The major impacts of abrupt climate change are most likely to occur when economic or ecological systems cross important thresholds. For example, high water in a river will pose few problems until the water runs over the bank, after which levees can be breached and massive flooding can occur. Many biological processes undergo shifts at particular thresholds of temperature and precipitation (Precht et al., 1973; Easterling et al., 2000). For example, many plants are adapted to a specific climate and limited by parameters such as frost and drought tolerance (Woodward, 1987; Andrewartha and Birch, 1954). Phenology, the progression of biological events throughout the year, is linked to climate. Several studies have demonstrated this link for climatic warmings over the last half century, For example, in southern Wisconsin a phenological study of over 61 years of springtime events showed that several of these events were occurring earlier in the year. The events that did not are probably linked to photoperiod or regulated by a physiological signal other than temperature (Bradley, 1999; Leopold, 2001). Data spanning 60 years in Britain show that breeding patterns of birds are linked to the North Atlantic Oscillation (NAO), with 53 percent of birds showing long-term, significant trends toward earlier breeding since 1939 (Crick et al., 1997; Crick and Sparks, 1999). The trends toward earlier nesting in birds are paralleled in studies of bird migration (Sparks et al., 1999), butterfly emergence (Roy and Sparks, 2000), and flowering (Schwartz and Reiter, 2000). In North America, similar studies have noted earlier egg laying in tree swallows (Dunn and Winkler, 1999), earlier robin migration, and earlier exit from hibernation by marmots (Inouye et al., 2000). Although these studies indicate a change in organism behavior relating to a steady shift in climate over time, abrupt shifts may exceed the limits of organisms’ ability to adapt.
Many important threshold effects occur at the boundaries of systems. Ecotones, the narrow zones where ecological communities overlap, are particularly susceptible to abrupt climate change, primarily because the species diversity is great and the vegetation is often limited by a sharp climatic gradient. Paleorecords of the decadal response of forest dieback (Peteet, 2000) demonstrate how rapidly boreal forest can be replaced by mixed hardwoods, as was observed in eastern United States ecotonal forests at the close of the Younger Dryas. European pollen records from the cold event about 8,200 years ago indicate significant species changes in fewer than 20 years (Tinner and Lotter, 2001). Furthermore, modern studies also point to