The rise in mean global surface temperature during the last century has been characterized by a faster increase in minimum temperatures compared to maximum temperatures (Easterling et al., 1997). Both the overall warming and the disproportionate increase in minimum temperatures have contributed to a lengthening of the warm (growing) season, especially in mid- to-high latitudes. Many plants and animals respond to a longer growing season by changing the timing of activities associated with the arrival of spring and onset of autumn such as flowering, leaf fall, breeding, and migration. There is growing evidence from sites in Europe and North America that such changes are occurring now.
Regional analyses show a trend toward earlier ice break-up and snow melting in spring that is consistent with the enhanced warming observed at higher latitudes (Nicholls et al., 1996). In tundra and boreal lands, river ice break-up dates are at least a week earlier compared to the last century (Fitzharris, 1996). Spring snow cover in the Northern Hemisphere declined significantly from 1988 to 1994 relative to the other seasons, coincident with a strong spring warming (Groisman et al., 1994). In the northeastern United States, the frost-free season now begins an average of 11 days earlier than in the 1950s (Karl et al., 1997). With continued global warming, many rivers in temperate regions may become ice-free or develop only intermittent or partial ice cover; in colder regions, the present ice season could be shortened by up to a month by 2050 (Fitzharris, 1996).
The life cycles of most plants and animals are critically tied to seasonal changes in temperature, precipitation, and light. The timing of bird migration and breeding is sensitive to changes in temperature, and global warming would be expected to lead to an earlier onset of those activities in the spring. Statistically significant trends toward earlier bird egg-laying and nesting have been reported for sites in Europe (Crick and Sparks, 1999; Crick et al., 1997) and the southern United States (Brown et al, 1999). The earlier nesting in Europe is attributed in part to earlier plant growth, which in turn causes earlier availability of the insects the birds feed upon (Crick et al., 1997). A trend toward earlier spring migration has also been observed for birds arriving in Michigan (Root, 1993).
Changes in European plants are consistent with the observations for birds. A recent study of European spring and autumn plant phases indicates that on average, spring events advanced by about six days and autumn events were delayed by about five days over the period from 1959 to 1993 (Menzel and Fabian, 1999). The long-term consequences of these changes are uncertain. The changing climate may impair the extent to which an animal s life cycle is synchronized with its food supply. Birds, for example, can adjust to warmer temperatures by flying to more northern areas in any given year, but the vegetation upon which they (or their prey) rely may take decades or longer to adjust (see Root, 1997).
A shorter duration of ice cover due to an earlier spring has the potential to affect the productivity, life history, and reproduction of aquatic organisms. Earlier break-up (and later freeze-up) of sea ice in the Arctic shortens the time available for feeding and rearing of certain marine mammals and may change the health or size of a population. A new study of polar bears in Hudson Bay indicates that because of earlier melting, the bears now have less time to hunt on the sea ice and must return to shore before they have adequately replenished their fat by feeding on seal pups (Stirling et al., 1999). Ice break-up in western Hudson Bay now occurs an average of three weeks earlier compared to the mid-1970s.
In temperate regions, reduced ice cover on rivers and lakes leads to increased turbulence and mixing of sediments, which can disturb incubating eggs of certain fish species (Arnell et al., 1996). On the other hand, an earlier spring ice-out lengthens the growing season of aquatic systems, and thus can increase the survival rate of young fish during the following winter. Because light attenuation by overlying ice is a major limiting factor for certain species, the shorter duration of ice cover also may alter the competitive balance of species in some systems (Arnell et al., 1996).
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