The rain forest of the Amazon accounts for roughly 15 percent of the Earth's terrestrial photosynthesis, and there is concern that anthropogenic global warming could lead to reduced rainfall and a "die-back" of what is arguably the world's greatest forest. Such a die-back could have serious implications for the biodiversity of the Amazon, and it could also lead to a substantial increase in global airborne carbon dioxide. But whether or not such die-back will occur is highly uncertain, and climate models show a wide range of projections for global warming-induced rainfall change over the Amazon. This project will address the question of future rainfall change by examing Amazon rainfall variability and associated dynamical mechanisms over the period of available observations, and determining the extent to which these mechanisms are correctly represented in climate models. The work has three main objectives: 1) to determine observationally whether or not rainfall seasonality and drought severity have changed significantly over the Amazon during the past few decades and, if so, what mechanisms have caused these changes; 2) to determine whether the observed rainfall changes are mainly attributable to natural climate variability or to forcing by anthropogenic emissions; 3) to validate the physical processes that control rainfall seasonality and drought severity in climate models, in order to reduce uncertainty in projections of future climate change in the Amazon. The work is based on the hypothesis that drought severity has increased over the past few decades due to more late onsets and early endings of the rainy season. A further hypothesis is that wet season onsets have been occurring later and wet season terminations earlier in recent decades over the southern Amazon due to warming of the tropical esastern Pacific and northern tropical Atlantic. Delays of the wet season onset are also hypothesized to occur as the result of a polward shift of the Southern Hemisphere subtropical jets, and because of agricultural burning.
Aside from the benefits of the research for understanding and anticipating drought in the Amazon, the project will support education and the advancement of science education in the United States and in countries of the Amazon region. The present work will 1) support climate research and education programs in Amazonian countries by training PhD students, especially women from Amazonian countries, and collaborating with newly formed climate programs in that region; 2) support outstanding minority high school students from rural Texas and undergraduate students in climate science; 3) communicate research results and general climate science to the public.
