Hurricane Katrina, Amazonian Drought Caused by Warmer Atlantic

Warmer Atlantic waters spawned more severe storms in the Caribbean in 2005, including Hurricane Katrina, and had an unexpected impact on the world’s largest tropical rain forest: drought.

The milder waters in the second-biggest ocean caused such arid conditions in the western and southern parts of the Amazon that younger trees died and growth in older ones slowed. That then turned a rain forest that normally absorbs 500 million tons of carbon dioxide a year into a net emitter of the greenhouse gas blamed for global warming, scientists said.

The Amazonian forest, in one dry year, released 900 million tons of CO2, more than reversing its usual role in climate change dynamics, ecologist Oliver Phillips of the University of Leeds said in an interview. There were few visible indications of the drought and the results run counter to the notion that intact forests are vast absorbers of CO2 at all times, he said.

“It didn’t take much for the Amazon to become a source of carbon emissions rather than a sink,” Phillips said. “This shows that it doesn’t take a catastrophic change of climate for the forest to become a net contributor of emissions.”

The results of the study of the impact of the drought on the Brazilian forest by a team that included geographers, biologists and climatologists appeared in the latest edition of Science. The scientists are examining whether the trees quickly returned to net absorbers of carbon after the drought.

Congo, Amazon Basins

“More than half the species on Earth and at least 2 billion people depend directly on tropical forests for survival,” said Sandy Andelman, co-author of the study titled “Drought Sensitivity of the Amazon Rain Forest.”

The “great remaining forests” of Africa’s Congo basin and the Amazon play a vital role in climate regulation by absorbing and storing huge amounts of atmospheric carbon, Andelman said. “Now the study reveals that increasing drought due to global climate change can cause potentially irreparable damage to the Amazon jungle and its ability to function as a carbon ‘sink.”

The sensitivity of forests to fluctuations in climate may harm efforts to reduce carbon emissions. Destruction of forests by logging and fires to clear land for farming contributes a fifth of annual CO2 emissions while at the same time the intact trees consume almost as much in the process of photosynthesis.

With the world’s forests shrinking daily by an area the size of Washington, D.C., envoys at ongoing UN-led climate- change talks are looking at ways to save tropical forests and include their carbon absorption in a new global-warming treaty to replace the Kyoto Protocol. Proposals include providing money or carbon credits for countries with large tracks of wooded areas such as Brazil and Indonesia for keeping the trees intact.

‘Call for Caution’

“There is a lot of uncertainty about how forests are adapting,” said David Huberman, an expert on conservation incentives at the International Union for the Conservation of Nature in Gland, Switzerland. “This study would be a call for caution. We don’t know how forests will react to climate change and conservation is clearly a solution.”

The findings by Phillips and 67 co-authors at 40 institutions come two weeks after University of Leeds researcher Simon Lewis said tropical rain forests will likely slow their absorption of carbon dioxide in the coming decades due to aging.

Older trees will probably stop growing for the same reasons they did in 2005 after Katrina devastated New Orleans -- warmer temperatures and less available water, the study showed.

Phillips and the other researchers examined more than 100 plots throughout the Amazon and measured 100,000 trees and recorded tree deaths. They also noted changes in weather.

Forests are under threat by logging as well. The world loses about 50,000 square kilometers (19,305 square miles) of primary forests annually, mostly in tropical regions of South America, Africa and Asia, the UN’s Environment Program has said.