Global Climate Change

Global climate change is expected to impact plants and animals worldwide (IPCC, 2001a; Malcolm and Pitelka, 2000). What these changes will be depends largely on the amount and rate at which the world's climate warms. New studies and improved models are increasing our understanding of global climate change. Over the past 140 years, the global average surface temperature has increased 0.6 (+/-0.2) degree centigrade (IPCC, 2001b). Most of the warming has occurred during two periods, from 1910 to 1945 and 1976 to 2000. In the last century, the 1990s were likely the warmest decade on record, and 1998 the warmest year. Although the average global surface temperature has increased by 0.6 (+/-0.2) degree centigrade, there has however been great regional variation, with some regions experiencing much larger increases, and others smaller or no increases in temperature. The average minimum temperature has also increased at a faster rate than the average maximum temperature. Although it is difficult to examine temperature change on longer time scales, recent studies indicate that the temperature increase in the Northern Hemisphere is likely the largest of any century in the past 1,000 years; unfortunately, less data is available for the Southern Hemisphere.

Besides increasing temperatures, there are other indications that the earth's climate is warming. Satellite data reveal that snow cover has declined by 10 percent since the 1960s. During the twentieth century, there has been a continued retreat of the world's mountain glaciers, and at mid and high latitudes in the Northern Hemisphere, the time that lakes and rivers remain frozen has decreased by an average of two weeks. In late summer through early autumn, the thickness of Arctic sea ice appears to have thinned by 40 percent. Sea level rose 0.1 to 0.2 meter during the twentieth century, largely because of thermal expansion (water expands at higher temperatures) and loss of land ice. It is also very likely that continental precipitation has increased by 5 to 10 percent in the Northern Hemisphere over the last century, although it has decreased in other regions, such as North and West Africa and the Mediterranean. Increasing temperatures have been accompanied by shifts in the period of seasons, with earlier springs and longer autumns. As a result, over the last forty years the growing season in the Northern Hemisphere has lengthened from one to four days per decade.

Current global circulation models predict that in the future, the globally averaged surface temperature will have increased by 1.4 to 5.8 degrees centigrade between 1990 and 2100, while sea level will have risen between 0.09 and 0.88 m. These averages are for the entire planet, and a large degree of regional variation is anticipated. Notably, climate change is expected to have a disproportionate effect at higher latitudes, which will have larger temperature increases. Although the mass media frequently mention increased storms as a consequence of climate change, models of global climate change cannot generally predict finer-scale climate events such as storms or hurricanes.

What does climate change mean for the world's ecosystems and species? Climate is central to the geographic distribution of the world's vegetation types and animal species. As the climate warms, we expect to see shifts in vegetation patterns and species distributions. These shifts may fundamentally alter ecosystem composition and function. An increase in temperature of 3 degrees corresponds to an altitude shift of 500 m or a latitude shift of 250 km; this speed of change is similar to the change in cli mate during the Pleistocene era, which was too rapid for many species to adapt to. Alpine species may disappear entirely as they are pushed to their distributional limits. Those species that can adapt fast enough may encounter other barriers, such as human development, that hinder their ability to adjust to climate change.

Parmesan et al. (1999) found that European butterflies were changing their ranges in response to climate change. Of the thirty-five species examined, 63 percent had shifted their range north by 35 to 240 km. Only 3 percent had shifted their range southward. This shift reflects changes in colonization and extinction rates at the boundaries of the species range. Europe has warmed an average of 0.8 degree centigrade this century, reflecting a northward shift in climate of about 120 km. Warming temperatures may also allow some insect pests to widen their range, such as the mosquitoes that transmit malaria and dengue fever.

Species that live close to their temperature limit are particularly vulnerable to climate change. Corals flourish at temperatures between 16 and 25 degrees centigrade. Excessive temperatures stress corals and at the extreme lead to so-called bleaching events. Bleaching occurs when corals expel their symbiotic dinoflagellates (zooxanthellae). Bleaching was once considered a rare, isolated event from which corals often recovered. During the 1980s, however, large-scale bleaching events caused extreme loss of coral, and since then bleaching as occurred somewhere in the world every year. The 1997-1998 bleaching was the most severe and widespread ever observed, affecting reefs in the Pacific and Indian Oceans, Red Sea, Persian Gulf, and the Caribbean (Wilkinson, 1998). Bleaching is usually confined to the surface areas (depths of less than 15 m); in this instance, the damage extended to depths of 50 m.

Similarly, species and communities that live at high elevations, adapted to the climatic conditions in those regions, need to move continually higher as warmer temperatures move up the mountains. At some point these communities will have nowhere else to go and they will perish.

Migratory species are also vulnerable to climate change. Sea birds time their migrations carefully to take advantage of prey resources along their route, such as the spawning of horseshoe crabs or krill. Warming temperatures alter this timing and may cause a species to miss these key resources on route. A long-term study in the American Southwest has revealed a trend toward earlier breeding in the Mexican jay (Aplelocoma ultramarine) (Brown et al., 1999). Similar patterns have been observed in the United Kingdom. It is unknown what effect this will have on their reproductive success.

At higher latitudes, increasing temperatures are altering the environment and affecting ecosystem function. In Canada's Hudson Bay, the weight and reproductive condition of polar bears have been declining since the early 1980s. At the same time, rising spring temperatures have led to earlier ice breakups. Polar bears need solid ice to hunt effectively, and in the spring their main prey are young ringed seal pups. Before the ice thaws in the spring, polar bears must gain sufficient weight to last through the fasting period in the summer, when they are unable to catch prey on the open water (Stirling et al., 1999).

Sea level has risen 10 to 20 cm in the last 100 years and is expected to increase by 0.09 to 0.88 m by 2100; the rise may have dramatic consequences for coastal environments. Rising sea levels are caused by a combination of thermal expansion and loss of glaciers in mountainous areas, and potentially the loss of polar ice sheets—though there is still much uncertainty about how ice sheets will respond to climate change. As with temperature change, sea level rise is expected to vary regionally. Low-lying coastal areas and small island states are particularly vulnerable to sea level change, because it increases the risk of coastal flooding and the impact of storm surge. Other consequences of sea level rise are the loss of beach, wetland, and mangrove habitats.

Finally, climate change may exacerbate already endangered systems. Species that are confined to a small fragmented habitat are particularly at risk to climate change, as these species will not be able to migrate as vegetation and habitats shift. For example, wetlands have the potential to migrate landward as sea level rises, but this migration is seriously hampered by coastal development.

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