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Unlike Arctic sea-ice, it is not possible to predict the loss of large areas of rainforest, says Dr Lewis

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Is the Amazon heading towards a 'tipping point' as a carbon sink?

Tom Levitt

27th May, 2011

The world's largest rainforest is ravaged by deforestation and two recent droughts. If they continue, says one expert, the Amazon risks entering a period where it can no longer be relied upon to absorb more greenhouse gas emissions than it produces

The Amazon rainforest is facing the combined threat of increasingly severe droughts and continuing deforestation that could wipe out large areas of the forest, warned a respected forest scientist this week.

In a groundbreaking study published in the journal Science earlier this year, Dr Simon Lewis, of Leeds University, found the 2010 drought in the Amazon was more widespread than the 2005 one, previously thought of as a once-in-a-century event.

In an interview with the Ecologist he now says if greenhouse gases are the cause of the severe droughts and such droughts are repeated three or more times a decade it could set in motion a vicious cycle by which droughts would lead to higher emissions of carbon dioxide from rotting trees and, in turn, potentially more frequent and severe droughts.

'If the climate changes in the Amazon to a regime with more severe and frequent droughts, then the dead trees may be numerous enough to cancel-out all the usual carbon uptake, and perhaps even add carbon dioxide to the atmosphere...our current emission pathways are, to be blunt, playing Russian roulette with a substantial portion of the world’s largest rainforest,' he says.

Dr Lewis says much of the forest has survived past climatic changes over millions of years, but what is different now is the interaction of other human interventions, such as deforestation, which together poses an even greater threat to the Amazon rainforest.

For example, he explained, droughts allow more forest to be burnt and cleared, while logged forest dry-out more easily making them more susceptible to fires. To make matters worse, large-scale deforestation can reduce local rainfall and potentially exacerbate future droughts.

Despite the fears over large-scale losses to the Amazon rainforest and the demise of its role as a major carbon sink, Dr Lewis says it was not possible to predict the loss of large areas of rainforest, unlike, say, the disappearance of the Arctic sea-ice.

'The Arctic sea-ice will certainly disappear, as the trend it already clear and physical mechanism – air temperature warming – will continue. So in terms of an irreversible shift in the average state, then the sea-ice is the place to watch.

'For both the Amazon and the Tundra the situation is much more complex, as they both include lots of living organisms, which adapt to changing conditions. For example, trees might die due to droughts, but more drought-tolerant species will likely take their place, so ‘living tipping points’ are much more difficult to predict than those more purely physical parts of the Earth system,' he says.

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