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Top2. Projected Climate Changes
Several different changes in the climate system are expected to lead to hotter, dryer and more-variable climate in Amazonia. On a global scale, the result of increased warming from greenhouse gases will be (and already is) increased atmospheric water content and intense rainfall events (Trenberth et al., 2007, pp. 254-265). This is because, as the oceans warm, more water evaporates. However, with the exception of coastal areas, average rainfall in Amazonia is expected to be less, rather than more.
One reason for the expectation of less rain in Amazonia is the frequency of major El Niño events. These events, which are triggered by a warming of surface waters in the tropical Pacific Ocean, invariably lead to droughts in Amazonia, especially in the northern portion of the region. Roraima is the best-known location for forest fires from El Niño events, such as the Great Roraima Fire of 1997-98 (Barbosa & Fearnside, 1999).
El Niño events are already becoming more frequent. Since 1976 the frequency of these events has been greater, a statistically significant change noted by the Intergovernmental Panel on Climate Change (IPCC) since its 1995 report (Nicholls et al., 1996, p. 165). However, the IPCC was unable to reach agreement on whether this trend was caused by global warming until its last report in 2007, when a major advance was made. Examination of 21 global climate models showed that all but three of them predicted more frequent “El Niño-like conditions” with continued global warming (Meehl et al., 2007, p. 779). “El Niño-like conditions” refers to the patch of warm water in the Pacific, as distinct from El Niño itself, which refers to the pattern of droughts and floods in different places around the world. The models still do not agree on the second part – the connection between the Pacific sea-surface temperatures and the pattern of droughts and floods. However, for Amazonia, the significant question is the connection between the warm water in the Pacific and droughts in the Amazon region. The fact that the entire global pattern, with floods in Brazil’s state of Santa Catarina and droughts in Ethiopia, Indonesia and other places, is not reproduced consistently in today’s climate models does not make us any safer in Amazonia. Direct experience indicates a connection between warm water in the Pacific and El Niño droughts in Amazonia, as in the major events in 1982, 1997-98, and 2003. In other words, recognition of this connection for Amazonia does not depend on a climate model, but rather on direct observation.
It is important to recognize that different climate models show a variety of results for Amazonia. However, most of them show significantly dryer climate (Kundzewicz et al., 2007, p. 183). The model with the most catastrophic result for Amazonia is the Hadley Center model from the UK Meteorological Office (Cox et al., 2000). Unfortunately, this is also the model that achieves the best reproduction of the connection between Pacific water temperatures and Amazonian droughts (Cox et al., 2004). Although the Hadley Center model is known to exaggerate the temperature and the dryness of Amazonia’s current climate (Cândido et al., 2007), discounting the exaggeration (which is by about 30% in the case of drought) would still be far from sufficient to keep climatic parameters within the range of tolerance of Amazonian trees. The model’s predictions of massive dieoff would therefore not be altered. A recent revised version of the model indicates less dieoff than its predecessors (Gornall et al., 2011).