A new NASA-funded study has found that a dried out Amazon, which has experienced a decline in rainfall over the last decade, could speed up global climate change due to the subsequent drop in vegetation.

And global climate models predict that things are only going to get more arid for the region in the future.

The Amazon's tropical rainforests are one of the largest sinks for atmospheric carbon dioxide (CO2) on the planet. They store an estimated 120 billion tons of Earth's carbon - that's about three times more carbon than humans release into the atmosphere each year. But without an abundance of green leafy vegetation to soak up this greenhouse gas, the consequences could be catastrophic.

"In other words, if greenness declines, this is an indication that less carbon will be removed from the atmosphere. The carbon storage of the Amazon basin is huge, and losing the ability to take up as much carbon could have global implications for climate change," lead author Thomas Hilker at Oregon State University said in a news release.

Using NASA satellites, researchers measured the "greenness" of plants and trees. They were astonished to find that rainfall had declined up to 25 percent across two thirds of the Amazon from 2000 to 2012, and consequentially the rainforest suffered a 0.8 percent decline in greenness, or vegetation. And while the lack of vegetation may seem miniscule, it affected a whopping 2.1 million square miles (5.4 million square kilometers) of the Amazon - that's over half the area of the continental United States.

In the past, scientists have had a difficult time using satellites to measure changes in vegetation greenness over the Amazon because cloud cover prevents the remote sensors from observing the surface.

"The wet season has typically 85 to over 95 percent cloudiness from late morning to early afternoon, when NASA satellites make measurements," explained co-author Alexei Lyapustin of NASA's Goddard Space Flight Center in Maryland. "Even during the dry season the average cloudiness can be on the order of 50 to 70 percent." Add in soot and other atmospheric particles and it makes for inaccurate measurements.

So this time around researchers tried a different approach. They look at the same location on Earth's surface day after day, and over time analysts were able to sift through and remove the atmospheric "noise" to pick out a stable pattern. Without the ever-changing clouds and aerosols in the way, they could better see the surface.

But what they saw was more serious than what past estimates had indicated.

"Our observations are too short to link drying to human causes," Hilker said. "But if, as global circulation models suggest, drying continues, our results provide evidence that this could degrade the Amazonian forest canopies, which would have cascading effects on global carbon and climate dynamics."

Researchers aren't exactly sure what is driving the Amazon's current drought, but they suspect changes in sea surface temperatures in the eastern Pacific Ocean, called the El Nino Southern Oscillation, have something to do with it. During warmer, dryer El Nino years, for example, the Amazon appears browner, whereas during cooler La Nina wet years, it's its typical green self.

Or, as another study suggests, the lack of the Amazon's "flying rivers" - the vapor clouds that normally bring rain to the center and south of Brazil - is to blame.

But aside from the 13-year decline in vegetation in the eastern and southeastern Amazon as a result of an incredibly long dry spell, deforestation is also contributing to the loss of carbon-storing leafy plants. The region saw a 29 percent increase in destruction last year, according to figures recently released by the Brazilian government.

And though efforts are being made to salvage this biodiverse ecosystem, this latest study indicates that in the future we may lose one of Earth's greatest combatants against climate change.

The findings were published in the journal Proceedings of the National Academy of Sciences.

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