IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Synthesis Report

3.3.5 Extreme events

Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems (Table 3.2). {WGII SPM}

Examples for selected extremes and sectors are shown in Table 3.2.

Table 3.2. 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. The likelihood estimates in column two relate to the phenomena listed in column one. {WGII Table SPM.1}

Phenomenona and direction of trend Likelihood of future trends based on projections for 21st century using SRES scenarios  Examples of major projected impacts by sector 
Agriculture, forestry and ecosystems {WGII 4.4, 5.4} Water resources {WGII 3.4} Human health {WGII 8.2, 8.4} Industry, settlement and society {WGII 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 snowmelt; 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 shortage 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  Likelyd  Salinisation of irrigation water, estuaries and fresh- water systems  Decreased fresh- water 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 {WGI 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. The effect of changes in regional weather systems on sea level extremes has not been assessed. {WGI 10.6}