Global Warming May Threaten Photosynthesis in Tropical Forests

On Wednesday, the University of Arizona published in Nature a study showing that the average critical temperature from which the photosynthetic machinery of tropical trees begins to fail is approximately 46.7 °C.

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Using empirical data modeling, the study estimates that 0.01 percent of all leaves on top of trees in the tropical forests studied exceed that critical temperature at least once per season.

Models suggest that tropical forests can withstand an increase of up to 3.9 degrees in air temperature, before reaching a possible tipping point in their metabolic function, which would imply large-scale leaf loss. However, there are uncertainties about the range of potentially critical temperatures in all tropical trees.

That temperature rise is within the worst-case scenario of climate crisis predictions, but the authors believe ambitious climate change mitigation targets and reduced deforestation are needed to help forests stay below thermally critical thresholds.

Tropical forests, which are critical carbon stores and home to most of the planet's biodiversity, can be especially sensitive to rising temperatures.

The research team led by Christopher Doughty used high-resolution measurements of global-scale land surface temperatures to estimate maximum canopy temperatures for tropical forests. The scientists also used data from Ecostress, a system aboard the International Space Station.

In this way, they discovered that the maximum temperatures of the treetops during dry periods were around 34 degrees on average, although a small proportion of those observed exceeded 40 degrees and 0.01 percent of the leaves of the upper cups exceed the temperature at which they begin to fail.

Various heating experiments suggest that 1.4 percent of the leaves in the upper tree canopy will exceed the critical temperature under future heating conditions.