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Impacts due to altered frequencies and intensities of extreme weather, climate and sea-level events are very likely to change.

Impacts of climate change will vary regionally but, aggregated and discounted to the present, they are very likely to impose net annual costs which will increase over time as global temperatures increase.

Since the IPCC Third Assessment, confidence has increased that some weather events and extremes will become more frequent, more widespread and/or more intense during the 21st century; and more is known about the potential effects of such changes. A selection of these is presented in Table SPM.1.

The direction of trend and likelihood of phenomena are for IPCC SRES projections of climate change.

Very large sea-level rises that would result from widespread deglaciation of Greenland and West Antarctic ice sheets imply major changes in coastlines and ecosystems, and inundation of low-lying areas, with greatest effects in river deltas. Relocating populations, economic activity, and infrastructure would be costly and challenging. There is medium confidence that at least partial deglaciation of the Greenland ice sheet, and possibly the West Antarctic ice sheet, would occur over a period of time ranging from centuries to millennia for a global average temperature increase of 1-4°C (relative to 1990-2000), causing a contribution to sea-level rise of 4-6 m or more. The complete melting of the Greenland ice sheet and the West Antarctic ice sheet would lead to a contribution to sea-level rise of up to 7 m and about 5 m, respectively [Working Group I Fourth Assessment 6.4, 10.7; Working Group II Fourth Assessment 19.3].

Based on climate model results, it is very unlikely that the Meridional Overturning Circulation (MOC) in the North Atlantic will undergo a large abrupt transition during the 21st century. Slowing of the MOC during this century is very likely, but temperatures over the Atlantic and Europe are projected to increase nevertheless, due to global warming. Impacts of large-scale and persistent changes in the MOC are likely to include changes to marine ecosystem productivity, fisheries, ocean carbon dioxide uptake, oceanic oxygen concentrations and terrestrial vegetation [Working Group I Fourth Assessment 10.3, 10.7; Working Group II Fourth Assessment 12.6, 19.3].

This Assessment makes it clear that the impacts of future climate change will be mixed across regions. For increases in global mean temperature of less than 1-3°C above 1990 levels, some impacts are projected to produce benefits in some places and some sectors, and produce costs in other places and other sectors. It is, however, projected that some low-latitude and polar regions will experience net costs even for small increases in temperature. It is very likely that all regions will experience either declines in net benefits or increases in net costs for increases in temperature greater than about 2-3°C [9.ES, 9.5, 10.6, T10.9, 15.3, 15.ES]. These observations confirm evidence reported in the Third Assessment that, while developing countries are expected to experience larger percentage losses, global mean losses could be 1-5% GDP for 4°C of warming [F20.3].

Many estimates of aggregate net economic costs of damages from climate change across the globe (i.e., the social cost of carbon (SCC), expressed in terms of future net benefits and costs that are discounted to the present) are now available. Peer-reviewed estimates of the SCC for 2005 have an average value of US$43 per tonne of carbon (i.e., US$12 per tonne of carbon dioxide), but the range around this mean is large. For example, in a survey of 100 estimates, the values ran from US$-10 per tonne of carbon (US$-3 per tonne of carbon dioxide) up to US$350 per tonne of carbon (US$95 per tonne of carbon dioxide) [20.6].

The large ranges of SCC are due in the 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. It is very likely that globally aggregated figures underestimate the damage costs because they cannot include many non-quantifiable impacts. Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time [T20.3, 20.6, F20.4].

It is virtually certain that aggregate estimates of costs mask significant differences in impacts across sectors, regions, countries and populations. In some locations and among some groups of people with high exposure, high sensitivity and/or low adaptive capacity, net costs will be significantly larger than the global aggregate [20.6,20.ES, 7.4].

Some large-scale climate events have the potential to cause very large impacts, especially after the 21st century.

Phenomenon9 and direction of trend

Likelihood of future trends based on projections for 21st century using SRES scenarios

Agriculture, forestry and ecosystems [4.4, 5.4]

Examples of major projected impacts by sector

Water resources Human health [8.2, Industry, settlement and [3.4] 8.4] society [7.4]

Over most land areas, warmer and fewer cold days and nights, warmer and more frequent hot days and nights

Virtually certainb

Increased yields in colder environments; decreased yields in warmer environments; increased insect outbreaks

Effects on water resources relying on snow melt; effects on some water supplies

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; increased danger of wildfire

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 socially-isolated

Reduction in quality of life for people in warm areas without appropriate housing; impacts on the 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 waterlogging of soils

Adverse effects on quality of surface and groundwater; contamination of water supply; water scarcity may be relieved

Increased risk of deaths, injuries and infectious, respiratory and skin diseases

Disruption of settlements, commerce, transport and societies due to flooding; pressures on urban and rural infrastructures; loss of property

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 food-borne 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 causing disruption of public water supply

Increased risk of deaths, injuries, water- and food-borne 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, loss of property

Increased incidence of extreme high sea level (excludes tsunamis)c

Likely1

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; migration-related health effects

Costs of coastal protection versus costs of land-use relocation; potential for movement of populations and infrastructure; also see tropical cyclones above

a See Working Group I Fourth Assessment Table 3.7 for further details regarding definitions. b Warming of the most extreme days and nights each year.

c Extreme high sea level depends on average sea level and on regional weather systems. It is defined as the highest 1 % of hourly values of observed sea level at a station for a given reference period.

d In all scenarios, the projected global average sea level at 2100 is higher than in the reference period [Working Group I Fourth Assessment 10.6]. The effect of changes in regional weather systems on sea level extremes has not been assessed.

a See Working Group I Fourth Assessment Table 3.7 for further details regarding definitions. b Warming of the most extreme days and nights each year.

c Extreme high sea level depends on average sea level and on regional weather systems. It is defined as the highest 1 % of hourly values of observed sea level at a station for a given reference period.

d In all scenarios, the projected global average sea level at 2100 is higher than in the reference period [Working Group I Fourth Assessment 10.6]. The effect of changes in regional weather systems on sea level extremes has not been assessed.

Table SPM.1. Examples of possible impacts of climate change due to changes in extreme weather and climate events, based on projections to the mid- to late 21st century. These do not take into account any changes or developments in adaptive capacity. Examples of all entries are to be found in chapters in the full Assessment (see source at top of columns). The first two columns of the table (shaded yellow) are taken directly from the Working Group I Fourth Assessment (Table SPM-2). The likelihood estimates in Column 2 relate to the phenomena listed in Column 1.

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