NASA scientists have mapped how green the Earth is. Their map shows how much vegetation covers the planet鈥檚 surface, and how much has disappeared in recent years due to wildfires, logging and crop failures.
The map also gives the first global glimpse of how much carbon these environmental disturbances release into the atmosphere. By comparing this figure with the carbon released from the fossil fuels we burn, scientists will be able to fine-tune their estimates of how human activity is driving global warming.
Vast areas of the Earth, particularly in the tropics and tundra, are too inaccessible to be mapped from the ground. So to create their map, a team from American universities led by Chris Potter of NASA Ames Research Center, reviewed vast amounts of data gathered since 1982 by satellites used by the National Oceanic and Atmospheric Administration. From records of radiation reflected from the Earth鈥檚 surface, the researchers calculated the extent of green leaf cover around the world, after making allowances for the changing seasons.
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The researchers found that, on average, 142.5 million square kilometres are covered by plants. Between 1982 and 1999 around 3 per cent of that has been disturbed by ecological events such as fire and storms.
This destruction of plant cover releases carbon from the vegetation and soil into the atmosphere. 鈥淧revious calculations of the impact of disturbances have been guesswork,鈥� says Potter. The map revealed that between 1982 and 1999 the environment released 9 billion tonnes of carbon into the atmosphere, slightly more than the 7 billion tonnes of carbon emitted in 1990 alone by the burning of fossil fuels. 鈥淗uman industrial activity is still a much worse emitter,鈥� Potter says. 鈥淏ut over time, ecological disturbances do contribute a significant amount.鈥�
Potter鈥檚 analysis reveals some surprises. Huge wildfires in the boreal forests of Siberia, southern Russia and China were even larger than previously thought. His analysis also picked up the massive crop failures in India linked to serious drought in 1986-87 (Global Change Biology, vol 9, p 1005)
These figures will allow researchers to model the carbon cycle more accurately, says Ramakrishna Nemani, a specialist in remote sensing at the University of Montana. 鈥淲ildfires are the second largest atmospheric carbon contributor, after fossil fuels. Up until now they were the missing link,鈥� he says.
The study could also lead to changes in the way each nation鈥檚 emissions budget is determined under the Kyoto Protocol, says Nemani. For example, if Brazil is planting trees to get carbon credits but also having lots of fires, there would be no net emission reduction, he says.
But Rob Jackson, an ecologist at Duke University in North Carolina, warns that Potter鈥檚 analysis does not tell the whole story. 鈥淪mall fires are taking place all over the world, all the time, and taken together, may have more of an impact than major disturbances,鈥� he says.
Potter鈥檚 team is planning to address this issue by looking at the data on a much smaller scale of about 1 square kilometre. 鈥淓very gram of carbon counts,鈥� he says.