Warmer global temperatures are expected to cause an intensification of the hydrologic cycle, with increased evaporation over both land and water. The higher evaporation rates will lead to greater drying of soils and vegetation, especially during the warm season. Climate models also project changes in the distribution and timing of rainfall. The combination of a decrease in summer rainfall and increased evaporation will lead to more severe and longer-lasting droughts in some areas. Increasing drought frequency has the potential to affect land-based natural and managed ecosystems, coastal systems, and both freshwater quality and quantity. Increasing drought frequency also has the potential to increase the likelihood of wildfires.
Predicted changes in the regional distribution of precipitation are uncertain, primarily because the scale of most climate models is too coarse to resolve important hydrologic processes such as the formation and distribution of clouds and storms. Nevertheless, model projections suggest increased vulnerability to drought for certain areas (Kattenberg et al., 1996). Simulations using higher atmospheric CO2 levels generally predict drier summers at northern high latitudes (e.g., Gregory et al., 1997). In these regions, less winter precipitation falling as snow and warmer temperatures leads to an earlier drying of soils in the spring, increasing the likelihood of drought. Other areas strongly affected by snow, such as mountainous watersheds, will be similarly affected. An increase in the ratio of rain to snow, accelerated spring snowmelt, and a shorter snow season will lead to more rapid, earlier, and greater spring runoff but reduced summer flow.
The observed climate trends over the last century have been dominated by increasing precipitation, rather than by drier conditions (see Downpours, Heavy Snowfalls, and Flooding). Some regional decreases in precipitation have been observed, notably in parts of Africa, the Caribbean, and tropical Asia (IPCC, 1998, Appendix A). There is presently no evidence for an increase in the frequency of droughts in North America over the past century, in part because the severe droughts of the 1930 s dominate the historical record (Karl et al., 1996). Studies of past drought variability in the US Great Plains, however, indicate the potential for more severe and frequent droughts in the future (Laird et al., 1996; Woodhouse and Overpeck, 1998), and human-induced warming may exacerbate this natural variability.
The environmental and ecological consequences of the summer 1999 drought in the eastern United States provide examples of situations that may become more frequent as climate changes. Without freshwater to rinse out rivers and streams, salt water encroached further up rivers in many areas of the mid-Atlantic coast (USGS, 1999). The high salt content threatened water supplies in cities that depend on freshwater from rivers and prevented farmers in some areas from irrigating their crops. Decreased freshwater runoff also led to increased salinity and low oxygen conditions in Chesapeake Bay, causing fish kills and other ecological changes. As future sea level rise shifts the saltwater-freshwater boundary farther inland, droughts will exacerbate the geographic extent and impacts of saltwater encroachment into coastal aquifers.
Susceptibility to wildfires increases during periods of drought. During 1997-98, the strong El Niño was associated with extremely dry conditions and large forest fires in many areas of the world, including Indonesia, eastern Russia, Brazil, Central America, and Florida. Although "slash-and-burn" land clearing methods and accidents triggered many of the fires, the severe drought conditions allowed them to spread out of control. Droughts have been relatively frequent since the late 1970s in some areas where drought usually accompanies El Niño events (e.g., north-east Australia) (Nicholls et al., 1996). This presumably reflects the relatively frequent El Niño events during this period. A new study suggests that climate change may produce a quasi-permanent/stable El Niño-like condition in the Pacific basin, interrupted by more extreme cold (La Nina) events (Timmerman et al., 1999). If the frequency of strong El Niños increases, the frequency and severity of droughts and forest fires may also increase, with potentially devastating effects on ecosystems if the increased fire frequency prevents the regeneration of certain species and ecosystems.
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Laird, K. R., S. C. Fritz, K. A. Maasch, and B. F. Cumming, 1996. Greater drought intensity and frequency before AD 1200 in the Northern Great Plains, USA. Nature 384, 552-554.
Nicholls, N., G.V. Gruza, J. Jouzel, T.R. Karl, L.A. Ogallo, and D.E. Parker, 1996. Observed climate variability and change, in Climate Change 1995: The Science of Climate Change, (Eds J.T. Houghton, L.G.M. Filho, B.A. Callander, N. Harris, A. Kattenberg, and K. Maskell), 133-192, Cambridge University Press, Cambridge, UK.
Timmerman, A.J. Oberhuber, A. Bacher, M. Esch, M. Latif and E. Roeckner, 1999. Increased El Nino frequency in a climate model forced by future greenhouse warming. Nature 398, 694-696.
USGS, 1999. 1999 Summer drought may become century s worst Mid Atlantic region experiencing most dramatic effects. News Release, U.S. Geological Survey. 2 August 1999.
Woodhouse, C.A. and J.T. Overpeck, 1998. 2000 years of drought variability in the central United States. Bulletin of the American Meteorological Society 79 (12), 2693-2714.
"Climate Enhanced Wildfires of 1998" 12 January 1999, National Climatic Data Center. http://www.ncdc.noaa.gov/ol/climate/research/1998/
"Florida Wild Fires and Climate Extremes" 29 June 1998, National Climatic Data Center, NOAA. http://www.ncdc.noaa.gov/ol/climate/research/1998/
Frederick, K.D. and P.H. Gleick, 1999. Water and Global Climate Change: Potential Impacts on U.S. Water Resources. Pew Center on Global Climate Change, Arlington, Virginia, 48p. http://www.pewclimate.org/projects/clim_change.pdf
National Drought Mitigation Center This site provides information on drought science and climatology, as well as links to additional drought-related resources.
Trenberth, Kevin E. 1999. The extreme weather events of 1997 and 1998. Consequences 5(1), 3-15. http://www.gcrio.org/CONSEQUENCES/introCON.html
Bibliography: Climate Change and Impact on US Water Resources Over 860 citations from the peer-reviewed literature on climate change and water in the U.S., compiled by the Pacific Institute. http://www.pacinst.org/CCBib.html