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Water Currents: Deforestation Reduces Hydropower and May Dry Out the Amazon

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Last week, scientists published a study in the journal PNAS that warned that deforestation in the Amazon could significantly decrease the power output of hydroelectric dams, which are a major source of energy in the region.

The study noted that although removal of trees tends to increase the amount of water that runs off the land, and therefore swells rivers, that short-term boost is outweighed by another force. Tropical trees transpire a large amount of moisture through their leaves, much of which eventually coalesces back into rain clouds, and then eventually returns to earth.

By cutting down trees, people disrupt that cycle, and that may result in less water in river systems. Less water means less power to turn turbines and generate electricity.

Deforestation scars a hill in Rio de Janeiro

Deforestation scars a hill in Rio de Janeiro. Photograph by Alex Rio Brazil, Wikimedia Commons

Sandra Postel, National Geographic’s Freshwater Fellow, told us that deforestation could eventually “cause the Amazon to dry out.”

In an email, Postel wrote, “The water cycle in the Amazon is in large part driven by the recycling of moisture from the eastern Amazon inland (toward the interior), and with less rainfall and less forest to recycle water to the interior, the Amazon could dry out.”

To find out more, Water Currents spoke with Thomas Lovejoy, a leading tropical ecologist and the first Biodiversity Chair of the H. John Heinz III Center for Science, Economics and the Environment. Lovejoy is also a professor of environmental science and policy at George Mason University and a fellow at National Geographic (see video of Lovejoy accepting the 2012 Blue Planet Prize).

The study published in PNAS found evidence that removing tropical forest cover results in less rainfall, because trees transpire so much moisture, which forms rain clouds. Is this consistent with what you know about the ecosystem?

Thomas Lovejoy: The particular paper is part of a story that goes back to the 1970s, when a Brazilian scientist named Eneas Salati proved definitively that the Amazon makes something like half of its own rainfall. He looked at the isotope ratios of oxygen in rainwater from the Atlantic coast to the Peruvian border, and it showed that there was water recycling going on.

Until that paper the dogma was that the vegetation you find anywhere in the world is a consequence of the climate but has no effect on the climate, but that just busted that old paradigm.

Inherent in all of this is the implication that if you eat away at the forest it will eat away at the hydrologic cycle, and at a certain point it won’t be able to support rainforest. [Last week's] paper had some particular examples of reduced rainfall.

What becomes even more interesting and worrisome is how it ties in with climate change. Brazil’s best climate scientist, Carlos Nobre, did some work for the World Bank looking at the dieback issue and found there is likely to be a tipping point at 20% deforestation. So this paper is in one sense old wine in a new bottle, but it is also new and fresh detail and has very specific implications for the energy sector.

Is the Brazilian government concerned about climate change?

Yes. I know the Brazilian government is actively considering reforestation, in part because the land has been so hammered with cattle ranching, and also because it builds back a margin of safety around that hydrological cycle.

Some scientists point out that reforestation doesn’t always work as well as intended, because the new growth isn’t the same ecosystem as what was lost. Is that a concern here?

Sure reforestation won’t be exactly what was there before, but it will be a functioning tropical forest that will feed the hydrological cycle. As one example, there’s a huge national park in Rio de Janeiro that is mostly reforested land, and it provides many benefits, so you can do it.

The recent PNAS study warns that by 2050 as much as 40 percent of Brazil’s hydropower could be lost because of the reduced rainfall caused by regional deforestation. How worried should governments be?

I think they are pretty close to the tipping point unless they really engage in integrated planning and reforestation. The solutions are there, they have to face up to the fact and get on with it. It’s always easier said than done of course.

Could this prediction help motivate people to conserve more forest, perhaps engaging business and other stakeholders, such as energy companies?

Yes absolutely, I mean the energy interests have a serious vested interest in maintenance of the hydrological cycle. So whether it is individual companies, or since most of those hydroprojects in Brazil are run by the government power agency, that agency (Electrobras) has a vested interest in maintaining the water flow.

Should farmers be concerned as well?

All the soybean farmers just south of the Amazon depend to a fair degree on rain that comes from the Amazon, so if they keep eating away at the Amazon to grow more soybeans all of a sudden they won’t be able to grow them at all. What this comes down to is the Amazon has to be managed as a system.

I know Brazil’s minister of the environment is really interested in getting integrated planning going. At the moment the transportation sector plans its roads, the energy sector plans power separately, agriculture wants to plan their activities separately, and so on, but it all has to be integrated to manage the Amazon as a system.


Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for TheDailyGreen.com and E/The Environmental Magazine, and has written for Popular Science, TheAtlantic.com, FastCompany.com, PopularMechanics.com, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVACGreen LightingBuild Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.

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