The world's forests take up roughly a third of the carbon dioxide emitted from burning fossil fuels each year.
But deforestation in the tropics sends about half that amount — equivalent to a sixth of global fossil fuel emissions — back into the atmosphere, reported a study by an international team of government and university researchers published Wednesday in Science.
Emissions of carbon dioxide into the atmosphere are linked to climate change, including an increase in global temperatures. Through the United Nations Framework Agreement on Climate Change and the Kyoto Protocol, many countries committed to trying to reduce emissions and climate change, and the resultant negative impacts, such as extreme weather and rising sea levels.
The recent study showed that Canada’s forests are not as good at absorbing carbon as might be expected based on the area they cover.
In fact, the study found that the amount of carbon absorbed annually by Canadian forests from 2000 to 2007 was about half the amount they absorbed annually from 1990 to 1999. That was because trees killed by forest fires and by insects such as mountain pine beetle were no longer taking in carbon.
"In short term, we don’t see impact on atmosphere," said Werner Kurz of the Canadian Forest Service, "but we know they’re a liability now. Once they’re dead, they can only decompose."
At that point, they will release their carbon back into the atmosphere.
However, deadwood and wood products can store carbon for a long time, Kurz pointed out.
Because younger forests absorb more carbon than older forests, Kurz predicts that in 20 or 30 years, when the damaged forests grow back, they will be stronger carbon sinks than existing forests.
Werner Kurz, a scientist with Natural Resources Canada's Canadian Forest Service who co-authored the paper, said the amount of carbon dioxide being absorbed by forests is "good news" and reinforces what scientists had previously estimated — that forests are the biggest carbon sinks among land ecosystems.
"Right now, forests are helping," he said, "but whether or not they will continue to help in the future will depend on the effect of human activities and climate change on the forest."
The amount of carbon taken in by the forests, 2.4 billion tonnes a year, is almost identical to the amount that scientists had previously estimated are taken up by all land ecosystems.
Kurz said that suggests other ecosystems, such as prairies, wetlands and tundra, don't make a net contribution to the carbon dioxide in the atmosphere. While individual components may be carbon sources or sinks, they all add up to zero.
"It reinforces that of the land ecosystems, it is the forests that are currently the biggest sinks," he said. "If they stop that uptake, then the rate of increase of CO2 in the atmosphere will greatly increase."
Deforestation a huge emitter
The paper represents the first time scientists have tried to calculate the amount of carbon taken up by land ecosystems, Kurz said. The discovery that deforestation releases 1.1 billion tonnes of carbon back into the atmosphere each year is also significant, he added, as under recent UNFCCC agreements, the world is about to spend "enormous amounts of money" to reduce emissions from deforestation.
"This study reaffirms why it's important," Kurz said.
In order to make their calculations, the researchers gathered data between 1990 and 2007 from sources around the world such as the Canadian national forest carbon monitoring, accounting and reporting system, which is headed by Kurz. The system combines data such as logging records, measurements of trees and forest litter, remote sensing data on wildfires and insects, and computer simulations that estimate the effects of carbon-emitting events such as forest fires and carbon-removing events such as forest growth.
It provides that information on Canada's 230 million hectares of "managed" forests to the United Nations each year as part of annual greenhouse gas reporting under the UNFCCC. The data is also used by researchers around the world.
Kurz said most of the sources for data used by the researchers were government forest services and universities, and the study shows how Canadian government science has contributed to the understanding of how the Earth works.
The study did not include Canada's 118 million hectares of northern, "unmanaged" forests, but Kurz said they wouldn't affect the data for the study very much because they make up such a small percentage of the world's four billion hectares of forest.