety of measures have been suggested for adaptation to global warming. These measures range from the trivial, such as the installation of air-conditioning equipment, to major infrastructure projects, such as abandoning settlements threatened by sea level rise.

Measures including water conservation,^[98] water rationing, adaptive agricultural practices,^[99] construction of flood defences,^[100] Martian colonization,^[101] changes to medical care,^[102] and interventions to protect threatened species^[103] have all been suggested. A wide-ranging study of the possible opportunities for adaptation of infrastructure has been published by the Institute of Mechanical Engineers.^[104

Geoengineering:

Geoengineering is the deliberate modification of Earth's natural environment on a large scale to suit human needs.^[105] An example is greenhouse gas remediation, which removes greenhouse gases from the atmosphere, usually through carbon sequestration techniques such as carbon dioxide air capture.^[106] Solar radiation management reduces insolation, such as by the addition of stratospheric sulfur aerosols.^[107] No large-scale geoengineering projects have yet been undertaken.

Debate and skepticism:

Increased publicity of the scientific findings surrounding global warming has resulted in political and economic debate.^[108] Poor regions, particularly Africa, appear at greatest risk from the projected effects of global warming, while their emissions have been small compared to the developed world.^[109] The exemption of developing countries from Kyoto Protocol restrictions has been used to rationalize non-ratification by the U.S. and criticism from Austria.^[110] Another point of contention is the degree to which emerging economies such as India and China should be expected to constrain their emissions.^[111] The U.S. contends that if it must bear the cost of reducing emissions, then China should do the same^[112][113] since China's gross national CO₂ emissions now exceed those of the U.S.^{[114][115][116]} China has contended that it is less obligated to reduce emissions since its per capita responsibility and per capita emissions are less that of the U.S.^[117] India, also exempt, has made similar contentions.^[118]

In 2007-2008 the Gallup Polls surveyed 127 countries. Over a third of the world's population were unaware of global warming, developing countries less aware than developed, and Africa the least aware. Awareness does not equate to belief that global warming is a result of human activities. 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.^[119] In the western world, the concept and the appropriate responses are contested. Nick Pidgeon of Cardiff University finds that "results show the different stages of engagement about global warming on each side of the Atlantic" where Europe debates the appropriate responses while the United States debates whether climate change is happening.^[120]

Debates weigh the benefits of limiting industrial emissions of greenhouse gases against the costs that such changes would entail.^[97] Using economic incentives, alternative and renewable energy have been promoted to reduce emissions while building infrastructure.^[121][122] Business-centered organizations such as the Competitive Enterprise Institute, conservative commentators, and companies such as ExxonMobil have downplayed IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls.^{[123][124][125][126]} 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.^[127] Some fossil fuel companies have scaled back their efforts in recent years,^[128] or called for policies to reduce global warming.^[129]

Some global warming skeptics in the science or political community dispute all or some of the global warming scientific consensus objecting to whether global warming is actually occurring, if human activity is truly to blame, and if the threat is as great a threat as has been alleged. Prominent global warming skeptics include Frederick Seitz, Freeman Dyson, Richard Lindzen, Fred Singer, Patrick Michaels, John Christy, Harrison Schmitt, and Robert Balling.^{[130][131][132]}

Effects of global warming

The effects of global warming and climate change^[2]are of concern both for the environment and human life. Scenarios studied by the Intergovernmental Panel on Climate Change (IPCC) predict that global warming will continue and get worse much faster than was expected even in their last report. Research by NOAA indicate that the effects of global warming are already largely irreversible.^[3] The IPCC reports attribute many specific natural phenomena to human causes. The expected long range effects of recent climate change may already be observed. Rising sea levels, glacier retreat, Arctic shrinkage, and altered patterns of agriculture are cited as direct consequences of human activities. Predictions for secondary and regional effects include extreme weather events, an expansion of tropical diseases, changes in the timing of seasonal patterns in ecosystems, and drastic economic impact. Concerns have led to political activism advocating proposals to mitigate, or adapt to it.

The 2007 Fourth Assessment Report by the IPCC includes a summary of the expected effects.

Overview

Climate changes characterized as global warming are leading to large-scale irreversible^[3] effects at continental and global scales. The likelihood and magnitude of the effects are observed and predicted to be increasing and accelerating.

Many consequences of global warming once controversial or thought to be unlikely are now being observed. Arctic shrinkage and Arctic methane release, alongside large reductions in the Greenland and West Antarctic Ice Sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments are accelerating.^[4][5]

The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change. Additionally, the United States National Academy of Sciences has stated, "greenhouse warming and other human alterations of the earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events…. Future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected."^[6]

The effects of global warming will be mixed across regions.^[7]For smaller values of warming (of up to 2°C relative to 1990 levels), the global economic effect is expected to be plus or minus a few percent of world GDP, with most people in the world negatively affected. Many developing countries would be expected to experience net market sector costs for 2°C warming, while many developed countries would experience net market sector benefits. Vulnerability to climate change is variable within countries, and in developed countries, some people are vulnerable to less than 2°C warming. Above 2 to 3°C warming, many developed and developing countries would be expected to suffer net negative impacts.^[8]

Most of the consequences of global warming would result from physical changes: sea level rise, higher local temperatures, and changes in rainfall patterns. Sea level is expected to rise 18 to 59 cm (7.1 to 23.2 inches) by the end of the 21st century, not including the unknown contribution from non-linear changes to large ice sheets.^[9]

It has also been proposed that the melting in the Arctic may bring fresh water to the North Atlantic to disrupt the Gulf Stream, which may cause a destabilisation or shutdown of the Thermohaline circulation.

Physical impacts:

Effects on weather

Increasing temperature is likely to lead to increasing precipitation ^[10][11] but the effects on storms are less clear. Extratropical storms partly depend on the temperature gradient, which is predicted to weaken in the northern hemisphere as the polar region warms more than the rest of the hemisphere.

Increased areas will be affected by drought
There will be increased intense tropical cyclone activity
There will be increased incidences of extreme high sea level (excluding tsunamis)

Storm strength leading to extreme weather is increasing, such as the power dissipation index of hurricane intensity.^[13] Kerry Emanuel writes that hurricane power dissipation is highly correlated with temperature, reflecting global warming.^[14] However, a further study by Emanuel using current model output concluded that the increase in power dissipation in recent decades cannot be completely attributed to global warming^[15]. Hurricane modeling has produced similar results, finding that hurricanes, simulated under warmer, high-CO₂ conditions, are more intense, however, hurricane frequency will be reduced.^[16] Worldwide, the proportion of hurricanes reaching categories 4 or 5 – with wind speeds above 56 metres per second – has risen from 20% in the 1970s to 35% in the 1990s.^[17] Precipitation hitting the US from hurricanes has increased by 7% over the twentieth century.^[18][19][20] The extent to which this is due to global warming as opposed to the Atlantic Multidecadal Oscillation is unclear. Some studies have found that the increase in sea surface temperature may be offset by an increase in wind shear, leading to little or no change in hurricane activity.^[21] Hoyos et al. (2006) have linked the increasing trend in number of category 4 and 5 hurricanes for the period 1970-2004 directly to the trend in sea surface temperatures.^[22]

Increases in catastrophes resulting from extreme weather are mainly caused by increasing population densities, and anticipated future increases are similarly dominated by societal change rather than climate change.^[23] The World Meteorological Organization explains that “though there is evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record to date, no firm conclusion can be made on this point.”^[24] They also clarified that “no individual tropical cyclone can be directly attributed to climate change.”^[24]

Thomas Knutson and Robert E. Tuleya of NOAA stated in 2004 that warming induced by greenhouse gas may lead to increasing occurrence of highly destructive category-5 storms.^[25] In 2008, Knutson et al. found that Atlantic hurricane and tropical storm frequencies could reduce under future greenhouse-gas-induced warming.^[26] Vecchi and Soden find that wind shear, the increase of which acts to inhibit tropical cyclones, also changes in model-projections of global warming. There are projected increases of wind shear in the tropical Atlantic and East Pacific associated with the deceleration of the Walker circulation, as well as decreases of wind shear in the western and central Pacific.^[27] The study does not make claims about the net effect on Atlantic and East Pacific hurricanes of the warming and moistening atmospheres, and the model-projected increases in Atlantic wind shear. ^[28]

A substantially higher risk of extreme weather does not necessarily mean a noticeably greater risk of slightly-above-average weather.^[29] However, the evidence is clear that severe weather and moderate rainfall are also increasing. Increases in temperature are expected to produce more intense convection over land and a higher frequency of the most severe

Increased evaporation

Over the course of the 20th century, evaporation rates have reduced worldwide ^[31]; this is thought by many to be explained by global dimming. As the climate grows warmer and the causes of global dimming are reduced, evaporation will increase due to warmer oceans. Because the world is a closed system this will cause heavier rainfall, with more erosion. This erosion, in turn, can in vulnerable tropical areas (especially in Africa) lead to desertification. On the other hand, in other areas, increased rainfall lead to growth of forests in dry desert areas.

Scientists have found evidence that increased evaporation could result in more extreme weather as global warming progresses. The IPCC Third Annual Report says: "...global average water vapor concentration and precipitation are projected to increase during the 21st century. By the second half of the 21st century, it is likely that precipitation will have increased over northern mid- to high latitudes and Antarctica in winter. At low latitudes there are both regional increases and decreases over land areas. Larger year to year variations in precipitation are very likely over most areas where an increase in mean precipitation is projected."^[10][32]

Cost of more extreme weather

As the World Meteorological Organization explains, “recent increase in societal impact from tropical cyclones has largely been caused by rising concentrations of population and infrastructure in coastal regions.”^[24] Pielke et al. (2008) normalized mainland U.S. hurricane damage from 1900–2005 to 2005 values and found no remaining trend of increasing absolute damage. The 1970s and 1980s were notable because of the extremely low amounts of damage compared to other decades. The decade 1996–2005 has the second most damage among the past 11 decades, with only the decade 1926–1935 surpassing its costs. The most damaging single storm is the 1926 Miami hurricane, with $157 billion of normalized damage.^[23]

The American Insurance Journal predicted that “catastrophe losses should be expected to double roughly every 10 years because of increases in construction costs, increases in the number of structures and changes in their characteristics.”^[33] The Association of British Insurers has stated that limiting carbon emissions would avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s. The cost is also increasing partly because of building in exposed areas such as coasts and floodplains. The ABI claims that reduction of the vulnerability to some inevitable effects of climate change, for example through more resilient buildings and improved flood defences, could also result in considerable cost-savings in the longterm.^[34

Local climate change

In the northern hemisphere, the southern part of the Arctic region (home to 4,000,000 people) has experienced a temperature rise of 1 °C to 3 °C (1.8 °F to 5.4 °F) over the last 50 years. Canada, Alaska and Russia are experiencing initial melting of permafrost. This may disrupt ecosystems and by increasing bacterial activity in the soil lead to these areas becoming carbon sources instead of carbon sinks.^[35] A study (published in Science) of changes to eastern Siberia's permafrost suggests that it is gradually disappearing in the southern regions, leading to the loss of nearly 11% of Siberia's nearly 11,000 lakes since 1971.^[36] At the same time, western Siberia is at the initial stage where melting permafrost is creating new lakes, which will eventually start disappearing as in the east. Furthermore, permafrost melting will eventually cause methane release from melting permafrost peat bogs.

Hurricanes were thought to be an entirely North Atlantic phenomenon. In late March 2004, the first Atlantic cyclone to form south of the equator hit Brazil with 40 m/s (144 km/h) winds, although some Brazilian meteorologists deny that it was a hurricane.^[37] Monitoring systems may have to be extended 1,600 km (1,000 miles) further south. There is no agreement as to whether this hurricane is linked to climate change,^[38][39] but one climate model exhibits increased tropical cyclone genesis in the South Atlantic under global warming by the end of the 21st century.^[40]

Glacier retreat and disappearance

In historic times, glaciers grew during a cool period from about 1550 to 1850 known as the Little Ice Age. Subsequently, until about 1940, glaciers around the world retreated as the climate warmed. Glacier retreat declined and reversed in many cases from 1950 to 1980 as a slight global cooling occurred. Since 1980, glacier retreat has become increasingly rapid and ubiquitous, and has threatened the existence of many of the glaciers of the world. This process has increased markedly since 1995.^[41]

Excluding the ice caps and ice sheets of the Arctic and Antarctic, the total surface area of glaciers worldwide has decreased by 50% since the end of the 19th century.^[42] Currently glacier retreat rates and mass balance losses have been increasing in the Andes, Alps, Pyrenees, Himalayas, Rocky Mountains and North Cascades.

The loss of glaciers not only directly causes landslides, flash floods and glacial lake overflow,^[43] but also increases annual variation in water flows in rivers. Glacier runoff declines in the summer as glaciers decrease in size, this decline is already observable in several regions.^[44] Glaciers retain water on mountains in high precipitation years, since the snow cover accumulating on glaciers protects the ice from melting. In warmer and drier years, glaciers offset the lower precipitation amounts with a higher meltwater input.^[42]

Of particular importance are the Hindu Kush and Himalayan glacial melts that comprise the principal dry-season water source of many of the major rivers of the Central, South, East and Southeast Asian mainland. Increased melting would cause greater flow for several decades, after which "some areas of the most populated regions on Earth are likely to 'run out of water'" as source glaciers are depleted.^[45]

According to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers—Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow—could disappear by 2035 as temperatures rise.^[46] Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.^[47] India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. In India alone, the Ganges provides water for drinking and farming for more than 500 million people.^[48][49][50] It has to be acknowledged, however, that increased seasonal runoff of Himalayan glaciers led to increased agricultural production in northern India throughout the 20th century.^[51]

The recession of mountain glaciers, notably in Western North America, Franz-Josef Land, Asia, the Alps, the Pyrenees, Indonesia and Africa, and tropical and sub-tropical regions of South America, has been used to provide qualitative support to the rise in global temperatures since the late 19th century. Many glaciers are being lost to melting further raising concerns about future local water resources in these glaciated areas. In Western North America the 47 North Cascade glaciers observed all are retreating.

Retreat of the Helheim Glacier, Greenland

Despite their proximity and importance to human populations, the mountain and valley glaciers of temperate latitudes amount to a small fraction of glacial ice on the earth. About 99% is in the great ice sheets of polar and subpolar Antarctica and Greenland. These continuous continental-scale ice sheets, 3 kilometres (1.9 mi) or more in thickness, cap the polar and subpolar land masses. Like rivers flowing from an enormous lake, numerous outlet glaciers transport ice from the margins of the ice sheet to the ocean.

Glacier retreat has been observed in these outlet glaciers, resulting in an increase of the ice flow rate. In Greenland the period since the year 2000 has brought retreat to several very large glaciers that had long been stable. Three glaciers that have been researched, Helheim, Jakobshavn Isbræ and Kangerdlugssuaq Glaciers, jointly drain more than 16% of the Greenland Ice Sheet. Satellite images and aerial photographs from the 1950s and 1970s show that the front of the glacier had remained in the same place for decades. But in 2001 it began retreating rapidly, retreating 7.2 km (4.5 mi) between 2001 and 2005. It has also accelerated from 20 m (66 ft)/day to 32 m (100 ft)/day.^[53] Jakobshavn Isbræ in western Greenland had been moving at speeds of over 24 m (79 ft)/day with a stable terminus since at least 1950. The glacier's ice tongue began to break apart in 2000, leading to almost complete disintegration in 2003, while the retreat rate doubled to over 30 m (98 ft)/day.^[5

Oceans:

The role of the oceans in global warming is a complex one. The oceans serve as a sink for carbon dioxide, taking up much that would otherwise remain in the atmosphere, but increased levels of CO₂ have led to ocean acidification. Furthermore, as the temperature of the oceans increases, they become less able to absorb excess CO₂. Global warming is projected to have a number of effects on the oceans. Ongoing effects include rising sea levels due to thermal expansion and melting of glaciers and ice sheets, and warming of the ocean surface, leading to increased temperature stratification. Other possible effects include large-scale changes in ocean circulation.

Sea level rise

With increasing average global temperature, the water in the oceans expands in volume, and additional water enters them which had previously been locked up on land in glaciers, for example, the Greenland and the Antarctic ice sheets. For most glaciers worldwide, an average volume loss of 60% until 2050 is predicted.^[55] Meanwhile, the estimated total ice melting rate over Greenland is 239 ± 23 cubic kilometres (57 ± 5.5 cu mi) per year, mostly from East Greenland.^[56] The Antarctic ice sheet, however, is expected to grow during the 21st century because of increased precipitation.^[57] Under the IPCC Special Report on Emission Scenario (SRES) A1B, by the mid-2090s global sea level will reach 0.22 to 0.44 m (8.7 to 17 in) above 1990 levels, and is currently rising at about 4 mm (0.16 in) per year.^[57] Since 1900, the sea level has risen at an average of 1.7 mm (0.067 in) per year;^[57] since 1993, satellite altimetry from TOPEX/Poseidon indicates a rate of about 3 mm (0.12 in) per year.^[57]

The sea level has risen more than 120 metres (390 ft) since the Last Glacial Maximum about 20,000 years ago. The bulk of that occurred before 7000 years ago.^[58] Global temperature declined after the Holocene Climatic Optimum, causing a sea level lowering of 0.7 ± 0.1 m (28 ± 3.9 in) between 4000 and 2500 years before present.^[59] From 3000 years ago to the start of the 19th century, sea level was almost constant, with only minor fluctuations. However, the Medieval Warm Period may have caused some sea level rise; evidence has been found in the Pacific Ocean for a rise to perhaps 0.9 m (2 ft 11 in) above present level in 700 BP.^[60]

In a paper published in 2007, the climatologist James Hansen et al. claimed that ice at the poles does not melt in a gradual and linear fashion, but that another according to the geological record, the ice sheets can suddenly destabilize when a certain threshold is exceeded. In this paper Hansen et al. state:

Our concern that BAU GHG scenarios would cause large sealevel rise this century (Hansen 2005) differs from estimates of IPCC (2001, 2007), which foresees little or no contribution to twentyfirst century sealevel rise from Greenland and Antarctica. However, the IPCC analyses and projections do not well account for the nonlinear physics of wet ice sheet disintegration, ice streams and eroding ice shelves, nor are they consistent with the palaeoclimate evidence we have presented for the absence of discernible lag between ice sheet forcing and sealevel rise.^[61]

A paper published in 2008 by a group of researchers at the University of Wisconsin lead by Anders Carlson used the deglaciation of North America at 9000 years before present as an analogue to predict sea level rise of 1.3 meters in the next century^[62][63], which is also much higher than the IPCC predictions. However, models of glacial flow in the smaller present-day ice sheets show that a probable maximum value for sea level rise in the next century is 80 centimeters, based on limitations on how quickly ice can flow below the equilibrium line altitude and to the sea.^[64]

Temperature rise

From 1961 to 2003, the global ocean temperature has risen by 0.10 °C from the surface to a depth of 700 m. There is variability both year-to-year and over longer time scales, with global ocean heat content observations showing high rates of warming for 1991 to 2003, but some cooling from 2003 to 2007.^[57] The temperature of the Antarctic Southern Ocean rose by 0.17 °C (0.31 °F) between the 1950s and the 1980s, nearly twice the rate for the world's oceans as a whole ^[65]. As well as having effects on ecosystems (e.g. by melting sea ice, affecting algae that grow on its underside), warming reduces the ocean's ability to absorb CO₂.

Acidification

Ocean acidification is an effect of rising concentrations of CO₂ in the atmosphere, and is not a direct consequence of global warming. The oceans soak up much of the CO₂ produced by living organisms, either as dissolved gas, or in the skeletons of tiny marine creatures that fall to the bottom to become chalk or limestone. Oceans currently absorb about one tonne of CO₂ per person per year. It is estimated that the oceans have absorbed around half of all CO₂ generated by human activities since 1800 (118 ± 19 petagrams of carbon from 1800 to 1994).^[66]

In water, CO₂ becomes a weak carbonic acid, and the increase in the greenhouse gas since the industrial revolution has already lowered the average pH (the laboratory measure of acidity) of seawater by 0.1 units, to 8.2. Predicted emissions could lower the pH by a further 0.5 by 2100, to a level probably not seen for hundreds of millennia and, critically, at a rate of change probably 100 times greater than at any time over this period.^[67][68]

There are concerns that increasing acidification could have a particularly detrimental effect on corals^[69] (16% of the world's coral reefs have died from bleaching caused by warm water in 1998,^[70] which coincidentally was the warmest year ever recorded) and other marine organisms with calcium carbonate shells.^[71]

Shutdown of thermohaline circulation:

There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region.^{[citation
needed]} This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.

The chances of this near-term collapse of the circulation are unclear; there is some evidence for the short-term stability of the Gulf Stream and possible weakening of the North Atlantic drift.^{[citation
needed]} However, the degree of weakening, and whether it will be sufficient to shut down the circulation, is under debate. As yet, no cooling has been found in northern Europe or nearby seas.^{[citation
needed]} Lenton et al. found that "simulations clearly pass a THC tipping point this century".^[72]

Economic and social :

The IPCC Fourth Assessment Report contains a review of the literature on the economics of climate change:

Impacts of climate change are very likely to impose net annual costs, which will increase over time as global temperatures increase. Peer-reviewed estimates of the social cost of carbon [Net economic costs of damages from climate change aggregated across the globe and discounted to the specified year] in 2005 average US$12 per tonne of CO₂, but the range from 100 estimates is large (-$3 to $95/tCO₂). This is due in large part to differences in assumptions regarding climate sensitivity, response lags, the treatment of risk and equity, economic and non-economic impacts, the inclusion of potentially catastrophic losses and discount rates. Aggregate estimates of costs mask significant differences in impacts across sectors, regions and populations and very likely underestimate damage costs because they cannot include many non-quantifiable impacts. ^[102]

In 2006, Nicholas Stern, the former Chief Economist and Senior Vice-President of the World Bank, presented the Stern Review on the Economics of Climate Change^[103]. Stern recommended that one percent of global GDP be invested to mitigate the effects of climate change, and that failure to do so could risk a recession worth up to twenty percent of global GDP^[104]. Stern’s report^[105] suggests that climate change threatens to be the greatest and widest-ranging market failure ever seen. The report has had significant political effects: Australia reported two days after the report was released that they would allott AU$60 million to projects to help cut greenhouse gas emissions^[106]. More recently, Stern has stated that his original report underestimated the damages of climate change ^[107][108].

The Stern Review has been criticized by some economists, saying that Stern did not consider costs past 2200, that he used an incorrect discount rate in his calculations, and that stopping or significantly slowing climate change will require deep emission cuts everywhere.^[109] Other economists have supported Stern's approach ^[110][111], or argued that Stern's estimates are reasonable, even if the method by which he reached them is open to criticism. ^[112].

In a 2004 comment on the economic effect of global warming in Copenhagen Consensus, Professor Robert O. Mendelsohn of Yale School of Forestry and Environmental Studies, stated that:

A series of studies on the impacts of climate change have systematically shown that the older literature overestimated climate damages by failing to allow for adaptation and for climate benefits (see Fankhauser et al. 1997; Mendelsohn and Newmann 1999; Tol 1999; Mendelsohn et al. 2000; Mendelsohn 2001; Maddison 2001; Tol 2002; Sohngen et al. 2002; Pearce 2003; Mendelsohn and Williams 2004). These new studies imply that impacts depend heavily upon initial temperatures (latitude). Countries in the polar region are likely to receive large benefits from warming, countries in the mid-latitudes will at first benefit and only begin to be harmed if temperatures rise above 2.5 °C (4.50 °F) (Mendelsohn et al. 2000). Only countries in the tropical and subtropical regions are likely to be harmed immediately by warming and be subject to the magnitudes of impacts first thought likely (Mendelsohn et al. 2000). Summing these regional impacts across the globe implies that warming benefits and damages will likely offset each other until warming passes 2.5C and even then it will be far smaller on net than originally thought (Mendelsohn and Williams 2004).

Insurance

n industry very directly affected by the risks is the insurance industry.^[114] According to a 2005 report from the Association of British Insurers, limiting carbon emissions could avoid 80% of the projected additional annual cost of tropical cyclones by the 2080s.^[115] A June 2004 report by the Association of British Insurers declared "Climate change is not a remote issue for future generations to deal with. It is, in various forms, here already, impacting on insurers' businesses now."^[116] It noted that weather risks for households and property were already increasing by 2-4 % per year due to changing weather, and that claims for storm and flood damages in the UK had doubled to over £6 billion over the period 1998–2003, compared to the previous five years. The results are rising insurance premiums, and the risk that in some areas flood insurance will become unaffordable for some.

Financial institutions, including the world's two largest insurance companies, Munich Re and Swiss Re, warned in a 2002 study that "the increasing frequency of severe climatic events, coupled with social trends" could cost almost US$ 150 billion each year in the next decade.^[117] These costs would, through increased costs related to insurance and disaster relief, burden customers, taxpayers, and industry alike.

In the United States, insurance losses have also greatly increased. According to Choi and Fisher (2003) each 1% increase in annual precipitation could enlarge catastrophe loss by as much as 2.8%.^[118] Gross increases are mostly attributed to increased population and property values in vulnerable coastal areas, though there was also an increase in frequency of weather-related events like heavy rainfalls since the 1950s.^[119]

Effects on agriculture

For some time it was hoped that a positive effect of global warming would be increased agricultural yields, because of the role of carbon dioxide in photosynthesis, especially in preventing photorespiration, which is responsible for significant destruction of several crops. In Iceland, rising temperatures have made possible the widespread sowing of barley, which was untenable twenty years ago. Some of the warming is due to a local (possibly temporary) effect via ocean currents from the Caribbean, which has also affected fish stocks.^[121]

There is evidence to suggest some areas like the US and Siberia will benefit from global warming.^[122] However, large-scale experiments have shown that "Rising atmospheric temperatures, longer droughts and side-effects of both, such as higher levels of ground-level ozone gas, are likely to bring about a substantial reduction in crop yields in the coming decades".^[123]

Moreover, the region likely to be worst affected is Africa, both because its geography makes it particularly vulnerable, and because seventy per cent of the population rely on rain-fed agriculture for their livelihoods. Tanzania's official report on climate change suggests that the areas that usually get two rainfalls in the year will probably get more, and those that get only one rainy season will get far less. The net result is expected to be that 33% less maize—the country's staple crop—will be grown.^[124]

Climate change may be one of the causes of the Darfur conflict. The combination of decades of drought, desertification and overpopulation are among the causes of the conflict, because the Arab Baggara nomads searching for water have to take their livestock further south, to land mainly occupied by farming peoples.^[125]

The scale of historical climate change, as recorded in Northern Darfur, is almost unprecedented: the reduction in rainfall has turned millions of hectares of already marginal semi-desert grazing land into desert. The impact of climate change is considered to be directly related to the conflict in the region, as desertification has added significantly to the stress on the livelihoods of pastoralist societies, forcing them to move south to find pasture

—UNEP report^[126]

Rice crops may be strongly affected by rising temperatures.^[127]

Flood defense

For historical reasons to do with trade, many of the world's largest and most prosperous cities are on the coast, and the cost of building better coastal defenses (due to the rising sea level) is likely to be considerable. Some countries will be more affected than others—low-lying countries such as Bangladesh and the Netherlands would be worst hit by any sea level rise, in terms of floods or the cost of preventing them. Still, in 180 of 192 littoral countries worldwide, coastal protection will cost less than 0.1% of the country's gross domestic product.^[128]

In developing countries, the poorest often live on flood plains, because it is the only available space, or fertile agricultural land. These settlements often lack infrastructure such as dykes and early warning systems. Poorer communities also tend to lack the insurance, savings or access to credit needed to recover from disasters.^[129]

Migration

Some Pacific Ocean island nations, such as Tuvalu, are concerned about the possibility of an eventual evacuation, as flood defense may become economically unviable for them. Tuvalu already has an ad hoc agreement with New Zealand to allow phased relocation.^[130]

In the 1990s a variety of estimates placed the number of environmental refugees at around 25 million. (Environmental refugees are not included in the official definition of refugees, which only includes migrants fleeing persecution.) The Intergovernmental Panel on Climate Change (IPCC), which advises the world’s governments under the auspices of the UN, estimated that 150 million environmental refugees will exist in the year 2050, due mainly to the effects of coastal flooding, shoreline erosion and agricultural disruption (150 million means 1.5% of 2050’s predicted 10 billion world population).^[131][132]

Ecosystems

Unchecked global warming could affect most terrestrial ecoregions. Increasing global temperature means that ecosystems will change; some species are being forced out of their habitats (possibly to extinction) because of changing conditions, while others are flourishing. Secondary effects of global warming, such as lessened snow cover, rising sea levels, and weather changes, may influence not only human activities but also the ecosystem. Studying the association between Earth climate and extinctions over the past 520 million years, scientists from University of York write, "The global temperatures predicted for the coming centuries may trigger a new ‘mass extinction event’, where over 50 per cent of animal and plant species would be wiped out."^[137]

Many of the species at risk are Arctic and Antarctic fauna such as polar bears^[138] and emperor penguins^[139]. In the Arctic, the waters of Hudson Bay are ice-free for three weeks longer than they were thirty years ago, affecting polar bears, which prefer to hunt on sea ice.^[140] Species that rely on cold weather conditions such as gyrfalcons, and snowy owls that prey on lemmings that use the cold winter to their advantage may be hit hard.^[141][142] Marine invertebrates enjoy peak growth at the temperatures they have adapted to, regardless of how cold these may be, and cold-blooded animals found at greater latitudes and altitudes generally grow faster to compensate for the short growing season.^[143] Warmer-than-ideal conditions result in higher metabolism and consequent reductions in body size despite increased foraging, which in turn elevates the risk of predation. Indeed, even a slight increase in temperature during development impairs growth efficiency and survival rate in rainbow trout.^[144]

Rising temperatures are beginning to have a noticeable impact on birds^[145], and butterflies have shifted their ranges northward by 200 km in Europe and North America. Plants lag behind, and larger animals' migration is slowed down by cities and roads. In Britain, spring butterflies are appearing an average of 6 days earlier than two decades ago ^[146].

A 2002 article in Nature^[147] surveyed the scientific literature to find recent changes in range or seasonal behaviour by plant and animal species. Of species showing recent change, 4 out of 5 shifted their ranges towards the poles or higher altitudes, creating "refugee species". Frogs were breeding, flowers blossoming and birds migrating an average 2.3 days earlier each decade; butterflies, birds and plants moving towards the poles by 6.1 km per decade. A 2005 study concludes human activity is the cause of the temperature rise and resultant changing species behaviour, and links these effects with the predictions of climate models to provide validation for them ^[148]. Scientists have observed that Antarctic hair grass is colonizing areas of Antarctica where previously their survival range was limited. ^[149]

Mechanistic studies have documented extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change.^[150] Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species"^[151] and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change."^[150] Daniel Botkin and other authors in one study believe that projected rates of extinction are overestimated.^[152]

Many species of freshwater and saltwater plants and animals are dependent on glacier-fed waters to ensure a cold water habitat that they have adapted to. Some species of freshwater fish need cold water to survive and to reproduce, and this is especially true with Salmon and Cutthroat trout. Reduced glacier runoff can lead to insufficient stream flow to allow these species to thrive. Ocean krill, a cornerstone species, prefer cold water and are the primary food source for aquatic mammals such as the Blue whale^[153]. Alterations to the ocean currents, due to increased freshwater inputs from glacier melt, and the potential alterations to thermohaline circulation of the worlds oceans, may affect existing fisheries upon which humans depend as well.

The white lemuroid possum, only found in the mountain forests of northern Queensland, has been named as the first mammal species to be driven extinct by man-made global warming. The White Possum has not been seen in over three years. These possums cannot survive extended temperatures over 30 °C (86 °F), which occurred in 2005. A final expedition to uncover any surviving White Possums is scheduled for 2009.^[154]

Forests