Future projections of changes in precipitation

For the future, the different IPCC models for the A2 scenario (Hulme and Sheard, 1999; Marengo and Soares, 2003) show negative rainfall anomalies in most of the Amazon Basin (up to 20% reduction), while for the lower emission scenario (B2) this reduction was 5%. In addition to these emission scenarios, there are climate change scenarios for the 21st century: new developments in dynamic vegetation schemes and coupled climate-carbon models (Cox et al., 2000, 2004; Betts et al., 2004) have shown an effect—named "die-back of the Amazon forest"—by means of which rising atmospheric CO2 is found to contribute to a 20% rainfall reduction and to more than a 30% increase in surface temperatures in the Amazon Basin, through physiological forcing of stomatal closure. They also show an increase in rainfall in southern Brazil and northern Argentina. These projections for drought in Amazonia after 2040 also show systematic warming in the tropical Pacific indicative of an El Nino-like mode of variability becoming persistent after 2040. However, the likelihood of this extended El Nino or more frequent/intense El Nino mode scenarios in a global warming world is still an open issue. Furthermore, these studies do not give any information on the likelihood of change in extreme rainfall events.

If we consider the climate change scenarios discussed by Marengo and Soares (2003) and the model results from Cox et al. (2004) and Betts et al. (2004), increase in the concentration of greenhouse gases in the atmosphere may result in increased warming and rainfall reduction rates in Amazonia, implying more intense respiration and the closing of stomata, leading to the decline of Amazon tropical forests. These projections also suggest that vegetation will be more susceptible to fires due to the drying of Amazonia. Figure 9.6 shows the observed increase in rainfall in southern Amazonia contrasting with the slight decrease of rainfall in northern Amazonia. Systematic increases in rainfall in southern Amazonia and southeastern South America, as detected by Obregon and Nobre (2003), are consistent with a tendency for more intense/frequent transport of moisture from Amazonia to the La Plata Basin by the South American Low-Level Jet (SALLJ) east of the Andes (Marengo et al., 2004c). Thus, it could be hypothesized that—following Betts' results from the HadCM3 model after the mid-2050s—the drying of the Amazon Basin and humidification of the southern Brazil and northern Argentina region in an extended El Nino mode predicted by this model could be explained by changes in regional circulation, with an increase in SALLJ frequency and/or intensity in a global warming world. However, not much can be said regarding changes in extreme rainfall events.

A reduction in rainfall will also have impacts on river and water levels, as well as on rainfall distribution in them, with longer dry periods and intense rainfall events concentrated on a few days. However, the uncertainty level is still high since these experiences were obtained from a model—in this case, the HadCM3 model—and other models are being tested now to see if they also simulate Amazon die-back. It is clear that models must improve or include the representation of natural processes, such as clouds and the impacts of aerosol.

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