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Graphical description of risks and impacts of climate change by IPCC (2001). A revision of this figure by Smith et al. (2009) shows increased risks.^[1]

This article is about the effects of global warming and climate change.^[2] The effects, or impacts, of climate change may be physical, ecological, social or economic. Evidence of observed climate change includes the instrumental temperature record, rising sea levels, and decreased snow cover in the Northern Hemisphere.^[3] According to IPCC (2007a:10), "[most] of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in [human greenhouse gas] concentrations". It is predicted that future climate changes will include further global warming (i.e., an upward trend in global mean temperature), sea level rise, and a probable increase in the frequency of some extreme weather events. Signatories of the United Nations Framework Convention on Climate Change have agreed to implement policies designed to reduce their emissions of greenhouse gases.

Overview

Global mean surface temperature difference from the average for 1961–1990

 

Mean surface temperature change for the period 1999 to 2008 relative to the average temperatures from 1940 to 1980

Over the last hundred years or so, the instrumental temperature record has shown a trend in climate of increased global mean temperature, i.e., global warming. Other observed changes include Arctic shrinkage, Arctic methane release, releases of terrestrial carbon from permafrost regions and Arctic methane release in coastal sediments, and sea level rise.^[4][5] Global average temperature is predicted to increase over this century, with a probable increase in frequency of some extreme weather events, and changes in rainfall patterns. Moving from global to regional scales, there is increased uncertainty over how climate will change. The probability of warming having unforeseen consequences increases with the rate, magnitude, and duration of climate change.^[6] Some of the physical impacts of climate change are irreversible at continental and global scales.^[7] With medium confidence, IPCC (2007b:17) concluded that with a global average temperature increase of 1–4°C, (relative to 1990–2000) partial deglaciation of the Greenland ice sheet would occur over a period of centuries to millennia.^[8] Including the possible contribution of partial deglaciation of the West Antarctic Ice Sheet, sea level would rise by 4–6 m or more.

The impacts of climate change across world population will not be distributed evenly (Smith et al., 2001:957).^[9] Some regions and sectors are expected to experience benefits while others will experience costs. With greater levels of warming (greater than 2–3°C by 2100, relative to 1990 temperature levels), it is very likely that benefits will decline and costs increase (IPCC, 2007b:17). Low-latitude and less-developed areas are probably at the greatest risk from climate change (Schneider et al., 2007:781).^[10] With human systems, adaptation potential for climate change impacts is considerable, although the costs of adaptation are largely unknown and potentially large. In a literature assessment, Schneider et al. (2007:792) concluded, with high confidence, that climate change would likely result in reduced diversity of ecosystems and the extinction of many species.

Definition of climate change

This article refers to reports produced by the IPCC. In their usage, "climate change" refers to a change in the state of the climate that can be identified by changes in the mean and/or variability of its properties, and that persists for extended periods, typically decades or longer (IPCC, 2007d:30).^[11] The climate change referred to may be due to natural causes or the result of human activity.

Physical impacts

This section describes physical impacts of climate change. For some of these physical impacts, their effect on social and economic systems are also described.

Effects on weather

Increasing temperature is likely to lead to increasing precipitation ^[12][13] 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.^[14]

Extreme weather

IPCC (2007a:8) predicted that in the future, over most land areas, the frequency of warm spells or heat waves would very likely increase.^[3] Other likely changes are listed below:

• 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)

Local climate change

The first recorded South Atlantic hurricane, "Catarina", which hit Brazil in March 2004

Regional effects of global warming vary in nature. Some are the result of a generalised global change, such as rising temperature, resulting in local effects, such as melting ice. In other cases, a change may be related to a change in a particular ocean current or weather system. In such cases, the regional effect may be disproportionate and will not necessarily follow the global trend.

There are three major ways in which global warming will make changes to regional climate: melting or forming ice, changing the hydrological cycle (of evaporation and precipitation) and changing currents in the oceans and air flows in the atmosphere. The coast can also be considered a region, and will suffer severe impacts from sea level rise.

Biogeochemical cycles

Climate change may have an effect on the carbon cycle in an interactive "feedback" process . A feedback exists where an initial process triggers changes in a second process that in turn influences the initial process. A positive feedback intensifies the original process, and a negative feedback reduces it (IPCC, 2007d:78).^[11] Models suggest that the interaction of the climate system and the carbon cycle is one where the feedback effect is positive (Schneider et al., 2007:792).^[10]

Using the A2 SRES emissions scenario, Schneider et al. (2007:789) found that this effect led to additional warming by 2100, relative to the 1990-2000 period, of 0.1 to 1.5 °C. This estimate was made with high confidence. The climate projections made in the IPCC Forth Assessment Report of 1.1 to 6.4 °C account for this feedback effect. On the other hand, with medium confidence, Schneider et al. (2007) commented that additional releases of GHGs were possible from permafrost, peat lands, wetlands, and large stores of marine hydrates at high latitudes.

Glacier retreat and disappearance

A map of the change in thickness of mountain glaciers since 1970. Thinning in orange and red, thickening in blue.

IPCC (2007a:5) found that, on average, mountain glaciers and snow cover had decreased in both the northern and southern hemispheres.^[3] This widespread decrease in glaciers and ice caps has contributed to observed sea level rise. With very high or high confidence, IPCC (2007d:11) made a number of predictions relating to future changes in glaciers:^[11]

• Mountainous areas in Europe will face glacier retreat
• In Latin America, changes in precipitation patterns and the disappearance of glaciers will significantly affect water availability for human consumption, agriculture, and energy production
• In Polar regions, there will be reductions in glacier extent and the thickness of glaciers.

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.

Acidification

Dissolving CO₂ in seawater increases the hydrogen ion (H⁺) concentration in the ocean, and thus decreases ocean pH. Caldeira and Wickett (2003) placed the rate and magnitude of modern ocean acidification changes in the context of probable historical changes during the last 300 million years.^[15] Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly less than 0.1 units (on the logarithmic scale of pH; approximately a 25% increase in H⁺), and it is estimated that it will drop by a further 0.3 to 0.5 units by 2100 as the oceans absorb more anthropogenic CO₂.^{[15] [16][17]}

Oxygen depletion

The amount of oxygen dissolved in the oceans may decline, with adverse consequences for ocean life.^[18][19]

Sea level rise

Sea level rise during the Holocene.

 

Sea level has been rising 0.2 cm/year, based on measurements of sea level rise from 23 long tide gauge records in geologically stable environments.

IPCC (2007a:5) reported that since 1961, global average sea level had risen at an average rate of 1.8 [1.3 to 2.3] mm/yr.^[3] Between 1993 and 2003, the rate increased above the previous period to 3.1 [2.4 to 3.8] mm/yr. IPCC (2007a) were uncertain whether the increase in rate from 1993 to 2003 was due to natural variations in sea level over the time period, or whether it reflected an increase in the underlying long-term trend.

IPCC (2007a:13, 14) projected sea level rise to the end of the 21^st century using the SRES emission scenarios. Across the six SRES marker scenarios, sea level was projected to rise by 18 to 59 cm (7.1 to 23.2 inches). This projection was for the time period 2090-2099, with the increase in level relative to average sea levels over the 1980-1999 period. Due to a lack of scientific understanding, this sea level rise estimate does not include all of the possible contributions of ice sheets.

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.^[20] This would affect in particular areas like Scandinavia and Britain that are warmed by the North Atlantic drift.

IPCC (2007b:17) concluded that a slowing of the Meridional Overturning Circulation would very likely occur this century. Due to global warming, temperatures across the Atlantic and Europe were still projected to increase.

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.^[21] 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 ^[22]. 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.^{[citation needed]}

Social systems

Food supply

Climate change will impact agriculture and food production around the world due to: the effects of elevated CO₂ in the atmosphere, higher temperatures, altered precipitation and transpiration regimes, increased frequency of extreme events, and modified weed, pest, and pathogen pressure (Easterling et al., 2007:282).^[23] In general, low-latitude areas are at most risk of having decreased crop yields (Schneider et al., 2007:790).^[10] With low to medium confidence, Schneider et al. (2007:787) concluded that for about a 1 to 3°C global mean temperature increase (by 2100, relative to the 1990-2000 average level) there would be productivity decreases for some cereals in low latitudes, and productivity increases in high latitudes. With medium confidence, global production potential was predicted to:

• increase up to around 3°C,
• very likely decrease above about 3 to 4°C.

Most of the studies on global agriculture assessed by Schneider et al. (2007:790) had not incorporated a number of critical factors, including changes in extreme events, or the spread of pests and diseases. Studies had also not considered the development of specific practices or technologies to aid adaptation.

Health

Human beings are exposed to climate change through changing weather patterns (temperature, precipitation, sea-level rise and more frequent extreme events) and indirectly through changes in water, air and food quality and changes in ecosystems, agriculture, industry and settlements and the economy (Confalonieri et al., 2007:393).^[24] According to a literature assessment by Confalonieri et al. (2007:393), the effects of climate change to date have been small, but are projected to progressively increase in all countries and regions.

With high confidence, Confalonieri et al. (2007:393) concluded that climate change had altered the seasonal distribution of some allergenic pollen species. With medium confidence, they concluded that climate change had:

• altered the distribution of some infectious disease vectors
• increased heatwave-related deaths

With high confidence, IPCC (2007d:48) projected that:^[11]

• the health status of millions of people would be affected through, for example, increases in malnutrition; increased deaths, diseases and injury due to extreme weather events; increased burden of diarrhoeal diseases; increased frequency of cardio-respiratory diseases due to high concentrations of ground-level ozone in urban areas related to climate change; and altered spatial distribution of some infectious diseases.
• climate change would bring some benefits in temperate areas, such as fewer deaths from cold exposure, and some mixed effects such as changes in range and transmission potential of malaria in Africa. Overall, IPCC (2007d:48) expected that benefits would be outweighed by negative health effects of rising temperatures, especially in developing countries.

With very high confidence, Confalonieri et al. (2007:393) concluded that economic development was an important component of possible adaptation to climate change. Economic growth on its own, however, was not judged to be sufficient to insulate the world's population from disease and injury due to climate change. The manner in which economic growth occurs was judged to be important, along with how the benefits of growth are distributed in society. Examples of other important factors in determining the health of populations were listed as: education, health care, and public-health infrastructure.

Water resources

In a literature assessment, Kundzewicz et al. (2007:175) concluded, with high confidence, that:^[25]

• the negative impacts of climate change on freshwater systems outweigh the benefits. All of the regions assessed in the IPCC Fourth Assessment Report (Africa, Asia, Australia and New Zealand, Europe, Latin America, North America, Polar regions (Arctic and Antarctic), and small islands) showed an overall net negative impact of climate change on water resources and freshwater ecosystems.
• Semi-arid and arid areas are particularly exposed to the impacts of climate change on freshwater. With very high confidence, it was judged that many of these areas, e.g., the Mediterranean basin, western USA, southern Africa, and north-eastern Brazil, would suffer a decrease in water resources due to climate change.

Migration and conflict

An argument can be made that rising ethnic conflicts may be linked to competition over natural resources that are increasingly scarce as a result of climate change (Wilbanks et al., 2007:365).^[26] According to a literature assessment by Wilbanks et al. (2007:365), other factors need to be taken into account. It was suggested that major environmentally-influenced conflicts in Africa have more to do with the relative abundance of resources, e.g., oil and diamonds, than with resource scarcity. On this basis, Wilbanks et al. (2007:365) suggested that predictions of future conflicts due climate change should be viewed with caution.

With high confidence, Schneider et al. (2007:787) predicted that stresses such as increased drought, water shortages, and riverine and coastal flooding would affect many local and regional populations.^[10] With medium confidence, it was predicted that these stresses would lead, in some cases, to relocation within or between countries. This might have the effect of exacerbating conflicts, and possibly impose migration pressures.

Aggregate impacts

Aggregating impacts adds up the total impact of climate change across sectors and/or regions (IPCC, 2007d:76).^[11] The impacts of climate change across world population will not be distributed evenly (Smith et al., 2001:957).^[9] IPCC (2007b:17) found that for increases in global mean temperature of less than 1-3 °C above 1990 levels, some impacts were projected to produce benefits in some places and sectors, and produce costs in other places and sectors.^[8] For some low-latitude and polar regions, net costs were projected for small increases in temperature. According to IPCC (2007b:17), a temperature increase of greater than about 2-3 °C would very likely result in all regions either experiencing reductions in net benefits or increases in net costs.

Regions

Some regions are likely to be especially affected by climate change (IPCC, 2007d:9):^[11]

• The Arctic, because of high rates of projected warming.
• Africa, especially in the sub-Saharan region. This is due to the continent's low capacity to adapt to climate change and projected impacts.
• Small islands, due to high exposure of population and infrastructure at risk to sea-level rise and increased storm surge.
• Asian megadeltas, due to large populations and high exposure to sea-level rise, storm surge and river flooding.

Within other areas, some people are particularly at risk, such as the poor, young children and the elderly.

Biological systems

With very high confidence, Schneider et al. (2007:792) concluded that regional temperature trends were already affecting species and ecosystems around the world.^[10] In a literature assessment, Rosenzweig et al. (2007:81) concluded that over the last three decades, human-induced warming had likely had a discernable influence on many physical and biological systems.^[27]

Schneider et al. (2007:792) concluded, with high confidence, that climate change would result in the extinction of many species and a reduction in the diversity of ecosystems.

• Terrestrial ecosystems and biodiversity: With a warming of 3°C, relative to 1990 levels, it is likely that global terrestrial vegetation would become a net source of carbon (Schneider et al., 2007:792). With high confidence, Schneider et al. (2007:788) concluded that a global mean temperature increase of around 4°C (above the 1990-2000 level) by 2100 would lead to major extinctions around the globe.
• Marine ecosystems and biodiversity: With very high confidence, Schneider et al. (2007:792) concluded that a warming of 2°C above 1990 levels would result in mass mortality of coral reefs globally.
• Freshwater ecosystems: Above about a 4°C increase in global mean temperature by 2100 (relative to 1990-2000), Schneider et al. (2007:789) concluded, with high confidence, that many freshwater species would become extinct.

See also

General Risks to civilization, humans and planet Earth Indigenous Peoples Climate Change Assessment Initiative Regional Arctic Climate Impact Assessment Effects of global warming on Australia Kivalina v. ExxonMobil Corp., Et al. National Assessment on Climate Change (United States)

Global warming portal Science Biological pump Oceanic anoxic event PRECIS (Providing Regional Climates for Impacts Studies) Solubility pump

Notes

Further reading

• NRC (2008). "Understanding and Responding to Climate Change". Board on Atmospheric Sciences and Climate, US National Academy of Sciences. http://dels.nas.edu/dels/rpt_briefs/climate_change_2008_final.pdf. Retrieved 2009-05-20. 
• NRC (2008). "Ecological Impacts of Climate Change". The National Academies Press, 500 Fifth Street, NW Washington, DC 20001, USA. pp. 70. http://books.nap.edu/catalog.php?record_id=12491. Retrieved 2010-03-13. 

External links

Physical impacts

• "Climate Change". World Meteorological Organization.
• The IPCC Working Group I (WG I) website. This body assesses the physical scientific aspects of the climate system and climate change.

Social, economic and ecological impacts

• Climate change on the United Nations Economic and Social Development (UNESD) Division for Sustainable Development website.
• The IPCC Working Group II (WG II) website – This body assesses the vulnerability of socio-economic and natural systems to climate change, negative and positive consequences of climate change, and options for adapting to it.

General

• List of United Nations Functional Commissions and Expert Bodies related to climate change
• IRIN, the humanitarian news and analysis service of the UN Office for the Coordination of Humanitarian Affairs: "What climate change does", "How climate change works", and "Gathering Storm - the humanitarian impact of climate change"
• Videos:
  • "Educational Forum: Arctic Climate Impact". Panel discussion with James J. McCarthy, Professor at Harvard University, and Author; Paul R. Epstein, M.D., instructor in medicine at Harvard Medical School; and Ross Gelbspan, Pulitzer-prize winning journalist and author. Massachusetts School of Law.
  • "How we know humans are changing the climate and Why climate change is a clear and present danger". Interviews with Christopher Field and Michael MacCracken. Christopher Field is the director of the Department of Global Ecology at the Carnegie Institution of Washington, professor of biology and environmental earth system science at Stanford University, and the Working Group II Co-Chair for the Intergovernmental Panel on Climate Change. Michael MacCracken is the chief scientist for Climate Change Programs at the Climate Institute and a co-author and contributing author for various chapters in the IPCC assessment reports. Climate Progress website, February 5, 2010.