REPORTS - ASSESSMENT REPORTS
Synthesis Report - Question 4

Climate Change 2001: Synthesis Report


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4.13 Most models project a weakening of the ocean thermohaline circulation, which leads to a reduction of the heat transport into high latitudes of Europe (see Figure 4-2). However, even in models where THC weakens, there is still a warming over Europe due to increased concentrations of greenhouse gases. The current projections do not exhibit a complete shutdown of THC by the year 2100. Beyond the year 2100, some models suggest that THC could completely, and possibly irreversibly, shut down in either hemisphere if the change in radiative forcing is large enough and applied long enough. Models indicate that a decrease in THC reduces its resilience to perturbations (i.e., a once-reduced THC appears to be less stable and a shutdown can become more likely).

WGI TAR SPM & WGI TAR Sections 7.3 & 9.3.4
4.14 The Antarctic ice sheet as a whole is likely to increase in mass during the 21st century. However, the West Antarctic ice sheet could lose mass over the next 1,000 years with an associated sea-level rise of several meters, but there is an incomplete understanding of some of the underlying processes. Concerns have been expressed about the stability of the West Antarctic ice sheet because it is grounded below sea level. However, loss of grounded ice leading to substantial sea-level rise from this source is widely agreed to be very unlikely during the 21st century. Current climate
and ice dynamic models project that over the next 100 years the Antarctic ice sheet as a whole is likely to gain mass because of a projected increase in precipitation, contributing to a relative decrease of several centimeters to sea level. Over the next 1,000 years, these models project that the West Antarctic ice sheet could contribute up to 3m to sea-level rise.

WGI TAR Section 11.5.4
4.15 The Greenland ice sheet is likely to lose mass during the 21st century and contribute a few centimeters to sea-level rise. Over the 21st century, the Greenland ice sheet is likely to lose mass because the projected increase in runoff will exceed the increase in precipitation and contribute 10 cm maximum to the total sea-level rise. The ice sheets will continue to react to climate warming and contribute to sea-level rise for thousands of years after climate has stabilized. Climate models indicate that the local warming over Greenland is likely to be one to three times the global average. Ice sheet models project that a local warming of larger than 3°C, if sustained for thousands of years, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea-level rise of about 7 m. A local warming of 5.5°C, if sustained for 1,000 years, would likely result in a contribution from Greenland of about 3 m to sea-level rise (see Question 3).

WGI TAR Section 11.5.4
4.16 Pronounced changes in permafrost temperature, surface morphology, and distribution are expected in the 21st century. Permafrost currently underlies 24.5% of the exposed land area of the Northern Hemisphere. Under climatic warming, much of this terrain would be vulnerable to subsidence, particularly in areas of relatively warm, discontinuous permafrost. The area of the Northern Hemisphere occupied by permafrost could eventually be reduced by 12 to 22% of its current extent and could eventually disappear from half the present-day Canadian permafrost region. The changes on the southern limit may become obvious by the late 21st century, but some thick ice-rich permafrost could persist in relict form for centuries or millennia. Thawing of ice-rich permafrost can be accompanied by mass movements and subsidence of the surface, possibly increasing the sediment loads in water courses and causing damage to the infrastructure in developed regions. Depending on the precipitation regime and drainage conditions, degradation of permafrost could lead to emission of greenhouse gases, conversion of forest to bogs, grasslands, or wetland ecosystems and could cause major erosion problems and landslides.

WGII TAR Sections 16.1-2

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