Synthesis Report - Question 9

Climate Change 2001: Synthesis Report

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Future Changes in Regional and Global Climate


The climate has changed during the 20th century; larger changes are projected for the 21st century.


Under all SRES scenarios, projections show the global average surface temperature continuing to rise during the 21st century at rates of rise that are very likely to be without precedent during the last 10,000 years, based on paleoclimate data (Figure 9-1b). It is very likely that nearly all land areas will warm more rapidly than the global average, particularly those at high northern latitudes in the cold season. There are very likely to be more hot days; fewer cold days, cold waves, and frost days; and a reduced diurnal temperature range.

Q3.7, Q3.11, & Q4.5
9.14 In a warmer world the hydrological cycle will become more intense. Global average precipitation is projected to increase. More intense precipitation events (hence flooding) are very likely over many areas. Increased summer drying and associated risk of drought is likely over most mid-latitude continental interiors. Even with little or no change in El Niño amplitude, an increase in temperatures globally is likely to lead to greater extremes of drying and heavy rainfall, and increase the risk of droughts and floods that occur with El Niño events in many different regions.

Q2.24, Q3.8, Q3.12, Q4.2, & Q4.6
9.15 In a warmer world the sea level will rise, primarily due to thermal expansion and loss of mass from glaciers and ice caps, the rise being continued for hundreds of years even after stabilization of greenhouse gas concentrations.
This is due to the long time scales on which the deep ocean adjusts to climate change. Ice sheets will continue to react to climate change for thousands of years. Models project that a local warming (annually averaged) of larger than 3ºC, sustained for many millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea-level rise of about 7 m.

Q3.9, Q3.14, Q4.15, & Q5.4
9.16 Key uncertainties that influence the quantification and the detail of future projections of climate change are those associated with the SRES scenarios, and also those associated with the modeling of climate change, in particular those that concern the understanding of key feedback processes in the climate system, especially those involving clouds, water vapor, and aerosols (including their indirect forcing). Allowing for these uncertainties leads to a range of projections of surface temperature increase for the period 1990 to 2100 of 1.4 to 5.8ºC (see Figure 9-1b) and of sea-level rise from 0.09 to 0.88 m. Another uncertainty concerns the understanding of the probability distribution associated with temperature and sea-level projections for the range of SRES scenarios. Key uncertainties also affect the detail of regional climate change and its impacts because of the limited capabilities of the regional models, and the global models driving them, and inconsistencies in results between different models especially in some areas and in precipitation. A further key uncertainty concerns the mechanisms, quantification, time scales, and likelihoods associated with large-scale abrupt/non-linear changes (e.g., ocean thermohaline circulation). Q3.6, Q3.9, & Q4.9-19

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