Global warming and severe weather activity

The concentration of atmospheric CO₂ has been significantly increasing mainly due to human consumption of fossil fuels since the middle of the 18th century. Although the recent increases of CO₂ could possibly give rise to some warming, serious problems remain unresolved in relation to the concept of global warming. The focus of this paper is directed at two areas: the first is the credibility of the global warming prediction, and the second is the long-term trend in severe weather activity associated with global warming. The global warming during the last century has been confirmed by observational data. The magnitude of the warming averaged globally is about 0.6°C during the last century, and is consistent with the computed increase in CO₂ concentration. The observational data indicate a large degree of hemispheric asymmetry; the warming of the Northern Hemisphere is slower and sometimes temporarily interrupted comparing with that of Southern Hemisphere. It is an open question why this hemispherical asymmetry is of opposite sense to the results of numerical simulations for the increases in CO₂ concentration. This question may possibly be resolved by consideration of the climatic effects of increases in anthropogenic aerosol levels in the troposphere in addition to greenhouse gas effects. One question raised by global warming is whether the frequency and intensity of extreme weather events have shown a long-term increased in association with global warming. Since extreme weather such as torrential rainfall and tropical cyclones are relatively small-scale, short-lived phenomena, they cannot be successfully simulated using general circulation models (GCM) at the present time. Furthermore, the limited period of the available observational data makes it difficult to produce any meaningful results with conventional statistical methods. A new statistical method, which was proposed by the present author and a colleague, is applied to the maximum daily precipitation data, and significant evidence is found for an interdecadal increasing trend in the intensity of extremely heavy rainfall. It is argued that the most likely cause of this trend is global warming. With regard to the relation between global warming and tropical cyclone activity, no significant results have yet been obtained either by numerical simulation or statistical analysis of historical data. Recent research analyzing historical data on the relation between tropical cyclone intensity and sea surface temperature has emphasized that the warmer sea surface accompanied by global warming could increase the maximum possible intensity of cyclones, although no appreciable change would be seen in the average intensity of regular storms. Presented at the International Conference on Technologies for Marine Environment Preservation (MARIENV’95), Tokyo, Japan, September 24–29, 1995.