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Advancing the Science of Climate Change
maximum range shifts over the past 30 years that approach the magnitude of those witnessed in the transition from last glacial maximum to the present (NRC, 2008b; Parmesan and Yohe, 2003). In the Northern Hemisphere, range shifts are almost wholly northward and up in elevation as species search for cooler temperatures (NRC, 2008b). Special stress is being placed on cold-adapted species located on mountain tops and at high latitudes where boreal forests are invading tundra lands and where Arctic and Antarctic sea ice is rapidly diminishing (e.g., polar bears and various species of seals and penguins [NRC, 2008b]). Warming of streams, rivers, and lakes also potentially affects cold-water fish, such as economically important salmon and trout, through impacts on reproduction, food resources, and disease. The IPCC estimates with medium confidence that approximately 20 to 30 percent of plant and animal species assessed so far are likely to be at increasingly high risk of extinction as global average temperatures exceed a warming of 3.6°F to 5.4°F (2°C to 3°C) above preindustrial levels (Fischlin et al., 2007).
The phenology of species (seasonal periodicity and timing of life-cycle events) is also changing with warming. Biological indicators of spring (e.g., timing of flowering, budding, and breeding) arrive in the Northern Hemisphere as much as 3 days earlier each decade, and the growing season is longer (Walther et al., 2002). Such changes can disrupt the synchronicity between species and their food and water sources, pollinators, and other vital interactions. It also affects the timing and severity of insect and disease outbreaks, wildfire, and other disturbances, challenging the capacity of ecosystems and those charged with managing them to deal with new disturbance patterns. For example, large and long-duration forest fires have increased fourfold over the past 30 years in the American West; the length of the fire season has expanded by 2.5 months; and the size of wildfires has increased several-fold (NIFC, 2008; Westerling and Bryant, 2008; Westerling et al., 2006). Recent research indicates that earlier snowmelt, temperature changes, and drought associated with climate change are important contributors to this increase in forest fire (Westerling et al., 2006). Climate change in the western United States is also increasing populations of forest pests such as the spruce beetle, pine beetle, spruce budworm, and wooly adelgid (Logan et al., 2003) and expanding their range into forested areas previously protected from insect attack. Climate change thus increases the complexity and costs of forest and fire management practices (Chapin et al., 2003; Spittlehouse and Stewart, 2003), which in turn are strongly affected by policy. These policies and practices can be better informed by linking downscaled climate models with hydrologic and fire-vegetation models to determine, under different projections of climate change, which ecosystems will be most vulnerable to wildfires (Westerling, 2009).
Climate change, including the higher levels of CO2 in the atmosphere that help to