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Intergovernmental Panel on Climate Change - Fourth Assessment Report

Observed Impacts Phenomena and direction of trends Effects of temperature increases Changes in physical and biological systems and surface temperature 1970-2004

Phenomena and direction of trend

Phenomena and trend*	Likelihood**		Examples of major projected impacts by sector
		Agriculture, forestry and ecosystems	Water resources	Human health	Industry/settlement/ Society
Warmer and fewer cold days and nights; warmer/more frequent hot days and nights over most land areas	Virtually certain	Increased yields in colder environments; decreased yields in warmer environments; increased insect outbreaks	Effects on water resources relying on snow melt; increased evapotranspiration rates	Reduced human mortality from decreased cold exposure	Reduced energy demand for heating; increased demand for cooling; declining air quality in cities; reduced disruption to transport due to snow, ice; effects on winter tourism
Warm spells/heat waves: frequency increases over most land areas	Very likely	Reduced yields in warmer regions due to heat stress; wild fire danger increase	Increased water demand; water quality problems, e.g., algal blooms	Increased risk of heat-related mortality, especially for the elderly, chronically sick, very young and sociallyisolated	Reduction in quality of life for people in warm areas without appropriate housing; impacts on elderly, very young and poor.
Heavy precipitation events: frequency increases over most areas	Very likely	Damage to crops; soil erosion, inability to cultivate land due to water logging of soils	Adverse effects on quality of surface and groundwater; contamination of water supply; water scarcity may be relieved	Increased risk of deaths, injuries, infectious, respiratory and skin diseases, post-traumatic stress disorders	Disruption of settlements, commerce, transport and societies due to flooding; pressures on urban and rural infrastructures
Area affected by drought: increases	Likely	Land degradation, lower yields/crop damage and failure; increased livestock deaths; increased risk of wildfire	More widespread water stress	Increased risk of food and water shortage; increased risk of malnutrition; increased risk of water- and foodborne diseases	Water shortages for settlements, industry and societies; reduced hydropower generation potentials; potential for population migration
Intense tropical cyclone activity increases	Likely	Damage to crops; windthrow (uprooting) of trees; damage to coral reefs	Power outages cause disruption of public water supply	Increased risk of deaths, injuries, water- and foodborne diseases; post-traumatic stress disorders	Disruption by flood and high winds; withdrawal of risk coverage in vulnerable areas by private insurers, potential for population migrations
Increased incidence of extreme high sea level (excludes tsunamis)	Likely	Salinisation of irrigation water, estuaries and freshwater systems	Decreased freshwater availability due to saltwater intrusion	Increased risk of deaths and injuries by drowning in floods; migrationrelated health effects	Costs of coastal protection versus costs of land-use relocation; potential for movement of populations and infrastructure; also see tropical cyclones above

Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases.

With regard to changes in snow, ice and frozen ground (including permafrost), there is high confidence that natural systems are affected.

Examples are:

enlargement and increased numbers of glacial lakes ;
increasing ground instability in permafrost regions, and rock avalanches in mountain regions;
changes in some Arctic and Antarctic ecosystems, including those in sea-ice biomes, and also predators high in the food chain.

Based on growing evidence, there is high confidence that the following types of hydrological systems are being affected around the world:

increased run-off and earlier spring peak discharge in many glacier- and snow-fed rivers ;
warming of lakes and rivers in many regions, with effects on thermal structure and water quality .
There is very high confidence, based on more evidence from a wider range of species, that recent warming is strongly affecting terrestrial biological systems, including such changes as:

earlier timing of spring events, such as leaf-unfolding, bird migration and egg-laying ;
poleward and upward shifts in ranges in plant and animal species.

Based on satellite observations since the early 1980s, there is high confidence that there has been a trend in many regions towards earlier "greening" of vegetation in the spring linked to longer thermal growing seasons due to recent warming.

There is high confidence, based on substantial new evidence, that observed changes in marine and freshwater biological systems are associated with rising water temperatures, as well as related changes in ice cover, salinity, oxygen levels and circulation . These include:

shifts in ranges and changes in algal, plankton and fish abundance in high-latitude oceans ;
increases in algal and zooplankton abundance in high-latitude and high-altitude lakes ;
range changes and earlier migrations of fish in rivers.

The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. However, the effects of observed ocean acidification on the marine biosphere are as yet undocumented.
A global assessment of data since 1970 has shown it is likely6 that anthropogenic warming has had a discernible influence on many physical and biological systems.Much more evidence has accumulated over the past five years to indicate that changes in many physical and biological systems are linked to anthropogenic warming. There are four sets of evidence which, taken together, support this conclusion:

The Working Group I Fourth Assessment concluded that most of the observed increase in the globally averaged temperature since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
Of the more than 29,000 observational data series, from 75 studies, that show significant change in many physical and biological systems, more than 89% are consistent with the direction of change expected as a response to warming.

Measured by the Normalised Difference Vegetation Index, which is a relative measure of the amount of green vegetation in an area based on satellite images. A subset of about 29,000 data series was selected from about 80,000 data series from 577 studies. These met the following criteria:

(1) Ending in 1990 or later;
(2) spanning a period of at least 20 years; and ...
(3) showing a significant change in either direction, as assessed in individual studies.

A global synthesis of studies in this Assessment strongly demonstrates that the spatial agreement between regions of significant warming across the globe and the locations of significant observed changes in many systems consistent with warming is very unlikely to be due solely to natural variability of temperatures or natural variability of the systems.

Finally, there have been several modelling studies that have linked responses in some physical and biological systems to anthropogenic warming by comparing observed responses in these systems with modelled responses in which the natural forcings (solar activity and volcanoes) and anthropogenic forcings (greenhouse gases and aerosols) are explicitly separated. Models with combined natural and anthropogenic forcings simulate observed responses significantly better than models with natural forcing only.

Limitations and gaps prevent more complete attribution of the causes of observed system responses to anthropogenic warming. First, the available analyses are limited in the number of systems and locations considered. Second, natural temperature variability is larger at the regional than the global scale, thus affecting identification of changes due to external forcing. Finally, at the regional scale other factors (such as land-use change, pollution, and invasive species) are influential.

Nevertheless, the consistency between observed and modelled changes in several studies and the spatial agreement between significant regional warming and consistent impacts at the global scale is sufficient to conclude with high confidence that anthropogenic warming over the last three decades has had a discernible influence on many physical and biological systems.

Other effects of regional climate changes on natural and human environments are emerging, although many are difficult to discern due to adaptation and non-climatic drivers.

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Effects of temperature increases have been documented in the following systems (medium confidence):

effects on agricultural and forestry management at Northern Hemisphere higher latitudes, such as earlier spring planting of crops, and alterations in disturbance regimes of forests due to fires and pests ;
some aspects of human health, such as heat-related mortality in Europe, infectious disease vectors in some areas, and allergenic pollen in Northern Hemisphere high and mid-latitudes ;

some human activities in the Arctic (e.g., hunting and travel over snow and ice) and in lowerelevation alpine areas (such as mountain sports).

Recent climate changes and climate variations are beginning to have effects on many other natural and human systems. However, based on the published literature, the impacts have not yet become established trends. Examples include:

Settlements in mountain regions are at enhanced risk to glacier lake outburst floods caused by melting glaciers. Governmental institutions in some places have begun to respond by ...

building dams and drainage works.
In the Sahelian region of Africa, warmer and drier conditions have led to a reduced length of growing season with detrimental effects on crops. In southern Africa, longer dry seasons and more ...

uncertain rainfall are prompting adaptation measures.

Sea-level rise and human development are together contributing to losses of coastal wetlands and mangroves and increasing damage from coastal flooding in many areas.

Source: Text IPCC; UNEP and WMO, 6 April 2007

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