Impacts of Climate Change on Land Based Ecosystems and Biodiversity

A series of place-based observations, meta-analyses, and models indicate that climate shifts have already begun to change the geographical range of plants and animal species on land (IPRC, 2007c). In the extreme, some plants and animals have experienced 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 (Fisch-lin 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 drive it, also affects the functioning of terrestrial ecosystems and their living communities (Loreau et al., 2001; Tilman et al., 1997); this, in turn, changes how ecosystems influence the atmosphere and climate system (Steffen et al., 2004). Experimental and modeling studies (e.g., Field et al., 2007b; Reich et al., 2006) reveal that, in general, exposure to elevated CO2 and temperatures leads to increases in photosynthesis and growth rates in many plants, up to a point; thereafter, the trend may reverse owing to processes not yet fully understood (Woodward, 2002). Decomposition and associated release of CO2 back to the atmosphere also increase as temperatures warm. However, ecosystem processes such as plant growth and decomposition are also determined by interactions with other factors such as nitrogen and carbon supplies, soil moisture, length of growing season, land use, and disturbance (Eviner and Chapin, 2003). Despite this complexity, projections suggest that forest productivity, especially in young forests on fertile soils where water is adequate, will increase with elevated CO2 and climate warming. Where water is scarce and drought is expected to increase, however, forest productivity is projected to decrease (Janetos et al., 2008).

Climate warming alone is projected to drive significant changes in the range and species composition of forests and other ecosystems. Generally, tree species are expected to shift their ranges northward or upslope, with some current forest types such as oak-hickory expanding, others such as maple-beech contracting, and still others such as spruce-fir disappearing from the United States altogether (Figure 9.1). Importantly, however, whole forest communities or ecosystems will not shift their ranges intact. Plant and animal species will respond independently, according to their physiology and sensitivity to climate, resulting in the breakup of existing communities and ecosystem types and the emergence of new ones. The consequences of such reshuffling are not clear, either for the plants and animals that now exist together, or for the services those systems provide to humanity.

In addition to climate change, ecosystems and biodiversity are already being impacted by human activities. For example, human infrastructure such as farms, settlements, and road networks have directly or indirectly affected more than 50 percent of the ice-free, terrestrial surface of the Earth (Ellis and Ramankutty, 2008; Foley et al., 2005; Vitousek et al., 1997). As much as 41 percent of the vast expanse of the oceans has been affected by human activities, for example through eutrophication or fish stock depletion (Halpern et al., 2008). Considering indirect impacts, such as ocean acidification, ground-level air pollution, and climate change, virtually all ecosystems on Earth are being affected in some way by climate change, and other human pressures on ecosystems are also growing significantly (Auffhammer et al., 2006; Chameides et al., 1994; Orr et al., 2005).

1960-1990

Hadley scenario 2070-2100

Canadian scenario 2070-2100

FIGURE 9.1 Potential changes in the geographic ranges of the dominant forest types in the eastern United States under projections of future climate change, based on the Hadley and Canadian climate models and a forest-type distribution model. Many forest types shift their ranges northward. Some types of forests, such as the loblolly-shortleaf pine in the Southeast (dark blue) or the maple-beech-birch forest type (red), shrink in area significantly or migrate to areas to the north and west. Oak-hickory (dark green) and oak-pine (light green) forest types expand their ranges. SOURCE: USGCRP (2001).

Hadley scenario 2070-2100

Canadian scenario 2070-2100

FIGURE 9.1 Potential changes in the geographic ranges of the dominant forest types in the eastern United States under projections of future climate change, based on the Hadley and Canadian climate models and a forest-type distribution model. Many forest types shift their ranges northward. Some types of forests, such as the loblolly-shortleaf pine in the Southeast (dark blue) or the maple-beech-birch forest type (red), shrink in area significantly or migrate to areas to the north and west. Oak-hickory (dark green) and oak-pine (light green) forest types expand their ranges. SOURCE: USGCRP (2001).

□ Wiiite-Red-Jack Pine U Spruce-Fir n Longleai-Slash Pine ' Loblolly-Shorlleaf Pine ^ Oak-Pine

^ Oak-Cjm-Cypress [_Elm-Ash-Colloiwood

■ Maple-Beech-Birch ' Aspen-Birch

D No Data

□ Wiiite-Red-Jack Pine U Spruce-Fir n Longleai-Slash Pine ' Loblolly-Shorlleaf Pine ^ Oak-Pine

^ Oak-Cjm-Cypress [_Elm-Ash-Colloiwood

■ Maple-Beech-Birch ' Aspen-Birch

D No Data

Managing the impacts of climate change on ecosystems and individual species already poses difficult challenges to land, resource, and conservation managers, and these challenges will undoubtedly increase. Past ecosystem conservation relied heavily on the assumption of a stable climate and focused on protecting individual species in place as well as preserving the habitat of entire species assemblages within protected areas. As climate change forces species to migrate to more suitable climates, ecosystems will be disassembled and reassembled in new locations, often outside the bounds of protection, and with new casts of characters. Some species will be lost, while other species will appear in new locations where they may become invasive and add to the pressures on existing species (NRC, 2008b).

Significant research is needed to better understand how climate change affects both individual species and entire ecosystems, and whether transitional or newly assembling ecosystems can continue to provide the ecosystem goods and services on which society depends (e.g., CCSP, 2008d; Fischlin et al., 2007). Moreover, social science research is needed to help land, resource, and conservation practitioners guide adaptive risk management in the face of altered species composition and a continually changing climatic and environmental baseline. In addition, very little is known yet about the social acceptability of new and evolving approaches to species conservation and land protection (including the Endangered Species Act under significant climate change, when many more species are at risk of extinction) or the social acceptability of a triage approach to species protection that may evolve as ecosystem functions are affected by climatic and species changes. Past experience with conservation management, however, indicates that societal values relative to species protection are significant to policy and practice. Integrated assessment and decision-support tools are also needed to help managers and the public understand and make wise judgments about the complex trade-offs that will be involved.

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