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Scientists find climate change tipping point for tropical forests

By Tess de la Mare
Tropical forests can still act as effective carbon sponges in a warmer world, research across three continents has found, but only if nations act quickly to cut greenhouse gas emissions.

A team of researchers coordinated by the University of Leeds found rainforests can continue to absorb huge volumes of carbon if global warming remains less than 2C (35.6F) above pre-industrial levels.

Increases above a threshold of 32C (89.6F) average daytime temperatures during the warmest month of the year was the point at which tropical forests’ ability to store carbon starts to diminish.

Currently 25% of tropical rainforests are above this 32C threshold, and store less carbon than their cooler counterparts.

Lead author Dr Martin Sullivan told the PA news agency that under the a 2C scenario of global warming, tropical forests will be 2.4C (36.3F) hotter than today due to the fact some regions warm faster than others.

This would push three-quarters of tropical forests above the 32C “safety zone”, and begin the rapid release of carbon back into the atmosphere.

The world’s tropical forests store an estimated 25 years worth of fossil fuel emissions in their trees.

Every degree of further warming above the 32C threshold releases four times as much carbon as would have been released compared to below this point, the research found.

Fragmentation of these forests through fire and logging could also impede tree species’ ability to adapt to a changing world, even if warming is kept below 2C.

Dr Sullivan, from the University of Leeds and Manchester Metropolitan University, said: “Our analysis reveals that up to a certain point of heating tropical forests are surprisingly resistant to small temperature differences.

“If we limit climate change they can continue to store a large amount of carbon in a warmer world.

“The 32C threshold highlights the critical importance of urgently cutting our emissions to avoid pushing too many forests beyond the safety zone.”

An international team of 225 researchers measured the height and diameter of trees in sample plots in 813 forests across the tropics to calculate how much carbon they stored.

Over the course of the study, nearly two million measurements were taken from 10,000 species of tree in 24 countries across South America, Africa and Asia.

Data gathered on some of the plots dates back to the early 1960s.

The sites were revisited every few years to measure how much carbon was being absorbed and how long it was stored before the tree died.

Co-author Professor Beatriz Marimon, from the State University of Mato Grosso in Brazil, said: “Our results suggest that intact forests are able to withstand some climate change.

“Yet these heat-tolerant trees also face immediate threats from fire and fragmentation.”

“Achieving climate adaptation means first of all protecting and connecting the forests that remain.”

The research suggested that in the long term, rising temperature has the greatest negative effect on forest carbon stocks by reducing growth, with drought killing off trees the second biggest factor.

South America’s forests are projected to see the greatest long-term reduction in carbon stocks, as baseline temperatures are highest in the region, and future warming predicted also to be highest.

Professor Oliver Phillips, of the University of Leeds, urged world leaders to take the opportunity offered by the current shutdown to transition towards a stable climate.

“Imagine if we take this chance to reset how we treat our Earth. We can keep our home cool enough to protect these magnificent forests – and keep all of us safer,” he said.

The paper, Long-Term Thermal Sensitivity Of Earth’s Tropical Forests, was published in the journal Science.

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