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By Roger Highfield on

Tropical forests closer to destructive ‘tipping point’ temperature than realised

Tropical forest canopies are edging closer to a critical high-temperature threshold of no return. Roger Highfield, Science Director, reports on a study published today.

The world’s tropical forests are the Earth’s lungs, also storing  25 years’ worth of fossil fuel emissions in trees alone. But these lungs are under threat of overheating, according to a new study, because they are closer to a critical temperature than previously thought.

Beyond that temperature, which averages at about 46.7 °C, photosynthesis will falter, the process by which trees use sunlight, water, and carbon dioxide to create oxygen and energy.

Today’s study, which is the first to show how close tropical forest canopies are to the limit, also found that there were disproportionately more leaves affected as the temperature rises.

As leaves brown and die, tree growth reduces and tree death increases, so tropical forests are more likely to pass a tipping point where their reduced capacity to store carbon will release more carbon to the atmosphere to accelerate the pace of climate change.

Tropical forests represent around 12 per cent of the Earth’s land surface and previous research has shown that trees are already dying at an accelerated rate in the Amazon, compared with Central Africa, suggesting high temperatures may already be partially to blame for this increased mortality: there are growing concerns the Amazon’s extraordinary ecosystem could pass a tipping point to turn into a savannah over a matter of decades.

The new study published today in the journal Nature by Christopher Doughty, who is an Associate Professor of Ecoinformatics at Northern Arizona University, and co-authors, blends data from an orbiting instrument with measurements and experiments made at ground level to study the variations of leaf temperatures within forest canopies in detail.

It is an unusual study, Doughty remarked, because it focuses on temperature extremes and not averages, as is usually the case.

Images of tropical forests
Images of tropical forests. Source: From top left clockwise, K. Middleby, NASA, L. Patrick, C. Doughty.

By measuring individual tropical leaf temperatures, conducting leaf warming experiments in Brazil, Puerto Rico, and Australia, and drawing on data from a new NASA instrument on the International Space Station, which can accurately measure the temperature of the trees from its altitude of 250 miles, they found individual leaves in a canopy can be much hotter than the average canopy temperature.

The result is that more than 1% of the leaves in the canopy warming experiments exceeded the critical temperature threshold of 46.7 °C at least once a year, increasing the percentage of leaves that surpass this threshold currently by two orders of magnitude.

Doughty said they were surprised to find that if they warmed leaves by 2, 3, or 4 °C, the highest leaf temperatures actually increased by a massive 8 °C and they suspect that a mechanism may be at work whereby the tree closes leaf pores, stomata, to curb water loss and, as a result, its leaves overheat.

‘This shows a concerning non-linear feedback that we were not expecting,’ said Doughty, where a familiar example of such positive feedback that accelerates change is the howl that results when a microphone is brought too close to a loudspeaker.

The fact that a small percentage of tropical leaves are already reaching and occasionally exceeding the temperatures at which they can no longer function suggests that as climate change continues, entire forest canopies could perish. Overheating leaves are akin to the ‘canary in the coal mine’, said co-author Dr Josh Fisher of Chapman University, California.

The team concludes that the combination of climate change and local deforestation ‘may already be placing the hottest tropical forest regions close to, or even beyond, a critical thermal threshold.’ The Amazon, for example, ‘has shown increased mortality that may be due to this combination,’ said Doughty.

‘Breaching the thresholds for thermal viability of the tropical forest biomes, home to most of the planet’s biodiversity, could be considered a major tipping point for the Earth’s biosphere’, said Yadvinder Malhi of the University of Oxford, who also worked on the study with colleagues in America, Brazil, Panama, France, Australia, along with the universities of Plymouth and Leeds.

The die-off of tropical forests would push the planet beyond a critical climate tipping point because these forests are key regulators of the Earth’s climate, as well as being home to most of the world’s biodiversity.

The team also simulated the fate of tropical forests under future climate change scenarios. Under the most likely warming scenarios, the model predicted that tropical forests could withstand up to 3.9 °C of additional climate change-driven warming before leaves experienced a change in metabolic function and potential death.

Temperatures above 3.9 °C are expected under global climate change scenario pathways if the world does nothing to curb carbon emissions.

However, their predictions could be complicated by local weather patterns such as drought and human activities such as land-use change, and there are unknowns, such as whether tropical trees can change their upper-temperature thresholds, or the risk of ‘hydraulic failure’, when high temperatures disrupt water transport within trees.

Doughty emphasized that he remains optimistic that these worst-case scenarios are preventable because the world still has the opportunity to act. ‘Our model projections are not destiny,’ he said, adding, ‘by avoiding high-emissions pathways and deforestation, we can protect the fate of these critical realms of carbon, water, and biodiversity.’