We know that global warming is a reality and that we humans are its primary cause. And we know that carbon dioxide emissions, in large part from burning fossil fuels, are one of the biggest contributors to global warming. But we still have much to learn about the Earth's mechanisms when it comes to regulating emissions and warming.

Forests -- along with grasslands, soils, and other ecosystems -- are an important part of the equation, and a new report published in the journal Nature sheds a bit more light on their role. We've known for a long time that forests are important carbon sinks. That is, they absorb carbon from the atmosphere thus preventing it from contributing to global warming.

But the Nature study shows that tropical forests absorb more carbon than we realized. Researchers from a number of institutions, including the University of Toronto, analyzed data from 79 intact forests in Africa from 1968 to 2007, along with similar data from 156 intact forests from 20 non-African countries. They concluded that tropical forests absorb about 4.8 billion tonnes of carbon a year, equivalent to about 18 per cent of the carbon dioxide added to the atmosphere each year by burning fossil fuels. The world's oceans are the other major carbon sink, absorbing about half the human-produced carbon that doesn't end up in the atmosphere.

That doesn't mean we can count on the forests or the oceans to save us from our folly. To start, about 15 billion of the 32 billion tonnes of carbon dioxide that humans produce is not reabsorbed on land or sea and ends up in the atmosphere. And the carbon stored in forests can be released back into the atmosphere with natural disturbances, such as fire or insect outbreaks, or if the forest is logged. This is because when trees are cut down, die, and decay naturally, or burn, some of the stored carbon is released back into the atmosphere. Even if carbon remains stored in the form of wood products taken from a logged forest, some is still released when soils in the forest floor are disturbed by logging. And many wood and pulp and paper products are discarded and destroyed in a much shorter time period than the life of an old-growth forest. This means that the carbon is released earlier than it would have been if the forests were left intact.

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