As reservoirs shrink and taps run dry in Brazil's worst ever water crisis, some scientists are making a connection between Amazonian deforestation and the monster drought.
Scientists have long known that rainforests sequester atmospheric carbon dioxide, making them important players in global climate dynamics. More recently, they've learned that forests influence regional climate conditions by pumping so much water into the air that they create their own rainy weather patterns, a dynamic scientists call the "biotic pump."
That means logging the Amazon in order to raise cattle or cultivate crops might be causing southern Brazil — most significantly, the 20-million-person megacity of São Paulo — to dry out.
The biotic pump works like this: Humid air rising from forests causes air pressure to drop, sucking moisture from surrounding areas, like the ocean, into the middle of continents, which adds to the forests' humidity. In turn, greater levels of precipitation fall on areas downwind. This dynamic is something that researchers think happens not only in the Amazon, but also in the Congo River Basin and in Russia.
"The real importance of this theory is that it has huge implications for how we think of forests and the threat of losing forests," Douglas Sheil, an ecologist at the Norwegian University of Life Sciences, told VICE News.
The biotic pump theory was controversial when two Russian physicists first published the idea in 2006. It has since gained traction with other scientists, though it remains absent from current climate models, says Sheil.
While Brazil has slowed deforestation in recent years, it's still occurring on a massive scale. That could damage Brazil's biotic pump because forest loss is happening on the country's Atlantic Ocean side, where moisture is higher. If the biotic pump is already broken, that's extra bad news for São Paulo.
"The south of Brazil right now is supposed to be in its rainy season and you have some reservoirs that are down to five or 10 percent of capacity," Louis Verchot, of the Center for International Forestry Research, told VICE News. "As they move into the dry season everyone is wondering where that water is going to come from to get them from the dry season into the next rainy season."
But deforestation doesn't mean Brazil will be stuck in a drought forever — though it's likely to see more of them than in the past, said Verchot.
"This drought will end; the climate is variable," he told VICE News. "But the science is suggesting that in all probability there will be a greater frequency in drought. This isn't a unique event that won't be seen for another hundred years."
Global climate change, not only deforestation, is likely playing a roll in Brazil's drought, says Antonio Donato Nobre of Brazil's National Institute of Amazonian Research.
"We have seen some phenomenal things in the Amazon in the last few years: record-breaking droughts and floods one after the other, to a degree that has never been seen before for at least 113 years," he said during a speech in New York last fall. "In 2005 and 2010 there were record-breaking droughts, and in 2009, 2012, and 2014 there were record-breaking floods. Five extremes in 10 years — this is very unlikely to be due to natural variability, natural occurrence of extremes."
Last fall, the link between deforestation and drought received an unusual amount of attention from Brazil's leaders, when Nobre released a report summarizing scientific understanding of the roll rainforests play in regulating temperature and precipitation patterns. In it, he called for a "war effort" to restore the Amazon.
Nobre acknowledged that it's tough to sort out how much of Brazil's water problems should be blamed on deforestation, compared to global climate change. But, he added: "There is no doubt that deforestation, forest degradation, and associated impacts have already affected the climate both near and far from the Amazon."
But Brazil's politicians skeptical. In December, São Paulo governor Geraldo Alckmin said he had a hard time making the connection between drought and deforestation, since other regions of South America are also facing intense dry spells.
Verchot disagrees that the link between the rainforest and water supplies should be so easily dismissed. He argues that it's in everyone's interest to acknowledge the problem and come up with some solutions such as reducing water waste, improving water storage, and restoring damaged rainforest.
"This isn't a one-off scientific discovery that may or may not be playing around the world," he says. "This is the way forests work around the world. To ignore that, I think, is short sighted."
Follow Sarah Jane Keller on Twitter: @sjanekeller