August 23, 2023

expert reaction to study suggesting a small percentage of leaves on trees in tropical forests may be approaching the maximum temperature for photosynthesis to work

A study published in Nature suggests that tropical forests are approaching critical temperature thresholds.

Dr Chloe Brimicombe, Climate Scientist and Extreme Heat Researcher, University of Graz (Austria), said:

“The study shows that in their model in a handful of cases leaves on trees in tropical regions have already died because of exceeding a ‘leaf thermal comfort threshold’ which they call T critical. And that we’d have to get to almost 4’C warming above average air temperatures before a tipping point where the number of leaves dying means trees themselves die. This suggests in theory tropical forests are quite resilient to climate change.

“This is a simple model and trees and forest dynamics are much more complex than this. In reality, we’ve seen in other studies that heat and drought negatively affects whole ecosystems including a forest and the health of trees. It’s really important as one of our main ways to absorb carbon emissions and a home to some of the most diverse species on earth, we protect tropical forests as part of planetary health.

“This is another nature study in as many weeks that is making claims about temperature related tipping points. But, you no longer need to read a scientific paper to see climate change is happening, it’s frequently visible outside your window. The studies all call for a reduction in climate emissions, policy action and more research.”

Dr Leslie Mabon, Lecturer in Environmental Systems, The Open University, said:

“The results of the study suggest that a very small proportion of leaves are surpassing the threshold at which photosynthesis starts to fail, and that this number may increase in future. Although the proportion of leaves being talked about may seem small at first glance, it is another indication that as climate change intensifies, we risk upsetting the natural environments on which we rely. This is why it is so important that we do everything we can to avoid the uncertainties associated with higher levels of global heating, by reducing the burning of fossil fuels and preventing deforestation. The authors are thus absolutely right to call for ambitious climate change mitigation goals and reduced deforestation.

“The study is also a reminder that there are limits to the extent to which we can rely on nature alone to prevent the worst extremes of climate change. It is true that trees and other kinds of vegetation can soak up emissions and provide cooling. However, the study illustrates that without concerted action by humans to reduce emissions and limit global heating at the same time as protecting and enhancing nature, some functions of nature may start to break down at higher temperatures.”

Dr Kevin Collins, Senior Lecturer Environment & Systems, The Open University, said:

What does this paper tell us and what is the significance?

“The press release indicates that vital environmental processes like photosynthesis are degraded and at risk from increased temperatures.  Almost all life – including humans – is dependent on photosynthesis for food either directly or indirectly.  For humans, a reduction in photosynthesis could lead to reduced harvests, especially in warmer climates. Tropical forests are also key climate regulators of regional and global climate and any stress on them adds to the effects of climate change.

“However, it is important to remember that while this science is important to help us understand changes in biological processes, there are more immediate concerns about tropical forests. Clearances and deforestation for agriculture, wildfires and droughts pose a greater short to medium-term risk to tropical forests rather than elevated leaf temperatures under a worse-case climate change scenario which is expected to be some decades away.”

Anything else you think it’s important to point out/consider?

“Photosynthesis also occurs on a global scale in algae in ocean environments.  If warmer seas degrade algal photosynthesis in similar ways to that found in tropical forests, then marine environments and ecosystems would be very significantly affected.  This would also have a direct impact on human food systems and nutrition.”

It seems like it is very unlikely that further leaves would tip above the threshold presented in the study, or at least only under RCP 8.5, which means around 4°C of warming, which is far above current projections of warming of around 2.4°C. Is that the case?

“The press release does suggest that the tipping point for further leaves is some way off in the future.”

(Not a third-party comment as Professor Yadvinder Malhi is one of the co-authors of the study)

Prof Yadvinder Malhi, Professor of Ecosystem Science, University of Oxford, said:

“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.”

“By looking at how leaf temperatures vary within and across forest canopies, this study offers novel new insights that this threshold is closer than we thought, but also that it is entirely within our collective means to navigate away from this dangerous threshold.”

‘Tropical forests are approaching critical temperature thresholds’ by Christopher E. Doughty et al. was published in Nature at 16:00 UK Time Wednesday 23 August 2023.

DOI: 10.1038/s41586-023-06391-z

Declared interests

Dr Chloe Brimicombe: “I have no competing interests to declare.”

Prof Yadvinder Malhi: “One of the co-authors of the study.”

Dr Leslie Mabon: “No conflicts of interest.”

Dr Kevin Collins: “No conflicts of interest.”