The undisturbed tropical forests of Africa—like the rainforest in Congo—remove 1.2 billion metric tons of carbon dioxide from the atmosphere every year. For the last few decades, that rate of removal has increased by 0.6 metric tons per hectare per year—simply because the trees are getting bigger and bigger in size, according to an analysis published in Nature today.

That's good news for those who are looking for ways to sequester carbon, which many say is necessary to curb global warming. "We are receiving a free subsidy from nature," said study co-author Simon Lewis of the University of Leeds, in a statement. He and his co-authors measured the girths of tropical trees in Africa—and hence CO₂ absorption.

All told, the world's tropical forests absorb some 4.8 billion metric tons, or 18 percent of manmade CO₂ from fossil fuel burning and only a little less than the entire emissions of the U.S. In effect, tropical trees are getting bigger globally, perhaps because the CO₂ is acting to promote growth as has been shown in other studies.

Based on current prices for metric tons of CO₂, all that avoided greenhouse gas is worth some $18 billion, according to Lee White, chief climate change scientist of Gabon and a co-author on the research.

Some policymakers and scientists have argued that countries with tropical forests should be paid to preserve them. That could somehow translate into healthier economies in those countries, and jobs to replace those previously created by cutting timber or clearing forest.

Ultimately, however, the trees can't do that much more than they already are to save us. "Even if we preserve all remaining tropical forest," Lewis said, "these trees will not continue getting bigger indefinitely."

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