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Carbon dioxide helps tropical rainforests grow

Tropical forests are growing faster than scientists thought due to rising levels of carbon dioxide in the atmosphere.

A Nasa-led study has found that tropical forests are absorbing 1.5 billion tons of carbon dioxide every year as they photosynthesise and grow. And this is far more than is absorbed by the vast areas of boreal forest that encircle the Arctic.

The researchers claim their findings show that rainforests like the Amazon are essential for soaking up excess greenhouse gases, and play a far greater role than had been previously realised.

Deforestation in the world's biggest rainforest, the Amazon, which extends over 6,1 million square kilometers with more than 60 percent within Brazil's borders, dropped by 18 per cent over the past year. Rain forest clearing is responsible for about 75 percent of Brazil's emissions, as vegetation is burned and felled trees rot.

However, scientist warn that deforestation in tropical rainforests could exacerbate climate change by leaving more carbon dioxide in the atmosphere. In total, they estimate that forests and other vegetation absorb around 2.7 billion tonnes of carbon dioxide, about 30 per cent of that emitted by humans.

As emissions add more carbon dioxide to the atmosphere, forests worldwide are using it to grow faster. However, the rate at which they absorb this has been hard to estimate with many studies producing contradictory results.

As many rainforests consist of mature trees that are often hundreds of years old, they were not thought to absorb much carbon dioxide. Young fast growing trees tend to absorb more carbon dioxide as they use the carbon as they grow.

Global air flows and data on deforestation also suggested tropical forests were releasing more carbon dioxide than they absorb. But this new study suggests the tropical forests are using far more of the carbon, and so growing far faster than previously believed.