ATMOSPHERIC SCIENCE LETTERS Atmos. Sci. Let. 12: 261–267 (2011) Published online 14 February 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/asl.329 Concurrent climate impacts of tropical South America land-cover change Marina Hirota, 1 * Marcos Daisuke Oyama 2 and Carlos Nobre 1 1 Center for Earth System Science, National Institute for Space Research (INPE), Cachoeira Paulista, SP, Brazil 2 Atmospheric Sciences Division, Institute for Aeronautics and Space (IAE), S˜ ao Jos´ e dos Campos, SP, Brazil *Correspondence to: Marina Hirota, Center for Earth System Science (CCST/INPE), Rod. Presidente Dutra, km 40, 12630-000 Cachoeira Paulista – SP, Brazil. E-mail: marina.hirota@inpe.br Received: 3 May 2010 Revised: 16 August 2010 Accepted: 23 December 2010 Abstract The climatic effects of concurrent land-cover changes in Amazonia and Northeast Brazil (NEB) were evaluated by simulations using the Center for Weather Forecasting and Climate Studies Atmospheric General Circulation Model (CPTEC-AGCM). Three experiments were performed: Amazon savannization, NEB desertification and both land-cover changes occurring concurrently. We found that land-cover change from adjacent areas do affect both Amazon and NEB regions and that the negative precipitation anomaly in NEB due to concurrent land-cover changes in Amazon and NEB is weaker than the linear addition of the anomalies considering the land-cover changes separately (synergistic behaviour). A simple mechanism was proposed to explain this behaviour. Copyright  2011 Royal Meteorological Society Supporting information may be found in the online version of this article. Keywords: land-cover change; Amazonia; Northeast Brazil; biomes; AGCM simulation 1. Introduction In the past three centuries, the replacement of natural ecosystems by croplands has been very significant. Particularly, two regions in tropical South America have been undergoing remarkable land-cover changes: Amazonia (AMZ), where current deforestation rates are close to 12 000 km 2 per year (2007–2008 mean value; http://www.obt.inpe.br/prodes), and Northeast Brazil (NEB), where environmental degradation pro- cesses have already affected large portions of the dry shrubland natural vegetation, also known as caatinga (Ministerio do Meio Ambiente, 2007, p. 27). Accord- ing to future scenarios for 2050, about 40% of AMZ area would be deforested (Soares-Filho et al., 2006) and more than half of NEB area would be highly degraded (Gon¸ calves, 2007, see the Supplementary material for an overview of the main goals of this study). Climate sensitivity to land-cover changes in AMZ and NEB has been evaluated by many studies. For AMZ, climatic impacts due to forest savannization (SAV; e.g. Dickinson and Henderson-Sellers, 1988; Nobre et al., 1991; Henderson-Sellers et al., 1993) or large-scale change from tropical forest to pasture (e.g. Costa et al., 2007; Sampaio et al., 2007) have been assessed; for NEB, replacement of caatinga by semi- desert (Dirmeyer and Shukla, 1996) or desert (Oyama and Nobre, 2004) has also been evaluated. For both regions, the majority of studies found a hydrological cycle weakening, i.e. precipitation, evapotranspiration and moisture convergence decrease in response to land-cover changes. However, an aspect not considered by the aforemen- tioned studies is that land-cover changes in AMZ and NEB have been occurring (and are expected to keep on occurring) concurrently. Climatic impacts due to land- cover changes in a region may teleconnect to other parts of the world (Werth and Avissar, 2002). There- fore, the climatic impacts over AMZ, for example, could be caused not only by local land-cover changes such as deforestation (local effect) but also by the effects of land-cover changes from other regions (non- local effect), such as from NEB desertification ((DES) Oyama and Nobre, 2004, p. 3207). One of the simplest procedures to assess the total or net climatic impact consists of linearly adding the local to all nonlocal effects (e.g. Stein and Alpert, 1993). This is analogous to linearly adding the effects of indi- vidual forcings to evaluate the net effect. For instance, Gillett et al. (2004) found that the combined climate effects due to different radiative forcings could be linearly added as both greenhouse gases, and the direct effect of sulfate aerosols are known to directly change the surface albedo and therefore the mean annual sur- face temperature. However, linear addition may not represent the net climatic effects completely; in this case, another term, which we call hereafter the syn- ergy term, would be necessary to include the impacts of nonlinear interactions. Let  I ,J be the climatic effect on a region J , J = AMZ, NEB, due to land-cover change(s) represented by I . The index I may refer to Amazon SAV, NEB DES, the linear sum of individual land-cover changes (SAV + DES) or the combined climatic effects (SD). Copyright  2011 Royal Meteorological Society