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) |
|
Higher (increasing) minimum temperatures; fewer cold days, frost days, and cold wavesd over nearly all land areas (very likelya) | |
More intense precipitation events (very likelya over many areas) |
|
Complex Extremes |
|
Increased summer drying over most mid-latitude continental interiors and associated risk of drought (likelya) |
|
Increase in tropical cyclone peak wind intensities, mean and peak precipitation intensities (likelya over some areas)e | |
Intensified droughts and floods associated with El Niño events
in many different regions (likelya) (see also under droughts and intense precipitation events) |
|
Increased Asian summer monsoon precipitation variability (likelya) |
|
Increased intensity of mid-latitude storms (little agreement between current models)d |
|
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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