2.6. The Potential for Large-Scale and Possibly Irreversible Impacts Poses Risks that have yet to be Reliably Quantified

Projected climate changes⁷ during the 21st century have the potential to lead to future large-scale and possibly irreversible changes in Earth systems resulting in impacts at continental and global scales. These possibilities are very climate scenario-dependent and a full range of plausible scenarios has not yet been evaluated. Examples include significant slowing of the ocean circulation that transports warm water to the North Atlantic, 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. The likelihood of many of these changes in Earth systems is not well-known, but is probably very low; however, their likelihood is expected to increase with the rate, magnitude, and duration of climate change (see Figure SPM-2). [3.5, 5.7, and 7.2.5]

If these changes in Earth systems were to occur, their impacts would be widespread and sustained. For example, significant slowing of the oceanic thermohaline circulation would impact deep-water oxygen levels and carbon uptake by oceans and marine ecosystems, and would reduce warming over parts of Europe. Disintegration of the West Antarctic Ice Sheet or melting of the Greenland Ice Sheet could raise global sea level up to 3 m each over the next 1,000 years ⁸ , submerge many islands, and inundate extensive coastal areas. Depending on the rate of ice loss, the rate and magnitude of sea-level rise could greatly exceed the capacity of human and natural systems to adapt without substantial impacts. Releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments, induced by warming, would further increase greenhouse gas concentrations in the atmosphere and amplify climate change. [3.5, 5.7, and 7.2.5]

Table SPM-1: Examples of impacts resulting from projected changes in extreme climate events.
Projected Changes during the 21st Century in Extreme Climate Phenomena and their Likelihooda	Representative Examples of Projected Impactsb (all high confidence of occurrence in some areasc)
Simple Extremes
Higher maximum temperatures; more hot days and heat wavesd over nearly all land areas (very likelya)	Increased incidence of death and serious illness in older age groups and urban poor [4.7] Increased heat stress in livestock and wildlife [4.2 and 4.3] Shift in tourist destinations [Table TS-4 and 5.8] Increased risk of damage to a number of crops [4.2] Increased electric cooling demand and reduced energy supply reliability [Table TS-4 and 4.5]
Higher (increasing) minimum temperatures; fewer cold days, frost days, and cold wavesd over nearly all land areas (very likelya)	Decreased cold-related human morbidity and mortality [4.7] Decreased risk of damage to a number of crops, and increased risk to others [4.2] Extended range and activity of some pest and disease vectors [4.2] Reduced heating energy demand [4.5]
More intense precipitation events (very likelya over many areas)	Increased flood, landslide, avalanche, and mudslide damage [4.5] Increased soil erosion [5.2.4] Increased flood runoff could increase recharge of some floodplain aquifers [4.1] Increased pressure on government and private flood insurance systems and disaster relief [Table TS-4 and 4.6]
Complex Extremes
Increased summer drying over most mid-latitude continental interiors and associated risk of drought (likelya)	Decreased crop yields [4.2] Increased damage to building foundations caused by ground shrinkage [Table TS-4] Decreased water resource quantity and quality [4.1 and 4.5] Increased risk of forest fire [5.4.2]
Increase in tropical cyclone peak wind intensities, mean and peak precipitation intensities (likelya over some areas)e	Increased risks to human life, risk of infectious disease epidemics, and many other risks [4.7] Increased coastal erosion and damage to coastal buildings and infrastructure [4.5 and 7.2.4] Increased damage to coastal ecosystems such as coral reefs and mangroves [4.4]
Intensified droughts and floods associated with El Niño events in many different regions (likelya) (see also under droughts and intense precipitation events)	Decreased agricultural and rangeland productivity in drought- and flood-prone regions [4.3] Decreased hydro-power potential in drought-prone regions [5.1.1 and Figure TS-7]
Increased Asian summer monsoon precipitation variability (likelya)	Increased flood and drought magnitude and damages in temperate and tropical Asia [5.2.4]
Increased intensity of mid-latitude storms (little agreement between current models)d	Increased risks to human life and health [4.7] Increased property and infrastructure losses [Table TS-4] Increased damage to coastal ecosystems [4.4]
a Likelihood refers to judgmental estimates of confidence used by TAR WGI: very likely (90-99% chance); likely (66-90% chance). Unless otherwise stated, information on climate phenomena is taken from the Summary for Policymakers, TAR WGI. b These impacts can be lessened by appropriate response measures. c High confidence refers to probabilities between 67 and 95% as described in Footnote 6 . d Information from TAR WGI, Technical Summary, Section F.5. e Changes in regional distribution of tropical cyclones are possible but have not been established.

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