The cutting of trees, scientists say, is hindering the immense jungle's ability to absorb carbon from the air — and to pull enough water through tree roots to supply gigantic "sky rivers" that move more moisture than the Amazon river itself. More than two-thirds of the rain in southeastern Brazil, home to 40 percent of its population, comes from these sky rivers, studies estimate. When they dry up, drought follows, scientists believe.

It's not just Brazil but South America as a whole for which these rivers in the sky play a pivotal meteorological role, according to a recent study by a top Brazilian climate scientist, Antonio Nobre of the government's Center for Earth System Science.