In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño-related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°-5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia.

[1] We studied the consequences of projected climate change on biome distribution in South America in the 21st century by forcing a potential vegetation model with climate scenarios from 15 climate models for two emission scenarios (A2 and B1). This analysis was carried out for the savanna and tropical forest biomes, which are the predominant biomes in tropical South America. In both scenarios, the results indicate reduction of tropical forest cover areas which would be replaced by savannas. This reduction of tropical forests increases with the time through the end of the 21st century, mostly over southeastern Amazonia. Considering the biome changes from current potential vegetation in the case when at least 75% of the calculations agree on the projected biome change (consensus), the decrease of the tropical forest area in South America is 3% for the period 2020-2029, 9% for 2050 -2059 and 18% for 2090-2099 for the A2 emission scenario.

The existence of multiple climate‐vegetation equilibria in Tropical South America is investigated under present‐day climate conditions with the use of an atmospheric general circulation model coupled to a potential vegetation model. Two stable equilibria were found. One corresponds to the current biome distribution. The second is a new equilibrium state: savannas replace eastern Amazonian forests and a semi‐desert area appears in the driest portion of Northeast Brazil. If sustainable development and conservation policies were not able to halt the increasing environmental degradation in those areas, then land use changes could, per se, tip the climate‐vegetation system towards this new alternative drier stable equilibrium state, with savannization of parts of Amazonia and desertification of the driest area of Northeast Brazil, and with potential adverse impacts on the rich species diversity in the former region and water resources in the latter.

In 2005, southwestern Amazonia experienced the effects of an intense drought that affected life and biodiversity. Several major tributaries as well as parts of the main river itself contained only a fraction of their normal volumes of water, and lakes were drying up. The consequences for local people, animals and the forest itself are impossible to estimate now, but they are likely to be serious. The analyses indicate that the drought was manifested as weak peak river season during autumn to winter as a consequence of a weak summertime season in southwestern Amazonia; the winter season was also accompanied by rainfall that sometimes reached 25% of the climatic value, being anomalously warm and dry and helping in the propagation of fires. Analyses of climatic and hydrological records in Amazonia suggest a broad consensus that the 2005 drought was linked not to El Niñ o as with most previous droughts in the Amazon, but to warming sea surface temperatures in the tropical North Atlantic Ocean.

[1] Tropical South America vegetation cover projections for the end of the century differ considerably depending on climate scenario and also on how physiological processes are considered in vegetation models. In this paper we use a potential vegetation model to analyze biome distribution in tropical South America under a range of climate projections and a range of estimates about the effects of increased atmospheric CO 2 . We show that if the CO 2 ''fertilization effect'' indeed takes place and is maintained in the long term in tropical forests, then it will avoid biome shifts in Amazonia in most of the climate scenarios, even if the effect of CO 2 fertilization is halved. However, if CO 2 fertilization does not play any important role on tropical forests in the future or if dry season is longer than 4 months (projected by 2/14 GCMs), then there is replacement of large portions of Amazonia by tropical savanna.