fire regimes on a subcontinental scale (Swetnam and Betancourt, 1990; Plate 9).

Droughts and floods are also responsible for changes in erosion patterns, as reduced vegetation due to fires results in greater soil loss (Allen, 2001). For example, in the Indonesian tropics, drought years have led to a greater frequency and magnitude of fires resulting in a loss of peatlands, increased erosion, and increased global air pollution. Globally, rates of soil erosion are 10 to 40 times greater than rates of soil formation (i.e., over 75 billion tons from terrestrial systems annually; Pimentel and Kounang, 1998).

Droughts have also been implicated in insect outbreaks and pulses (births and deaths), with impacts on tree demography (Swetnam and Betancourt, 1998). Episodic outbreaks of pandora moth (Coloradia pandora), a forest insect that defoliates ponderosa pine (Pinus ponderosa) and other western US pine species, have been linked to climatic oscillations (Speer et al., 2001). Drought years have been linked to insect crashes as well as booms (Hawkins and Holyoak, 1998).

Tundra systems are highly susceptible to the effects of climate change because of their sensitivity to water table fluctuations, snow-albedo feedbacks, fire frequency, and permafrost melting (Gorham, 1991). Tundra areas are significant in the terrestrial/atmospheric carbon balance because tundra peat is a large source and sink of greenhouse gases, most notably carbon dioxide and methane (Billings, 1987). Furthermore, 15 to 33 percent of the global soil carbon is contained in the northern wetlands (Gorham, 1991; Schlesinger, 1991). The sensitivity of tundra ecosystems to abrupt climate change, and the widespread influence these areas have on feedbacks in the climate system, make these key areas for climate change monitoring. For example, field measurements and modeling have shown that even vegetation changes induced by summer warming could result in climatic feedbacks (Chapin et al., 2000).

Paleoclimatic investigations have shown that changes in biodiversity are correlated to climate variations. Abrupt climate shifts have been linked to increases in biodiversity as well as to species extinction, although extinctions can occur much more rapidly than can origin of diversity by evolution. The evolution of large beaks among Darwin’s finches (Geospiza fortis) is thought to be the result of ecological stress from El Niño drought years, while extremely wet years favored evolution of small beaks (Boag and Grant, 1984). But recent research suggests that the rate of species extinctions is on the order of 100 to 1,000 times higher than before humans were dominant (Lawton and May, 1995; Vitousek et al., 1997). For example,