The New York Times reports: Like California, much of Brazil is gripped by one of the worst droughts in its history. Huge reservoirs are bone dry and water has been rationed in São Paulo, a megacity of 20 million people; in Rio; and in many other places.

Drought is usually thought of as a natural disaster beyond human control. But as researchers peer deeper into the Earth’s changing bioclimate — the vastly complex global interplay between living organisms and climatic forces — they are better appreciating the crucial role that deforestation plays.

Cutting down forests releases stored carbon dioxide, which traps heat and contributes to atmospheric warming. But forests also affect climate in other ways, by absorbing more solar energy than grasslands, for example, or releasing vast amounts of water vapor. Many experts believe that deforestation is taking place on such a large scale, especially in South America, that it has already significantly altered the world’s climate — even though its dynamics are not well understood.

“A lot of people are scrambling to make observations in the Amazon this year, with the expected big El Niño coming,” said Abigail L. S. Swann, an eco-climatologist at the University of Washington. “It’s expected to drive significant drought over the Amazon, which will change how much water trees have available.”

Humans have long settled in places where there is adequate and predictable precipitation, and large forests play a crucial role in generating dependable amounts of rainfall. Trees take up moisture from the soil and transpire it, lifting it into the atmosphere. A fully grown tree releases 1,000 liters of water vapor a day into the atmosphere: The entire Amazon rain forest sends up 20 billion tons a day. [Continue reading…]