Research warns two degree rise will halve rainforest "carbon sink"

The impact of global warming on tropical rainforests will be so severe that even increases in temperature that are widely regarded as "safe" could raise tree mortality rates to such a level that almost 50 billion tons of carbon dioxide is released into the atmosphere.

That is the sobering warning contained in new research from a team of Australian scientists, which suggests that even a two degree increase in average global temperatures will see the "carbon sink" effect currently provided by the world's rainforests cut in half.

It also calculates that should temperatures reach four degrees above pre-industrial levels, the rate of forest die-off will reach a level where rainforests become a net contributor to carbon dioxide in the atmosphere, potentially triggering runaway climate change.

The research, which was funded by the Australian government's Marine and Tropical Sciences Research Facility, is based on data from 116 plots of rainforests around the tropics.

Echoing the findings of an international study recently published in Nature, the new research found that individual trees would be stimulated by increased concentrations of CO2. But it warned that this effect would be more than offset by the increase in tree mortality resulting from higher temperatures.

David Hilbert, principal research scientist at the CSIRO Tropical Forest Research Centre in Australia, who worked on the study, said that the data suggested that the previous hope that the rates at which forests absorb CO2 would increase as concentrations of CO2 in the atmosphere increase are overly optimistic.

"The significant thing about this [research] is that we expect to see a pattern of reduced uptake of carbon in rainforest around the tropics," he said.

The researchers calculated that for each degree Celsius global temperatures rise, the rainforests will shrink at such a rate that 24.5 billion tons of carbon is released to the atmosphere. In comparison, man-made emissions of greenhouse gases in 2007 reached a peak of 10 billion tons CO2 equivalent.

The findings are being released just a week ahead of a landmark scientific conference in Copenhagen where the world's leading climate scientists are to provide political and business leaders with an update on the latest global warming research, and recommendations on how to mitigate the risks presented by climate change.