climatic changes and human activites seriusly affect the speed intensity equilibrium of bio geo chemical cycle
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Biological systems have responded to 20th-century changes in climate with range shifts and alterations in the timing of key life events, such as flowering, bud burst, and seasonal migration patterns (Parmesan and Yohe 2003, Root et al. 2003). The climate changes expected over the next 100 years exceed those of the past century (IPCC 2001) and are likely to lead to additional range shifts as the narrow climate characteristics required by many individuals, populations, and species move throughout the world. How successfully biological systems shift their ranges will depend on the rate and magnitude of climate changes to come and on the rate at which species are able to migrate in response to those climate changes.
Fossil pollen data from the early Holocene have been widely interpreted to demonstrate that long-lived plant species migrated extremely quickly in response to postglacial warming: up to 100 to 1000 meters (m) per year (10 to 100 kilometers per century) for tree species (Higgins and Richardson 1999, Tinner and Lotter 2001). Pollen analysis cannot accurately map present-day tree ranges, however, in part because the approach used to analyze pollen does not account for extensive areas where species exist at low densities (McLachlan and Clark 2004). Furthermore, lodgepole pine is still expanding northward, which demonstrates that the species is not yet in equilibrium with climate after a minimum of 300 to 400 years (Johnstone and Chapin 2003). Such findings suggest that the estimates of rapid migration in the past may be overly optimistic.
Similarly, results from diffusion models lead to somewhat ambiguous conclusions about migration rates. Recent advances in model development, such as the inclusion of the potential for occasional long-distance dispersal, help explain how migration rates could be fast (Clark 1998). On the other hand, the inclusion of potentially realistic model assumptions, such as discrete individuals and stochasticity, leads to spread rates that appear to be much slower (Clark et al. 2003). These contradictory interpretations suggest that scientists really do not yet know how rapidly vegetation can migrate in response to changes in climate.
Furthermore, the large and rapid climate changes expected over the next century, even under moderate greenhouse gas (GHG) emissions scenarios (IPCC 2001), would require much faster rates of species migration than those optimistically supposed for postglacial warming (Solomon and Kirilenko 1997, Malcolm et al. 2002). Yet migration rates (and our ability to predict migration patterns) are inherently uncertain even for undisturbed conditions (Clark et al. 2003). Land-use patterns associated with urban, suburban, rural, and agricultural development very likely further complicate ecosystem adaptation to climate change by hindering migration.
The ability of biological systems to migrate and disperse in response to climate change will also depend on the magnitude and rate at which climate changes occur over the next century and beyond. Larger changes in climate imply the need for species to migrate longer distances. A broad range of GHG emissions scenarios are now considered possible for the next century (Nakicenovic et al. 2000), leading to a range in expected globally averaged temperature increases of 1.4 degrees Celsius (°C) to 5.8°C by the year 2100 (IPCC 2001).