For the past few weeks, the world has watched in horror as massive fires engulf large sections of the Amazon. If forest ecologists are correct, the fires may set off a feedback loop that threatens the very existence of the Amazon rainforest itself.

The risks start far from the Amazon. As Daniel Nepstad et al. outline in a 2008 paper, demand for agricultural products keeps growing in places like the United States, Europe, and China. The Amazon has the potential to provide a lot of space for agriculture, once the trees are removed. Through the decades the forest has been cleared for corn, soybeans, cattle, and sugar, not to mention timber and mining.

All this deforestation takes place against the backdrop of climate change, which increases the frequency and intensity of drought in the region. Rainforests are fairly drought-resistant. Deep roots tap underground moisture, and the thick canopy of leaves and densely-packed trees helps maintain shade and ground moisture. When drought gets severe enough, according to Nepstad et al., a threshold is reached where trees begin to die. Leaves fall, allowing more sunlight to penetrate the canopy, drying out and warming the forest floor, and increasing the growth of grasses. Fragmentation of the forest with roads exacerbates the problem by increasing edges where light can penetrate from the side. Trees on forest edges are also more vulnerable to other dangers.

As drought stresses increases, the thinned forest, speckled with gaps and dead trees, becomes more vulnerable to fire. Fires rarely start naturally in the Amazon. As with the current fires, they are frequently set deliberately. Farmers, ranchers, and others often burn fields to clear out brush and fallen logs. These fires spread into the drought-stressed forest and can kill huge numbers of trees. The invasive grasses provide additional fuel, and can prevent trees from growing back. As more forest burns, more grasses grow, drought stress expands in the weakened forest, and what is left becomes even more susceptible to fire. The cleared land is suited for agriculture and unlikely to be reforested.

It gets worse. There is some evidence that smoke from the fires may directly inhibit rainfall, further perpetuating the fires. Evaporation from the wet forest creates rain over the Amazon. As the forest decreases, so does the available rainfall. If enough forest is lost (from all sources combined)—conservatively estimated at 30%—the loss in rainfall will become self-perpetuating and the conversion of forest into savannah becomes inevitable. Nepstad at al. believe this “tipping point” may occur as early as 2100.

Unfortunately, the forces that Nepstad et al. hoped would slow this process, e.g. protected areas or consumer choices, have not been effective. The march toward the tipping point has accelerated, and the Amazon as we know it may be gone within decades, with global consequences.

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Philosophical Transactions: Biological Sciences, Vol. 363, No. 1498, Climate Change and the Fate of the Amazon (May 27, 2008), pp. 1737-1746
Royal Society