"It's a bit like if you're injured at work you have lost earnings for years into the future," says co-author of new study.
The amount of planet-warming carbon emitted by the world's lost tropical forests has been under-reported as estimates failed to take into account the longer-term effects of tree destruction, researchers said.
A new international study re-evaluated the carbon impacts of forests that were destroyed or degraded between 2000 and 2013, adding up to 49 million hectares (121 million acres), roughly the size of Spain.
The carbon released from losses to those "intact forests" will amount to more than six times previous estimates when additional emissions caused by changes to the forest up to 2050 are included, it found.
Intact forests are large areas of continuous forest with no signs of intensive human activity, like agriculture or logging.
"Once you've caused the initial round of damage, you have committed to a lot of further emissions in the future once the forest has opened up," said study co-author Tom Evans of the New York-based Wildlife Conservation Society (WCS).
"It's a bit like if you're injured at work you have lost earnings for years into the future," he told the Thomson Reuters Foundation. "Those carbon lost earnings turn out to be the biggest part of the picture for these intact forests."
Trees suck carbon dioxide from the air, and store carbon, the main greenhouse gas heating up the Earth's climate. But they release it when they are cut down and are burned or rot.
Environmentalists say protecting existing forests and restoring damaged ones prevents flooding, limits climate change and protects biodiversity.
The tropics lost 12 million hectares of tree cover in 2018, the fourth-highest annual loss since records began in 2001, according to forest monitoring service Global Forest Watch.
Of greatest concern, it said, was the disappearance of 3.6 million hectares of old-growth rainforest, an area the size of Belgium, much due to fires, land-clearing for farms and mining.
Areas analysed in the new study, published in the journal Science Advances, included intact forests lost in the Amazon and Congo basins and on Southeast Asia's islands.
While previous research largely focused on stored carbon losses directly linked to deforestation, the new study also took into account the knock-on and future effects.
Those include indiscriminate logging around newly opened up forest areas as they become more accessible and a rise in hunting that limits the spread of seeds and hinders forest regeneration.
Over time, the carbon-storing abilities of the new forest edges created by deforestation also decline as human and grazing activities increase and temperatures change.
"Once you open up a forest, there are several forms of damage that lead to that dwindling of carbon stocks," said Evans, adding that undamaged forests also become more carbon-rich each year.
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These effects tend to get overlooked in carbon accounting used to work out how and where to target conservation incentives and provide payments for preventing forest loss, he added.
In 2013, the world had about 549 million hectares of intact tropical forests left, the study said.
Ensuring their future should now be a priority, with increased efforts and policies to keep them safe, Evans said.
That should include better recognition of indigenous land rights and a halt to the expansion of mining, fossil fuel extraction, agriculture and infrastructure which often drives forest loss, he added.
"Our results revealed that continued destruction of intact tropical forests is a ticking time bomb for carbon emissions," the study's lead author Sean Maxwell, a scientist with Australia's University of Queensland, said in a statement.
"There is an urgent need to safeguard these landscapes because they play an indispensable role in stabilising the climate."