As part of an effort to monitor extreme weather events around the globe, some
recent studies have examined the intensity of rainfall events. Karl et al. (1995)
found a trend toward higher frequencies of extreme (>50.8 mm) 1-day rainfalls
over the United States. The results pertained to the period 1911-92; the increasing
frequency of such events was found to be a product mainly of heavier warm-season
rainfall. Karl et al. (1996) also found a steady increase from 1910 to 1995
in the percentage area of the contiguous United States with a much above-normal
(defined as the upper decile of all daily precipitation amounts) proportion
of total annual precipitation coming from these extreme 1-day events (Figure
8-6). This area increased by 2-3%, and it was determined that there is less
than 1 chance in 1000 that this change could occur in a quasi-stationary climate.
To date, however, there is no similar evidence of an increase in the proportion
of Canadian precipitation from extreme 1-day events (Hogg and Swail, 1997).
|Figure 8-6: Percentage of the conterminous U.S. area with a much above normal proportion of total annual precipitation from 1-day extreme (more than 2 in. or 50.8 mm) events (Karl et al., 1996).|
Although sea-level rise usually is not considered a climatic variable, it is arguably one of the most important potential impacts of global climate change in terms of environmental and social consequences (IPCC 1996, WG I, Section 7.1). Therefore, a brief summary of sea-level trends is appropriate. Global mean sea level is estimated to have risen 10-25 cm over the past 100 years (IPCC 1996, WG I, Section 7.2). These estimates are based on tide gauge records; the increase is thought to result largely from the concurrent increase observed in global temperatures, which causes thermal expansion of the ocean and contributes to the melting of glaciers, ice caps, and ice sheets. In general, there is broad agreement that both thermal expansion and glaciers have contributed to the observed sea-level rise, but there are very large uncertainties regarding the role of the ice sheets and other hydrological factors (IPCC 1996, WG I, Section 7.4). There are differences in century-scale sea-level trends across regions of the globe because of vertical land movements such as "postglacial rebound." Figure 8-7 depicts sea-level trends for several North American sites. Sea level has risen 2.5-3.0 mm/year along parts of the U.S. Gulf coast and along the Atlantic coast south of Maine. Along the Texas-Louisiana coasts, sea level has been increasing about 10 mm/year as a result of rapid land subsidence in this region. Sea level is stable or dropping along much of the Canadian and Alaskan coasts because of postglacial rebound.
|Figure 8-7: Relative sea-level trends for selected North American sites (adapted from Titus, 1997).|
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