The impacts of past landcover changes on climate

Changes in land-cover affect climate through, for example, changes in albedo, in moisture retention and water vapor flux, and in gas and fine particulate flux into the atmosphere, with implications for temperature, cloud formation, and precipitation regimes. All these and other interlinked processes are being observed, measured, and modeled at the present day (summarized in Chhabra et al. 2006; Lawrence and Chase 2006). Observational (Lim et al. 2005) and experimental studies (Sturm et al. 2005) of present-day vegetation changes confirm the sensitivity of surface climate to changes in land-cover. Model sensitivity studies suggest that biomass burning can increase cloud condensation nuclei, which can seriously reduce the efficiency of rain production in convective clouds. This in turn changes the energy budget, convective heating, and vertical temperature gradients, with likely effects on atmospheric circulation (Nober et al. 2003). Pielke (2005) claims that at the scale at which thunderstorms are generated, for example, the effects of spatially heterogeneous land-cover may be at least as important in altering weather as changes in climate patterns associated with greenhouse gases. A growing number of modeling studies designed to estimate the likely impact of hypothesized land-cover change on future climate (Bounoua et al. 2002; Snyder et al. 2004; Fedemma et al. 2005; Crucifix, this volume; Claussen, this volume) highlight their importance. This carries with it the implication that these processes should be taken into account in evaluating the climatic significance of past land-cover changes.

Several studies point to the importance of past changes in land-use and landcover as modulators of climate at least at the regional scale. Moreover, land-cover change is being increasingly incorporated in climate/Earth system models, especially those of intermediate (reduced) complexity (Claussen, this volume). Recent studies by Werth and Avissar (2005) and Schneider and Eugster (2005) attempt to assess the likely effects of past land-cover change on climate in south-east Asia and the Swiss Plateau, respectively, and identify significant local effects on temperature and precipitation regimes. Fu (2003) estimates that over the past 3000 years over 60 percent of East Asia has been affected by various forms of land-cover conversion, e.g. forest to farmland, grassland to semi-desert, and various types of land degradation. By comparing the effects of inferred "natural" and the existing, transformed land-cover in climate simulations, he suggests that major changes in the hydrologic balance have resulted, with important implications for precipitation, runoff, and soil water content. He cites these changes as likely contributors to a decline in atmospheric and soil humidity, and the consequent trend to aridification over the same period, especially in northern China. On a global scale, Myhre et al. (2005) use comparison of MODIS-derived land-surface cover and inferred "natural" vegetation to estimate radiative forcing due to surface albedo changes resulting from anthropogenic vegetation changes. They infer weaker cooling on a global scale than do earlier modeling studies (Bauer et al. 2003; Matthews et al. 2004), but identify regionally important effects. They claim that 25-33 percent of the temperature change forced by land-cover change pre-dated industrialization (see also Goosse et al. this volume). As hinted above there are conflicting views on the extent to which aridification trends in the Sahel region of Africa have been reinforced by anthropogenic changes in vegetation. Taylor et al. (2002) suggest that these changes are not large enough to have been the main cause of Sahel drought in recent decades, although they may have a larger impact in the future. Govaerts and Lattanzio (2007), however, use satellite-derived surface albedo data from the past two decades to show that the feedback from reduced precipitation and effects on land-cover sustains drought conditions arising initially from changes in sea-surface temperatures in the tropical Atlantic. Another view of Sahel desertification, on a longer time-scale, is presented by Foley et al. (2003). They envisage the possibility that nonlinear shifts from wet to dry states might have been triggered by stochastic events (e.g. a period of drought) superimposed on trends that included land degradation. One of the very few attempts to apply a modeling approach to the issue of land-cover-climate feedbacks in Europe during prehistoric times is that of Reale and Dirmeyer (2000) in their study of the effects of vegetation on Mediterranean climate during the Roman classical period.

From the research published so far, it is clear that further work on feedbacks from changing land-cover to the climate system (see e.g. Koster et al. 2006) have important implications for interpreting past ecosystem changes and deepening our understanding of past human-environment interactions. Given sufficiently robust reconstructions of past land-cover change (see below) there should also be scope for using them to test the credibility of models used to assess the extent to which land-cover feedback will affect future climate change, especially at the regional scale.

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