Counting carbon in the Amazon

Published online on October 21, 2009, in Nature.

If the next climate treaty tackles deforestation, tropical nations will need to monitor the biomass of their forests. One ecologist has worked out a way to do that from the sky, finds Jeff Tollefson.

Greg Asner peers out an open window, taking stock of the jungle as the single-engine prop plane chugs over a pair of scarlet macaws gliding among the treetops 120 metres below. The Peruvian Amazon stretches in all directions, painted in countless shades of green, accented here and there by patches of purple, pink and yellow. Occasionally, naked white trunks rise amid the leaves, a reminder that even the rainforest has deciduous tendencies.

Forty-five minutes into the flight, Asner spots his quarry: narrow red trails, barely visible, then a fallen tree in the middle of an otherwise intact canopy. The cause isn’t immediately clear to the untrained eye, but Asner knows all too well. “When trees die in the tropics, they don’t just fall over,” he says as the plane passes over more downed trees, a road, then a small clearing that contains stacked logs and a bulldozer. It is a legal concession, authorized by the Peruvian government to extract just three species of hardwood trees. As the plane veers away from the clearing, Asner gives his verdict. “The biomass levels are going to be a lot lower here,” he says, “but it really is low-impact logging compared with the mayhem of Brazil.”

As a tropical ecologist with the Carnegie Institution for Science’s global ecology department in Stanford, California, Asner has developed a keen ability to interpret the rainforest from great heights. Frequently operating with oxygen masks at high altitude, his team uses a powerful laser system to map trees and calculate the biomass of the forest. Satellites extend his view across the tropics, and he has developed automated software that can track annual changes in forest cover and calculate the biomass of the vegetation. The system can even spot small logging operations like the one he just passed, which escape detection in most satellite studies.

The fully integrated system is designed to measure the amount of carbon locked up in forests and to track changes over time — an exercise that may become a crucial foundation of the new climate treaty that global leaders are hoping to sign at the United Nations Climate Change Conference in Copenhagen this December. Tropical deforestation accounts for up to 20% of the carbon dioxide emitted by humanity each year and there is broad agreement on the need to include a forest-protection element in the new treaty. This component — known as REDD, for Reducing Emissions from Deforestation and Forest Degradation — would allow developed nations to meet their required emissions-reduction targets in part by paying tropical countries to preserve their forests, which keeps carbon in trees and out of the atmosphere.

But first, tropical nations must determine how much carbon is in the forest, a notoriously difficult task. A leading researcher on remote sensing in the tropics, Asner is out to prove that developing countries can quickly and cheaply perform their own analyses, then move on to long-term carbon monitoring. He has come to Peru to demonstrate the technology and he will present the results of this proof-of-concept test at a REDD meeting he is planning to coincide with the negotiations in Copenhagen. Success in Peru, he hopes, will bolster efforts to include a strong forest carbon component in the agreement.

The European Union last year called for a halving of deforestation by 2020, a goal that has since picked up political momentum. Estimates range widely, but reaching that target could mean pumping some US$20 billion into tropical countries each year, according to the Union of Concerned Scientists in Cambridge, Massachusetts. Venture capitalists see profits in forest carbon and have approached Asner with business offers. Instead he has licensed his technology, dubbed CLASLite, for Carnegie Landsat Analysis System Lite, and is providing it for free to governments and others. Starting in Latin America, his team is training scientists, officials and advocacy groups on how to use the software. He likes to say that he is putting himself and other scientists out of business by injecting a decade’s worth of work into the public sphere.

All this effort in testing and training has meant devoting less time to academic research, but Asner says that his work on REDD has been nothing short of rejuvenating. “This is more fun than anything I’ve ever done as a scientist,” he says. “These forests are really special. We need to get these people some cash and protect them.”

Sweeping the forest

For Asner’s crew in Peru, work starts early each morning. Today, a three-man team heads to a small airport in the southeastern part of the country before sunrise in an effort to get as much mapping done as possible before the jungle pumps enough water into the air to form midday clouds. Ty Kennedy-Bowdoin runs the Light Detection and Ranging system, or LIDAR, which sweeps a laser back and forth, blasting the forest with 70,000 laser pulses per second. A sensor continuously records the signals as they bounce off leaves, branches and other objects. These data enable the researchers to calculate the height, structure and density of the forest, and they use this information to determine how much biomass it holds.

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