Until recently, Brazil stood out as a hopeful outlier in the plague of deforestation. Between 1990 and 2010 clearing of tropical forests increased 62 percent worldwide, but in Brazil, such destruction plummeted from 2004 to 2011, in part because of tough environmental regulations and a ban on the sale of soybeans grown on rain-forest-cleared land. Since August 2014, however, tree cutting more than doubled in the country compared with the same period a year earlier, according to a satellite analysis released this spring by the independent research institution Imazon.

The report may signal a new round of challenges facing the world's largest rain forest. Most of the land cleared in the uptick will serve as cattle pasture, spurred by higher global prices for beef. (Cutting the forest for ranches is the largest driver of deforestation in the Brazilian Amazon, accounting for nearly 70 percent of clearing.) And Brazil's recently reelected president, Dilma Rousseff, has called for several new hydroelectric dams and a major highway that, if built, will slice through the pristine heart of the Amazon. Her administration also supports legislation that weakens environmental protections and offers amnesty to those who illegally cut down trees, citing the need for economic growth.

A 2014 study by Brazil's National Institute for Space Research found that deforestation, especially extensive cutting along the southern edge of Amazonia, has decreased the movement of atmospheric moisture to the south. Climate scientists at the institute say the change is a possible factor in a severe drought that has necessitated rationing of water in Brazil's largest metropolis, São Paulo. And if clearing of the Amazon continues, says Phillip Fearnside, a biologist at Brazil's Amazon research institute INPA, “you will end up with a permanent drought, not just a one-year thing.”

Tree loss in the Amazon reverberates beyond Brazil's boundaries. It reshuffles the climate deck for the entire Western Hemisphere: the rain forest pumps 20 billion tons of water vapor daily into the atmosphere through leaf transpiration, an influx that has ripple effects in weather systems a continent away. The Amazon is currently nearly 20 percent deforested, which may be close to a tipping point in terms of its ability to maintain the climate system and rains that it helps to support, says pioneering Amazon researcher Thomas Lovejoy. A perfect storm of deforestation, fire and climate change, he fears, could potentially transform vast swaths of the southern and eastern Amazon into savanna.

One 2013 study, for example, predicts that a fully deforested Amazon would mean 50 percent less snowfall in California's Sierra Nevada, quashing spring runoff vital to the region's agriculture. (Whether the present level of deforestation factors in the current West Coast drought is unknown.) To avoid further damage, many players will need to come together, but Brazil now appears to be moving in the opposite direction.