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5.4 Climate Change and Extreme Weather-Related Event Results

5.4.1 Wildfires

Because climate change may alter the frequency, intensity, distribution or extent of wildfires, species regeneration patterns may be disturbed, with species or communities at the edges of their natural range experiencing potentially severe effects.

Model results from the fire distribution module of MC1 predict great variation in future fire-weather patterns for the northern portion of North America (Bachelet and Neilson 2000). The seasonal severity rating (SSR) of fire hazard increases over much of North America under both the HadCM2Sul and the CGCM1 scenarios. The wetter HadCM2Sul scenario predicts smaller (<10 percent) increases in SSR by 2060 for most of the United States. The warmer and drier CGCM1 scenario produces a 30-percent increase in SSR for the South. Expected increases in area burned in the Unites States are between 25 and 50 percent by 2060, with most of the increase occurring across the South and in Alaska (Flannigan and others 2000).

In addition, recent results from the MC1 model, described by Neilson and Drapek (1998), show increases in biomass burned. This model includes an interaction with CO₂ and increased WUE that produces more biomass and thus more fuel, contributing to more and larger fires under a highly variable climate having dry years interspersed with wet periods.

5.4.2 Hurricanes

Global climate change may speed up the hydrologic cycle by evaporating more water, transporting that water vapor to higher latitudes, and producing more intense and possibly more frequent storms (Royer and others 1998, Walsh and Pittock 1998). Hurricane formation could be influenced by changes in temperature and the global hydrologic cycle, but neither the magnitude nor direction of the change can be predicted at this time. Sea-surface temperatures (SSTs) are predicted to increase, with warmer SSTs expanding to higher latitudes (Royer and others 1998, Walsh and Pittock 1998). Even if hurricane frequency does not increase, the intensity and duration of storms may increase with air and ocean temperatures, which are energy sources for hurricanes (Walsh and Pittock 1998).

5.4.3 Tornadoes

Berz (1993) suggested that the frequency and intensity of tornadoes (and hailstorms) might be accelerated with increased intensity of atmospheric convective processes. Karl and others (1995c) found that the proportion of precipitation occurring in extreme thunderstorms has increased in the United States from 1910 to 1990, and their research suggested that precipitation and temperature anomalies have become extreme in recent decades (Karl and others 1995a). The thunderstorm conditions that contribute to tornado formation have increased, and this trend is expected to continue with projected changes in climate. It can be inferred from this relationship that warmer temperatures will increase tornado frequency. Despite the data on thunderstorms and the indirect inferences about tornado frequencies, the understanding of tornado genesis is still inadequate for forecasting climate change impacts on tornado frequency or severity in the coming decades.

5.4.4 Floods

Climate change predictions include increased frequency of heavy precipitation events and severe flooding (IPCC 1998). From 1987 to 1997, there were ten times as many catastrophic floods globally than in the previous decade (Hileman 1997).

Over the last century, sea level has risen 3 to 10 inches. Predicted increases in global air temperatures may result in sea level rises of 15 to 25 inches by 2100 (Gornitz 2001). Current trends in sea level have been confirmed to be higher than those found in long-term geologic records (Gornitz 2001).

5.4.5 Drought

Global circulation model predictions of future precipitation patterns are particularly problematic for the South. While the HadCM2Sul scenario predicts increased precipitation throughout the United States, a Canadian Centre for Climate Modelling and Analysis GCM, CGCM1, predicts significant reductions in both summer and winter precipitation across the South by 2100. To address the potential impacts of drought on forests, the net effect of precipitation changes on soil water must be understood; unfortunately, global scale climate models are not designed to predict this information (Hanson and Weltzin 2000).

5.4.6 Ice Storms

Unfortunately, there is no consistent historic record of ice storms over broad scales with rigorous measurements of ice accumulation. Neither are historical data on climatology associated with ice storms sufficient to correlate past storm frequency and severity with past climate changes. Effects of future climate change on location, extent, and impacts of ice storms, therefore, are also unknown (Irland 2000).

5.4.7 Climate Change

Southern forest productivity, as predicted by the PnET-II model and the HadCM2Sul climate scenario, is shown in Figure 5, Figure 6, and Figure 7 for the decades centered on 2000, 2040, and 2090. Predicted productivity increased by 12 percent from 2000 to 2100 (NAST 2001). Changes in forest productivity resulting from climate change were different for hardwood and pine forest types. By 2040, hardwood and mixed pine-hardwood forest productivity increased by 22 percent, while plantation pine forest productivity increased by 11 percent. By 2100, hardwood and mixed pine-hardwood forest productivity increased by 25 percent, and plantation pine forest productivity increased by 8 percent (NAST 2001). A review of over 50 studies found an average increase in plant dry mass of 32 percent under a doubling of CO₂ concentrations. WUE, examined in another review, increased between 30 and 40 percent (IPCC 1998).

Both MAPSS and MC1 models predict that moderate temperature increases produce increased vegetation density and carbon sequestration across most of the United States small changes in vegetation types result. If temperature increases are more severe, the models predict shifts in vegetation types and reductions in carbon storage. The South is predicted to have expanded forest area (national average of 20 percent) under the more moderate climate scenarios but forest decline under more severe climate scenarios (including CGCM1), with catastrophic fires potentially causing rapid vegetation conversion from forest to savanna (Figure 8) (Bachelet and others 2001). MC1 predicts a return to forest by the end of the 21^st century, albeit with lower forest biomass than before the fires occurred. The same moderate-increase, severe-decrease trend is true for leaf area index (LAI), a measure of leaf area per unit of ground area, and vegetation density of forests in the South. MAPSS and MC1 predict an increased presence of tropical forests along the Gulf Coast (Bachelet and others 2001).