Modeling studies suggest that ecosystem responses to elevated CO2 result in a net carbon sink (that is, some of the elevated carbon generated by human activities is taken up and stored in plant tissues and soils, and the amount stored exceeds the carbon released through plant respiration and decomposition) and that this sink will persist through the twenty-first century (Schimel et al., 2000, 2001). When the models include temperature change as well as elevated CO2, however, they project that these carbon sinks could decrease, thereby increasing concentrations of CO2 in the atmosphere and reinforcing climate warming (e.g., Field et al., 2007a). Indeed, recent analyses suggest that the reduction in efficiency of land ecosystem sinks may already be in decline (Canadell et al., 2007). Several major carbon sinks in terrestrial ecosystems face a high degree of risk from projected climate and land use changes (Fischlin et al., 2007). One of these is permafrost—frozen soil that covers vast areas of the northern latitudes and has locked away vast quantities of carbon. Permafrost temperatures are already rising due to high-latitude warming, creating a potential feedback that could drive further warming. Permafrost could also switch from a carbon sink to a source with thawing, releasing more carbon than in takes up (Dutta et al., 2006; Field et al., 2007a; McGuire et al., 2006; Norby et al., 2005; Zimov et al., 2006) and thus accelerating the pace of climate change. The potential for such a switch is one of several tipping points of concern in ecosystem-climate interactions (Barbier et al., 2008; Lenton et al., 2008; see also Chapter 6). Many other factors will ultimately determine whether terrestrial ecosystems provide a net feedback that enhances or slows the pace of climate change. Species redistributions, changes in major growth forms (e.g., from grass to woody plants, or from coniferous to deciduous trees), drought, length of growing seasons, air pollution, fire, insects and pathogens, deforestation and reforestation (Canadell and Raupach, 2008), and land use (Tilman et al., 2001) will influence uptake or release of CO2 and other greenhouse gases (GHGs) such as N2O and CH4 (Canadell and Raupach, 2008; Swann et al., 2010; Tilman et al., 2001).
Globally, as much as 35 percent of human-induced CO2 emitted over time to the atmosphere has had its origins in changes in land systems (both use and vegetative cover), principally deforestation (Foley et al., 2005). Biomass burning is also a major source of atmospheric aerosols (Andreae and Merlet, 2001). As discussed in Chapter 6, aerosols have direct effects on climate through scattering and absorbing solar radiation, and indirect changes in the properties and propensity for formation of clouds and hence precipitation, all of which can affect ecosystems (Lohmann and Feichter, 2005; Menon et al., 2002). Biomass burning is one of the largest sources of black carbon (soot) aerosols, a particularly potent warming agent that has been implicated in changing precipitation patterns and rapid ice melting in the Arctic (Flanner et al., 2007; McConnell et al., 2007; Wang, 2007). Finally, the emission of various trace gases by plants and from biomass burning leads to the formation of ground-level ozone, a gas that is both a climate-influencing agent and a pollutant that directly affects human and ecosystem health (Auffhammer et al., 2006; Chameides et al., 1994; Orr et al., 2005).
Land use change also influences climate by changing the reflective characteristics of the land surface and the exchange of water between the surface and the atmosphere. Deforestation, arid land degradation, and the transformation of ecosystems into built-up areas, for example, tend to increase reflectivity of the land surface and decrease evapotranspiration, leading to both local climate changes and, in combination with other land use changes, influencing large-scale climate forcing, feedbacks, and atmo spheric circulation patterns (Chapin et al., 2002; Pielke et al., 1998; Zhao et al., 2001). Deforestation tends to lead to warmer and drier climate conditions in the humid tropics, apparently due to reductions in evapotranspiration, (Bounoua et al., 2002; DeFries and Bounoua, 2004). Reductions in vegetation at high latitudes, on the other hand, tend to exert a cooling effect because more snow cover is exposed, increasing the reflection of solar radiation back to space (see Chapter 6 and Bonan, 1999). Afforestation (planting trees where they do not naturally occur), replanting forests in previously deforested areas, or shifts in evergreen species into previously shrub or forb areas could lead to increased absorption of solar radiation and thus increases in temperature (Bala et al., 2007). All these factors are important to the critical question of whether changes in terrestrial ecosystems accelerate or decelerate climate change, yet their combined role has not been evaluated. Importantly, these and other facets of ecosystem change not only influence the global climate system but also generate large local to regional climate implications as well (Cook et al., 2009; Durieux et al., 2003; Li et al., 2006; Malhi et al., 2008; Pielke et al., 1998).
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