"This is really a timely breakthrough with which we can now clearly demonstrate how forests and changes in landscape such as wildfire or forest regrowth impact the removal or release of atmospheric carbon dioxide (CO2)," says Dr Pep Canadell, CSIRO co-author of the paper: A Large and Persistent Carbon Sink in the World's Forests. "What this research tells us is that forests play a much larger role as carbon sinks as a result of tree growth and forest expansion."
“The new information suggests forests alone account for the most significant terrestrial carbon sink, and that non-forest lands collectively cannot be considered a major carbon absorption sink,” said Dr. Yude Pan, a U.S. Forest Service scientist and a lead author of the study.
In 2009 a total of 8.4 billion tonnes of carbon was emitted by humans to the atmosphere - about 30.8 billion tonnes CO2. (1 tonne of carbon is equivalent to 3.67 tonnes of CO2). Each year about 26 percent of CO2 emissions are absorbed by the world's oceans making them progressively more acidic (See Ocean acidification); 47 percent accumulate in the atmosphere; and 27 percent are absorbed in the world's forests and to a much lesser extent agricultural land.
The study used on-the-ground measurements of trees worldwide and statistical models to provide an updated picture of atmospheric carbon uptake and loss by boreal, temperate and tropical forests spanning a total of 3.9 billion hectares. Dr Pan said, “By breaking the large-scale carbon dynamics of forests into their many components, we were able to separately analyse the large magnitudes of carbon fluxes, both sinks and sources, and gain an idea of the potential capacity for carbon sequestration by the world’s forests.”
The study also confirms the importance of tropical forests as carbon sinks, separating out the carbon emissions from tropical deforestation and the carbon sink provided by tropical regrowth. Deforestation is still a huge problem in many tropical countries including Indonesia and Brazil. In Indonesia deforestation, illegal logging of tropical peatland forests and palm plantation establishment have been the subject of protests in 2009 by indigenous communities and Greenpeace.
“By separating them out, we see that carbon emissions from tropical deforestation are much larger than we thought, while uptake from forest re-growth in the tropics is also probably very large,” said Dr Simon Lewis, a tropical ecologist from the University of Leeds and co-author of the study.
“Humans are altering the world’s forests in a number of ways, from their outright destruction to the much more subtle impacts on even the most remote forests caused by global changes to the environment." said Dr Lewis. “Our research shows these changes are having globally important impacts, which highlights the critical role forests play in the global cycling of carbon and therefore the speed and severity of future climate change."
Carbon sequestration is also reversible through increased drought, wildfire and forest degradation. As the climate warms forests will be more susceptible to drought and wildfire which may turn them from net carbon sinks to net carbon sources of emissions accelerating climate change.
“The practical importance of this new information is that if schemes to reduce deforestation are successful they would have significant positive global impacts, as would similar efforts promoting forest restoration.” said Dr Lewis. This will be a major consideration for carbon offset schemes such as the United Nations-backed Reduced Emissions from Deforestation and Degradation (REDD) scheme. But the REDD scheme has also come under fire for it's narrow focus on carbon storage.
"We estimate that tropical forest regrowth is removing an average of 1.6 billion tonnes of carbon per year. Unfortunately, some countries have not looked on forest regrowth as a component of REDD, and so are missing a very important opportunity to gain even further climate benefits from the conservation of forests. Combining the uptake by established and forest re-growth plus emissions from deforestation, the world’s forests have a net effect on atmospheric CO2 equivalent to the removal of 1.1 billion tonnes of carbon every year. Carbon exchanges from tropical forests have the highest uncertainties in this analysis and this research has required a concerted effort to refine them to our best knowledge," said CSIRO forests researcher Dr Canadell.
Study co-author Oliver Phillips, Professor of Tropical Ecology at the University of Leeds, emphasised the need for more research, especially in the tropics, “We know the tropics are the most dynamic area of the world when considering the exchange of carbon between the land and atmosphere. Trees grow fast in the tropics, and widespread deforestation is the norm, yet our collective research effort is smaller in the tropics than elsewhere. What we need is serious investment in monitoring the world’s tropical forests to better understand their role in our rapidly changing global environment.”
Dr Lewis provided a word of warning for continuing to rely on the carbon sinks of the world's forests and oceans in place of mitigation action in reducing greenhouse gas emissions. “Forest management can help, but reducing fossil fuel emissions is essential. And, of course, forests won’t keep removing CO2 from the atmosphere forever. The world’s remaining forests are providing a time-limited benefit to humanity. We can't rely on forest management alone to halt the increase of CO2 in the atmosphere and solve the problem of climate change. There is simply not enough land on Earth to store all the carbon released from ongoing fossil fuel emissions in trees."