Want to save the planet? Plant a tree.

Or maybe a lot of them. Or maybe don't cut down so many.

These are the implications of a new study, which found that the world's forests play an unexpectedly large role in climate change, vacuuming up the greenhouse gas carbon dioxide (CO2) and storing the carbon in wood, according to research published online Thursday by the journal Science.

That, in turn, helps regulate CO2 concentrations in the atmosphere – and keeps the planet from overheating.

About one-quarter of the earth's land surface is covered by forest. But while scientists and schoolchildren have long known that trees absorb carbon dioxide, no one was sure how significant their role was, overall. Oceans, the atmosphere, and other terrestrial ecosystems also absorb carbon.

So how much is due to forests? Forests are incredibly diverse across different regions – tropical, boreal, temperate – and different conditions: growing fast, being cut back, dying off, or being replanted. Researchers have struggled to get a complete picture of how much impact forests alone had on climate. Until now.

Earth's forests, it turns out, play a dominant role in absorbing greenhouse gases in the atmosphere, acting like a giant sponge and soaking up on average about 8.8 billion tons of carbon dioxide each year, the new study led by the US Forest Service shows – or about one-third of fossil fuel emissions annually during the 1990-2007 study period. In the end, about 2.4 billion tons of solid carbon were locked away in wood fiber each year over that period – a surprise to scientists.

“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 Yude Pan, a US Forest Service scientist and a lead author of the study, in a statement. That finding could have big implications for national forest policies worldwide, implying that as forests go, so too does the planet.

Tropical forests are critical. Tropical forests untouched by deforestation absorb huge amounts of carbon, more than all other northern hemisphere forests combined, the study found. Yet scientists also discovered a surprisingly large amount of carbon (1.6 billion tons per year) was absorbed by re-growth of tropical forests recovering from deforestation and logging, which partly compensates for the large amount of carbon emitted into the atmosphere from tropical slash-and-burn deforestation.

But with that see-saw battle going on in the tropics, the result was that overall, tropical forests' impact on atmospheric carbon dioxide was a wash - deforestation emitted about the same amount that was captured in forest growth.

"Our estimates suggest that, currently, the global established forests which are outside the [tropics] alone can account for the terrestrial carbon sink," the study found.

So where are the forests regrowing? It turns out Siberia's massive boreal forest has been growing back, filling in areas cut down for agriculture under the old Soviet Union. At the same time, beetle damage and drought have devastated Canada's boreal forests, causing it to slip into being a net emitter of carbon, as decaying wood releases carbon to the atmosphere.

Temperate forests in the US and Europe, on the other hand, have been regrowing across areas once cleared for agriculture. These temperate forests are helping to tip the balance in the right direction, absorbing about one-third of the total contribution by forests globally – about 800 million tons of carbon annually during the study period.

In the US, for instance, the fast regrowth of temperate forest land has increased its carbon uptake by nearly a third since 1990, the study found. China has seen a similar rate of increase, as massive tree-planting programs have accelerated the rate of CO2 absorption by that nation's forests.

But it’s too simplistic to rely on forests to continue removing carbon from the atmosphere, the study says, noting that forest carbon sequestration is reversible if drought, wildfire and insect damage increase. Further study is key, researchers say.

Breaking the data down by region, “we were able to explain – in much greater detail than before – what's happening with the earth's forests," says Richard Birdsey, a study co-author and program manager for climate change research at the US Forest Service.

"Before, we knew that carbon dioxide was being absorbed on the land, but we didn't know for sure just why,” says Dr. Birdsey. “The story is a lot clearer now."