The world’s tropical forest canopies may be closer to critical high-temperature thresholds than previously thought but moderately ambitious climate-change mitigation can avoid these dangerous thresholds. That’s the findings of a team of scientists including many associated with the Environmental Change Institute (ECI).

The team included ECI co-authors Yadvinder Malhi, Ecosystems programme lead and Professor of Ecosystem Science and Dr Imma Oliveras Menor, Senior Researcher in Disturbance Ecology and Global Change  as well as former postdoc researchers Dr Chris Doughty (current Associate Professor at Northern Arizona University and who led the study), Dr Josh Fisher (Ass Professor at Chapman University and who created the space station mission used in the study), and Dr Greg Goldsmith (Ass. Professor at Chapman University).

The study combined satellite thermal and experiment data from the world’s tropical forests to look at the variation of leaf temperatures within forest canopies.

It found that a small percentage of tropical leaves are already reaching and exceeding the temperatures at which they can no longer function. As climate change continues, entire canopies could die.

Prof Yadvinder Malhi 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.”

Photo by MChe-lee on Unsplash

The study, Tropical forests are approaching critical temperature thresholds, published in Nature estimated the number of leaves approaching critical temperatures under future increases in air temperatures of 2°, 3° and 4°C. These are the various warming scenarios under climate change.

The highest leaf temperatures increased by 8°C. Dr Doughty said: “We were really surprised that when we warmed leaves by 2, 3 or 4°C, the highest leaf temperatures increased by 8°C. This is a non-linear feedback that we were not expecting,”

More than 1% of the leaves in the warming experiments exceeded critical temperature thresholds, estimated at 46.7°C, at least once a year. This is a times-two increase in the percentage of leaves that currently exceed the threshold.

In order to measure the canopy temperatures the team combined ground-based measurements of individual tropical leaf temperatures, leaf warming experiments from three continents and data from a new NASA thermal imaging instrument on the International Space Station.

The researchers built a comprehensive understanding of tropical forest leaf temperatures and created a model of expected temperature change alongside global warming.

Dr Doughty said: “Given that tropical forests’ key role in housing species diversity and regulating the planet’s climate, insights into their future can build an understanding of the trajectory of the planet. But our model is not fate. It suggests that with some basic climate mitigation, we can address this issue, and helps pinpoint a few key areas that need further research. It also shows that by avoiding high-emissions pathways and deforestation, we can protect the fate of these critical realms of carbon, water, and biodiversity.”

Dr Oliveras said: “This study combines a unique multi-scale approach using different observational and experimental methods, showing the importance of scientific multidisciplinary collaboration. Importantly, the study highlights the potential risks of inaction in tackling climate change and helps identify important knowledge gaps where further urgent research is needed.”