Tropical forests may be absorbing far more carbon dioxide in response to rising atmospheric levels of the greenhouse gas than many scientists had thought, a new NASA-led study has found.
The study estimates that tropical forests absorb 1.4 billion metric tonnes of carbon dioxide out of a total global absorption of 2.5 billion - more than is absorbed by forests in Canada, Siberia and other northern regions, called boreal forests.
"This is good news, because uptake in boreal forests is already slowing, while tropical forests may continue to take up carbon for many years," said David Schimel of NASA's Jet Propulsion Laboratory, Pasadena, California, lead author of a paper on the new research.
Forests and other land vegetation currently remove up to 30 per cent of human carbon dioxide emissions from the atmosphere during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up in return.
The new study is the first to devise a way to make apples-to-apples comparisons of carbon dioxide estimates from many sources at different scales: computer models of ecosystem processes, atmospheric models run backward in time to deduce the sources of today's concentrations (called inverse models), satellite images and data from experimental forest plots.
The researchers obtained their new estimate of the tropical carbon absorption from the models they determined to be the most trusted and verified.
"Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry and modelling communities," said co-author Joshua Fisher of JPL.
The question of which type of forest is the bigger carbon absorber "is not just an accounting curiosity," said co-author Britton Stephens of the National Centre for Atmospheric Research, Boulder, Colorado.
"It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change," Stephens said.
As human-caused emissions add more carbon dioxide to the atmosphere, forests worldwide are using it to grow faster, reducing the amount that stays airborne. This effect is called carbon fertilisation.
"All else being equal, the effect is stronger at higher temperatures, meaning it will be higher in the tropics than in the boreal forests," Schimel said.
Tropical rainforests experience high average temperatures and a significant amount of rainfall. This ecosystem can be found in Asia, Australia, Africa, South America, Central America, Mexico and on many of the Pacific, Caribbean, and Indian Ocean islands.
The research appears in the journal PNAS.
