Act Now To Save Tropical Forests From Climate Change Die-Offs, Researchers Warn

Leaves in tropical forest canopies may be closer to reaching critical temperature thresholds than previously thought. Data from new satellite imaging technology, confirmed using additional ground measurements, appear to suggest that a small fraction of leaves are already beginning to reach the temperatures at which photosynthesis begins to breakdown.

The research is published in Nature.

Climate change and its impact on the tropics

July 2023 was hottest month on record globally, approximately 0.24 °C hotter than any other July since records began in 1880. According to analysis by NASA, parts of South America, North Africa, North America, and the Antarctic Peninsula were especially hot – recording temperature increases around 4 °C more than average.

High temperatures are a concern for many reasons.  In tropical regions where mean temperatures are already very high with only minor variation, significant temperature increases could disrupt the optimum conditions that plants have grown accustomed to.

Above a certain critical temperature (T_crit), a plant’s stomata – tiny pores that are vital for photosynthesis – will begin to close up. This reduces the plant’s transpirational cooling ability, resulting in extensive leaf damage and the possible heat death of the entire plant.

Current heat tolerance estimates suggest an average T_crit of 46.7 °C for tropical forest tree species. Tropical forests are a vitally important carbon sink and host a significant proportion of the world’s biodiversity, which is why the prospect of tree death in this region raises such alarm bells.

So, how close are our forests to reaching this threshold? Previously, data of this nature was particularly hard to come by, as measurements were slow and difficult, though the advent of affordable drone technologies did provide some breakthroughs. Now, the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) sensor hosted on the International Space Station can provide researchers with insights more easily than ever before.

Satellite data

In this new study, researchers used high-resolution ECOSTRESS data from forest canopies in Brazil, Puerto Rico and Australia, and leaf warming experiments from the Amazon basin, Central Africa and Southeast Asia to build a comprehensive picture of current tropical forest leaf temperatures.

The ECOSTRESS data showed that during hot periods with low soil moisture content, canopy temperatures in tropical forests average around 34 °C, with some pixels in the temperature measurement data exceeding 40 °C. Currently, 0.01% of upper canopy leaves have been measured exceeding T_crit at least once per season.

In the leaf warming experiments, the researchers were surprised to find that when leaves were warmed by even just a few degrees, the highest leaf temperatures recorded jumped by around eight degrees. This kind of non-linear feedback is a concern, as it would mean that leaf temperatures could exceed critical levels even if environmental conditions only warm by a couple of degrees.

Leaves are approaching critical temperatures under global warming

Using the leaf warming and satellite datasets, the researchers also constructed a predictive model capable of simulating what might happen to tropical forests under different future climate change scenarios.

The model showed that tropical forests could likely withstand up to 3.9 °C of additional climate change-driven warming before a significant number of leaves would exceed their critical temperatures and fall into dysfunction.

“If tropical forest air temperatures increase 4 °C above current temperatures, we could start to see massive leaf mortality and possible tree mortality in many tropical tree species,” said lead author Christopher Doughty, associate professor of ecoinformatics at Northern Arizona University. “This may shift the composition of the forest towards more heat tolerant tree species or to shrubs/grasses. This would likely reduce the tropical forest carbon sink accelerating climate change and putting many animal species at extinction risk.”

To enable the study of future scenarios, the Intergovernmental Panel on Climate Change has developed a set of standard Representative Concentration Pathways (RCPs) that describe 4 different potential warming scenarios through the 21st century. Temperature shifts of above 3.9 °C are expected within the “worst-case scenario” estimates in RCP 8.5, where no efforts are made to reduce emissions and so they continue to rise.

Based on their new model projections – which are still somewhat limited by variability in weather patterns such as drought and unforeseen changes in human activity in the region – the researchers say that action must be taken now in order to save the tropical forests. If emission reduction projects are successful and the world ends up following a path closer to RCP 4.5, a scenario in which renewable power is more widely adopted and carbon emissions begin to fall by the mid-century, such a disaster could be avoided.

“By enforcing moderate climate mitigation strategies and reduced deforestation, we could prevent this worst-case scenario,” said Doughty.

Reference: Doughty CE, Keany JM, Wiebe BC, et al. Tropical forests are approaching critical temperature thresholds. Nature. 2023:1-7. doi: 10.1038/s41586-023-06391-z

Christopher Doughty was speaking to Alexander Beadle, Science Writer and Editor for Technology Networks.