The long hot summer: researchers are starting to make sense of a severe drought that ravaged the Amazon rainforest four years ago. Their findings are terrifying.

• Author: Brown, Paul
• Position: SPECIAL REPORT

In the summer of 2005, the Amazon rainforest suffered the worst drought it had seen in a century. Whole tributaries of the Amazon River dried up, leaving ferries stranded on dried-up mud and hundreds of thousands of dead fish in stagnant pools, robbing local people of their main source of protein. Local farmers, their river routes too parched to transport goods, watched their crops rot on the docks. So dire was the situation that in October of that year, the governor of the Brazilian state of Amazonas, in the heart of the rainforest, declared the situation a "public calamity."

But for one group of people--scientists--the crisis provided an opportunity. Although the Amazon is vital to the future health of the climate, and therefore mankind, it remains a region of unanswered scientific questions, and the drought was a chance to pry loose some of its secrets. No one was better positioned to do this than Oliver Phillips, a professor of geography at the University of Leeds in the United Kingdom. Working with sixty-five collaborators, many of them from Amazonian countries, Phillips has spent a quarter century studying how the world's largest forest reacts to changing weather patterns, a project that entails monitoring 136 sites in forty-four distinct ecosystems and collecting heaps of data on everything from tree diameter to wood density and species mix. This means he was able to bring rigorous scientific analysis to bear on a crucial question that had previously been relegated to the realm of informed speculation: How does the rainforest respond to extreme drought?

Phillips's findings, which were published earlier this year in the journal Science, are sobering. The world's forests are an enormous carbon sink, meaning they absorb massive quantities of carbon dioxide, through the processes of photosynthesis and respiration. In normal years the Amazon alone absorbs three billion tons of carbon, more than twice the quantity human beings produce by burning fossil fuels. But during the 2005 drought, this process was reversed, and the Amazon gave off two billion tons of carbon in stead, creating an additional five billion tons of heat-trapping gases in the atmosphere. That's more than the total annual emissions of Europe and Japan combined.

The drought was not evenly spread across the vast expanses of the Amazon, but in the worst affected areas there was severe dieback. Some trees stopped growing, others lost their leaves, and many of the...