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Climate change a ‘tipping point’ for tropical rainforests

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Tropical forests will be resilient to global warming, but only if nations act quickly to cut greenhouse gas emissions, new research suggests.

A team of researchers, coordinated by the University of Leeds, found that rainforests can continue to absorb huge volumes of carbon if global warming remains less than 2°C above pre-industrial levels, as specified by the 2015 Paris Agreement.

Increases above a threshold of 32°C average daytime temperature during the warmest month of the year was found to be the point at which tropical forests’ ability to store carbon starts to diminish. However, 25 per cent of tropical rainforests are currently above this 32°C threshold and store less carbon than their cooler counterparts.

Dr Martin Sullivan, University of Leeds, said that under a 2°C scenario of global warming, tropical forests will be 2.4°C hotter than today due to the fact that some regions warm faster than others. This would push three-quarters of tropical forests above the 32°C “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. However, the research found every degree of further warming above the 32°C threshold releases four times as much carbon as would have been released compared to below this point.

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 2°C, the study disclosed.

“Our analysis reveals that up to a certain point of heating, tropical forests are surprisingly resistant to small temperature differences,” said Sullivan. “The 2°C above pre-industrial levels 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, while other 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.

University of Leeds of scientists measuring a giant Ceiba in the Choco rainforest, Colombia

University of Leeds of scientists measuring a giant Ceiba in the Choco rainforest, Colombia.

Image credit: Pauline Kindler/Col-Tree

“Our results suggest that intact forests are able to withstand some climate change,” said Beatriz Marimon, a professor from the State University of Mato Grosso in Brazil. “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, the rising temperature has the greatest negative effect on forest carbon stocks by reducing growth, with drought killing off trees being 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 also predicted to be highest.

Professor Oliver Phillips, also 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.

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