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 peoplescientiststhe 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 worlds 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?

Phillipss findings, which were published earlier this year in the journal
Science, are sobering. The worlds 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 instead, creating an additional five billion tons of heat-trapping gases in the atmosphere. Thats 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 fastest-growing trees and creepers died altogether. Perhaps more surprising, comparing exact measurements of tree diameter, wood density, and biomass against measurements taken in earlier years, Phillips and his colleagues found that even in places that seemed to emerge relatively unscathedwhere the forest looked no different to the naked eyethere had been a loss of biomass. Rainforests, it seems, are more sensitive to drought than was previously understood.

Significantly, Phillips also found that the 2005 drought was not the result of El Nio, the cause of previous smaller episodes, but of a regional rise in sea temperaturesone of the expected early signs of global warming. Taken together, these findings suggest that climate change could trigger the worst kind of vicious cycle, with climbing temperatures causing the rainforests to dry out and give off massive quantities of greenhouse gases, which in turn causes the planet to warm more rapidlya dynamic with harrowing implications. An article last year in Proceedings of the National Academy of Science identified rainforest-wide dieback in the Amazon, along with the melting of the Arctic sea ice, as among the nine most crucial “tipping points” that must be staved off to prevent catastrophic climate change.

As if thats not enough bad news, new research presented in March at a conference organized by the University of Copenhagen, with the support of the Intergovernmental Panel on Climate Change, says that as much as 85 percent of the Amazon forests will be lost if the temperature in the region increases by just 7.2 degrees Fahrenheit. To keep from hitting that mark, we will have to curb global carbon emissions by at least 80 percent. At the same meeting, Vicky Pope, a researcher from Britains Met Office (a government agency that tracks climate and weather data) showed that a temperature increase of 2.2 degrees above current levels would trigger a 20 to 40 percent Amazon die-off within 100 years. A rise of 5.4 degrees would kill 75 percent of the trees. “The forest as we know it would effectively be gone,” she says. Other Met researchers have found that the loss of the forest would be irreversible.

Nor is the Amazon the only forest at risk. In African countries like Congo, where Phillips is conducting research with African scientists, forests “also suffer from attack around the fringes,” he says. “Parts are so dry they are in danger of degenerating into savannah.”

The good news, Phillips says, is that much of the damage has yet to be done. “About four-fifths of Amazonia is still mature forest,” he explains. “It is the biggest wilderness left on the planet. There is a lot to fight for; there is a lot to save. I am not disheartened.” But if his and others predictions are reliableand the careful measurement of the 2005 Amazon drought certainly lends them some credencethere is a real risk of drought accelerating climate change to a point where it would be beyond our ability to control it. All the more reason, then, to take action now to prevent that from happening.

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