Forests are chock-full of carbon. Some three-quarters of the stuff on the Earth's surface lies trapped in leaves, branches, stems and roots. Two to three times more is buried in the soil but it is hard to dislodge
Vegetal carbon, by contrast, is released into the atmosphere whenever woods are engulfed by fire, pests or tree-uprooting winds. Or humans: some experts reckon that deforestation accounts for as much as 17% of global manmade emissions. Others, though, put the figure at as little as 6%.
The discrepancy arises because the data for exactly how much carbon is stored in forests is inconclusive. The generally accepted measure, the United Nations' "Global Forest Resources Assessment", is calculated from figures about forest size reported by individual countries. But some countries do not undertake such inventories. Much of the data from those that do is outdated. And the original purpose of the inventories—to estimate how much timber could be commercially useful—means that not all types of wood are tallied. It is hard putting a number on the amount of carbon stored in forests when nobody is quite sure how much forest there is.
A new study from the Woods Hole Research Centre uses remote-sensing technology (along with old-fashioned fieldwork) to measure more accurately how much carbon is stored in tropical forests. Researchers trudge into a forest to measure trees in a 500 square metre plot. This data is used to calibrate a technique called light detection and ranging (lidar). Like its cousins radar and sonar, lidar, works by broadcasting electromagnetic waves towards a target and then building up a picture from the reflection. In the case of lidar, the waves are in the form of an infra-red laser beam sent from a satellite. As a result, fewer boots on the ground are needed.
The researchers put the total amount of carbon in tropical forests at 229 billion tonnes. This is 21% higher than UN's latest estimates. They also calculated that the net emission of carbon into the atmosphere thanks to tropical deforestation from 2001-10 amounts to 1 billion tonnes per year, 10% less than previously thought.
Crucially, the data allowed the researchers to chart tropical regions' carbon density in unprecendented detail on a large-scale map: each pixel corresponds to a square patch 500 metres on each side. It offers a picture of how carbon is distributed not just across different countries, but also across different types of flora. For example, nearly half of tropical Africa’s carbon turns out to be locked up in non-forest vegetation like shrublands or savannah.
These findings ought to help tropical countries to meet the reporting requirements set out in the UN Framework Convention on Climate Change. More importantly, they may have far-reaching consequences for efforts like a scheme under which developing countries would be paid not to cut down trees. The latest findings bolster the idea that this scheme, known as "Reducing Emissions from Deforestation and Forest Degradation", or REDD, should cover not just forests but other plant patches, too.