Working with data from 116 plots of rain forests around the tropics, researchers from Australia’s CSIRO Climate Adaptation Flagship and Sustainable Ecosystems, as well as CSIRO’s Global Carbon Project, have found that increased levels of atmospheric carbon dioxide is killing trees at a faster rate than it is stimulating growth in the trees that survive warmer climes.
Tropical rain forests have so far absorbed an estimated one billion tons of carbon a year from CO2 in the atmosphere, and “hopes have been high,” according to the researchers, that the forests would soak up even more in the future as the amount of CO2 in the atmosphere grows. However, they found that the opposite is happening: although individual tree growth is stimulated by increased atmospheric CO2, the warmer climate that comes with increased greenhouse gases is causing more trees to die.
This increased mortality in rain forests trees is projected to release 24.5 billion tons of carbon to the atmosphere over time for every degree Celsius the temperature rises. Man-made emissions of greenhouse gases reached a peak of 10 billion tons CO2 equivalent in 2007.
“The significant thing about this is that we expect to see a pattern of reduced uptake of carbon in rain forest around the tropics,” says David Hilbert, Principal Research Scientist at CSIRO Tropical Forest Research Centre, Queensland Australia. The research indicates that even if warming can be contained within the two degrees that is popularly cited as relatively safe, the world’s rain forests will nevertheless lose almost 50 billion tons of carbon to the atmosphere, in effect cutting its mitigation efficiency in half. The authors of the paper further estimate that a warming of 4 or more degrees Celsius (7.2 ºF) may increase tropical rain forest tree mortality enough to shift the forests from carbon sinks to carbon sources.
The paper will be presented in full at the International Alliance of Research Universities (IARU) International Scientific Congress on Climate Change in Copenhagen next week. The congress was created in response to an acceleration of key climate change indicators subsequent to the publishing of the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change. Work has begun on the Fifth Assessment Report, scheduled to be published in 2013.