A NASA-led research team has created a new map using ground and satellite data that accurately quantifies the amount and location of carbon stored in Earth's tropical trees and forests. Based on data from the early 2000s, the map focuses on 2.5 million hectares of tropical forest in seventy-five countries. Data shows that tropical forests contain 247 billion tons of carbon, and of this carbon stock, almost half is held in Latin American forests. Almost the same carbon stock is stored in sub-Saharan Africa in its entirety, compared with 61 billion tons of carbon stored in Brazilian forests alone.

"These patterns of carbon storage, which we really didn't know before, depend on climate, soil, topography and the history of human or natural disturbance of the forests," said leading researcher Sassan Saatchi of NASA's Jet Propulsion Laboratory in Pasadena, California. "Areas often impacted by disturbance, human or natural, have lower carbon storage."

Up to a fifth of global carbon emissions are caused by deforestation and forest degradation, mainly in tropical areas. Carbon is stored in the wood and roots, but when trees are cut, burned or decompose, carbon is released into the atmosphere. Previous studies have only taken carbon measurements from individual countries or continents, and usually by calculating carbon from ground-based observations of tree heights. This is the first time researchers have achieved a pan-continental overview, across varying forest types and structures using the same methodology.

NASA's map shows carbon storage in tropical forests in 75m countries, and the level of certainty (Image: NASA)

The map was created using data from the Geoscience Laser Altimeter System lidar on NASA's ICESat satellite. The satellite identified tree heights using three million measurements which were then combined with ground data to calculate carbon of above-ground biomass. This calculation was then extrapolated across the various study landscapes and a visual map created from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra spacecraft, the QuikScat scatterometer satellite and the Shuttle Radar Topography Mission.

The map will provide a baseline for comparison in future carbon monitoring, and also show the forests' health and longevity. Each forest's contribution to the global carbon cycle can be calculated, and, compared with satellite observations of deforestation, sources of carbon dioxide can be identified. It can also help countries planning to participate in the United Nations-led international effort Reducing Emissions from Deforestation and Degradation (REDD+) which aims to quantify the financial value of forest-stored carbon.

"This is a benchmark map that can be used as a basis for comparison in the future when the forest cover and its carbon stock change," said Saatchi. "The map shows not only the amount of carbon stored in the forest, but also the accuracy of the estimate."

The study was published May 30th in the Proceedings of the National Academy of Sciences.

