GREENBELT, Md., Dec. 11 (UPI) -- As revealed by a newly concluded 13-year NASA study, vegetation in the Amazon is becoming smaller and less green as precipitation totals have tapered. The research, showcased in the latest issue of PNAS, features a new technique for analyzing vegetation health via satellite imagery.

The technique involves the observation of a forest from low Earth orbit -- measuring the change in "greenness" of plants and trees over time. While a separate study measured a 25 percent decline in rainfall across two-thirds of Amazon between 2000 and 2012, the new NASA analysis calculated a corresponding 0.8 percent decline in greenness.

The decline may seem small in comparison, but the nearly one-percent de-greening effect can be seen across a swath of rainforest stretching 2.1 million square miles.

That's not just bad news for the astronauts aboard the International Space Station who enjoy the Amazon's rich hues; it's also worrisome for those who are hoping South America's massive rainforest can be a strong ally in the fight against climate change. The massive canopy, most of which lies in Brazil, acts as an important heat sink -- capturing excess greenhouse gases, the active ingredient in global warming.

"In other words, if greenness declines, this is an indication that less carbon will be removed from the atmosphere," lead study author Thomas Hilker, a remote sensing specialist at Oregon State University, explained in a press release. "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."

The study's new greenness-measuring technique utilizes a new algorithm that enables NASA's satellites to better pick out the gaps in the clouds and pull data about vegetation health. The new strategy cancels out atmospheric noise caused by clouds and aerosols and allows the satellite to hone in on ground-level information.

"We're much more confident that this is a gap between clouds where we can measure greenness, but standard algorithms would call it a cloud," explained Alexei Lyapustin of NASA's Goddard Space Flight Center. "We can get more data about the surface, and we can start seeing more subtle changes."

The research also helped tease out the relationship between greenness, precipitation and sunlight. Which is most important to vegetation health and greenness? The answer is both. On the one hand, more sunlight improves greenness; on the other, trees and plants without an adequate stockpile of water can't take full advantage of a longer dry (sun-filled) season.

"With our work," Hiker said, "we have shown that there is a dry season greening but that under extended drought we get a decline in vegetation greenness."

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NASA study: Vegetation decline seen in wake of drying Amazon