"Trends in Extreme Weather and Climate Events: Issues Related to Modeling Extremes in Projections of Future Climate Change"

   In this paper, Zwiers et al provide an overview of significant simulations that have been published thus far on possible future changes in extreme weather events under circumstances of increased greenhouse gas concentrations and higher global mean temperatures. At the time of this paper’s publishing, some computer and simulation technology advancements had allowed for more accurate simulation experiments to be undertaken, however they are still far from definite in their conclusions.

   The first weather aspect to be discussed is temperature. The authors find that a higher mean global temperature will increase the number of extreme warm days as compared to the number of extreme cold days. These results have generally held weight so far, although changes in extreme temperatures vary by region: for example, much smaller increases in minimum temperatures were found at the boundaries of the polar oceans, while a simulation in Australia found that the number of extreme minimum temperature days were cut in half when this minimum temperature increased by 2⁰C.

   Next, the authors move on to the topic that has been a source of more certainty in its future changes: precipitation. There is some evidence that an increase in precipitation as a result of anthropogenic forces is already taking place in some regions of the world. It is projected that higher greenhouse gas concentrations in the atmosphere lead to more intense precipitation, though this varies strongly by geographical region. As precipitation levels increase in variability as a result of higher global temperatures, there is also more likely to be an increase in drought and floods during the monsoon seasons in regions such as Asia-Pacific.

   The possible future changes in extratropical storm frequency and intensity have been more difficult to predict. One simulation showed that when CO₂ and sulfate aerosol levels in the atmosphere were increased, the number of storms in the Northern Hemisphere actually decreased, although their intensity increased. ENSO events also come into play here. As temperatures rise, the pacific climate will be more likely to move into more El Niño-like state whose events can create more precipitation variability. However, since ENSO events are already extremely variable, it is very difficult to predict at all if changes in these events are anthropogenic in nature.

   Tropical cyclone future simulations are also very difficult to assess with any certainty. Some simulations have found that fewer cyclones will occur in a more greenhouse gas rich environment, while the intensity of these events might increase. Tropical cyclone occurrences, however, often rely on ENSO events and therefore are difficult to predict.

Meehl G, Zwiers F, Evans J, Knutson T, Mearns L, and Whetton P (2000) “Trends in Extreme Weather and Climate Events: Issues Related to Modeling Extremes in Projections of Future Climate Change.” Bulletin of the American Meteorological Society 81(3): 427-436.