Climate Change 2001:
Synthesis Report
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4.1

This answer focuses on projected changes in the frequency and magnitude of climate fluctuations as a result of increasing concentrations of greenhouse gases and aerosols. Particular emphasis is placed on changes in the frequency, magnitude, and duration of climatic extremes, which represent important climate change risks for ecological systems and socio-economic sectors. Projected abrupt or other non-linear changes in the biophysical system are discussed here; the gradual changes in the physical, biological, and social systems are discussed in Question 3.

  
4.2 Models project that increasing atmospheric concentrations of greenhouse gases will result in changes in daily, seasonal, inter-annual, and decadal variability. There is projected to be a decrease in diurnal temperature range in many areas, with nighttime lows increasing more than daytime highs. A number of models show a general decrease of daily variability of surface air temperature in winter and increased daily variability in summer in the Northern Hemisphere land areas. Current projections show little change or a small increase in amplitude for El Niño events over the next 100 years. Many models show a more El Niño-like mean response in the tropical Pacific, with the central and eastern equatorial Pacific sea surface temperatures projected to warm more than the western equatorial Pacific and with a corresponding mean eastward shift of precipitation. Even with little or no change in El Niño strength, global warming 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. There is no clear agreement between models concerning the changes in frequency or structure of other naturally occurring atmosphere-ocean circulation pattern such as the North Atlantic Oscillation (NAO).

 WGI TAR Sections 9.3.5-6, & WGII TAR Section 14.1.3
4.3 The duration, location, frequency, and intensity of extreme weather and climate events are likely to very likely to change, and would result in mostly adverse impacts on biophysical systems.

  
4.4 Natural circulation patterns, such as ENSO and NAO, play a fundamental role in global climate and its short-term (daily, intra- and inter-annual) and longer term (decadal to multi-decadal) variability. Climate change may manifest itself as a shift in means as well as a change in preference of specific climate circulation patterns that could result in changes in the variance and frequency of extremes of climatic variables (see Figure 4-1).

 WGI TAR Sections 1.2 & 2.7
4.5 More hot days and heat waves and fewer cold and frost days are very likely over nearly all land areas. Increases in mean temperature will lead to increases in hot weather and record hot weather, with fewer frost days and cold waves (see Figure 4-1a,b). A number of models show a generally decreased daily variability of surface air temperature in winter and increased daily variability in summer in Northern Hemisphere land areas. The changes in temperature extremes are likely to result in increased crop and livestock losses, higher energy use for cooling and lower for heating, and increased human morbidity and heat-stress-related mortality (see Table 4-1). Fewer frost days will result in decreased cold-related human morbidity and mortality, and decreased risk of damage to a number of crops, though the risk to other crops may increase. Benefits to agriculture from a small temperature increase could result in small increases in the GDP of temperate zone countries.

 WGI TAR Sections 9.3.6 & 10.3.2, & WGII TAR Sections 5.3, 9.4.2, & 19.5
4.6 The amplitude and frequency of extreme precipitation events is very likely to increase over many areas and the return period for extreme precipitation events are projected to decrease. This would lead to more frequent floods and landslides with attendant loss of life, health impacts (e.g., epidemics, infectious diseases, food poisoning), property damage, loss to infrastructure and settlements, soil erosion, pollution loads, insurance and agriculture losses, amongst others. A general drying of the mid-continental areas during summer is likely to lead to increases in summer droughts and could increase the risk of wild fires. This general drying is due to a combination of increased temperature and potential evaporation that is not balanced by increases in precipitation. It is likely that global warming will lead to an increase in the variability of Asian summer monsoon precipitation.

 WGI TAR Section 9.3.6 & WGII TAR Sections 4.3.8, 9.5.3, 9.7.10, & 9.8
 
Figure 4-1: Schematic diagrams showing the effects on extreme temperatures when (a) the mean increases, leading to more record hot weather, (b) the variance increases, and (c) when both the mean and variance increase, leading to much more record hot weather.		 WGI TAR Figure 2.32

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