Working Group I: The Scientific Basis

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Much of what climate model studies show could happen to weather and climate extremes in a future climate with increased greenhouse gases is what we would intuitively expect from our understanding of how the climate system works. For example, a warming of the surface supplies more water vapour to the atmosphere, which is a greater source of moisture in storms and thus we would expect an increase in intense precipitation and more rainfall from a given rainfall event, both results seen in climate model simulations. There are competing effects of decreased baroclinicity in some regions due to greater surface warming at high latitudes, and increasing mid-tropospheric baroclinicity due to greater mid-tropospheric low latitude warming (Kushner et al., 2001). Additionally, a number of changes in weather and climate extremes from climate models have been seen in observations in various parts of the world (decreased diurnal temperature range, warmer mean temperatures associated with increased extreme warm days and decreased extreme cold days, increased rainfall intensity, etc.). Though the climate models can simulate many aspects of climate variability and extremes, they are still characterised by systematic simulation errors and limitations in accurately simulating regional climate such that appropriate caveats must accompany any discussion of future changes in weather and climate extremes.

Recent studies have reproduced previous results in the SAR and this gives us increased confidence in their credibility (although agreement between models does not guarantee that those changes will occur in the real climate system):

  • An increase in mean temperatures leads to more frequent extreme high temperatures and less frequent extreme low temperatures.
  • Night-time low temperatures in many regions increase more than daytime highs, thus reducing the diurnal temperature range.
  • Decreased daily variability of temperature in winter and increased variability in summer in Northern Hemisphere mid-latitude areas.
  • There is a general drying of the mid-continental areas during summer in terms of decreases in soil moisture, and this is ascribed to a combination of increased temperature and potential evaporation not being balanced by precipitation.
  • Intensity of precipitation events increases.

Additional results since 1995 include:

  • Changes in temperature extremes noted above have been related to an increase in a heat index (leading to increased discomfort and stress on the human body), an increase in cooling degree days and a decrease in heating degree days.
  • Additional statistics relating to extremes are now being produced. For example, in one model the greatest increase in the 20-year return values of daily maximum temperature is found in central and Southeast North America, central and Southeast Asia and tropical Africa where there is a decrease in soil moisture content, and also over the dry surface of North Africa. The west coast of North America is affected by increased precipitation, resulting in moister soil and more moderate increases in extreme temperature. The increases in the return values of daily minimum temperature are larger than those of daily maximum temperature mainly over land areas and where snow and sea ice retreat.
  • Precipitation extremes increase more than the mean and that means a decrease in return period for the extreme precipitation events almost everywhere (e.g., 20 to 10 years over North America).

Aspects which have been addressed but remain unresolved at this time include:

  • There is no general agreement yet among models concerning future changes in mid-latitude storms (intensity, frequency and variability), though there are now a number of studies that have looked at such possible changes and some show fewer weak but greater numbers of deeper mid-latitude lows, meaning a reduced total number of cyclones.
  • Due to the limitations of spatial resolution in current AOGCMs, climate models do not provide any direct information at present regarding lightning, hail, and tornadoes. Results derived from earlier models used empirical relationships to infer a possible future increase in lightning and hail, though there have been no recent studies to corroborate those results.
  • There is some evidence that shows only small changes in the frequency of tropical cyclones derived from large-scale parameters related to tropical cyclone genesis, though some measures of intensities show increases, and some theoretical and modelling studies suggest that upper limit intensities could increase.

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