Positive Feedback in the Amazon
One of the most alarming aspects of climate change is the existence of positive feedback loops. For example, as polar ice melts, less sunlight is reflected back into space, thus heating up the ocean and causing more ice to melt. Rinse and repeat. Another one: warming causes the permafrost in the Siberian tundra to melt, releasing CO2 into the atmosphere, thus warming the earth and causing yet more tundra to melt.
Here's still another, from the latest issue of the Washington Monthly. Oliver Phillips, a professor of geography at the University of Leeds, has studied a 2005 drought in the Amazon rainforest and come to a frightening conclusion:
In normal years the Amazon alone absorbs three billion tons of carbon....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. That’s more than the total annual emissions of Europe and Japan combined.
....Significantly, Phillips  found that the 2005 drought was not the result of El Niño, the cause of previous smaller episodes, but of a regional rise in sea temperatures — one 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 rapidly — a dynamic with harrowing implications.
Read the whole thing for more. The Monthly's entire special package on tropical deforestation is here.