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Rainforest leaves feel the burn of climate change as temperature limit approaches
Tropical forests could be approaching their temperature limit.
While only a fraction of leaves are thought to be at imminent risk of dying, the wider ecosystem could be under threat if the worst case predictions of climate change come to pass.
Rainforests are the lungs of the world – but the heat is making it difficult to breathe.
As the average temperature of the planet creeps ever higher, it’s getting closer to the point where plants will no longer be able to photosynthesise. In tropical plants, this threshold is 46.7⁰C on average, with 0.01% of their leaves currently estimated to breach this temperature at least once a year.
In the future, however, more and more leaves might overheat. A new study estimates that a tipping point could be reached if the air temperature in forests warms by around 3.9⁰C, a scenario that might happen if the world’s dependence on fossil fuels remains high.
Dr Christopher Doughty, the study's lead author, says ‘As a society, we still have a chance to choose the fate of these forests. If air temperatures rise by 4⁰C, there is the potential that these forests will pass a tipping point.’
‘While there is a great degree of uncertainty over the exact threshold, we’re less likely to hit these temperatures if we reduce deforestation and make steps towards cutting emissions now.’
The findings of the study were published in the journal Nature.
The ECOSTRESS instrument (the whitest box in the image) can measure surface temperature from orbit. Public domain image by JSC from NASA images.
Studying rainforests from space
The origins of this study have their roots not in Earth’s soil, but in its orbit. Much of the research data comes from a NASA instrument known as ECOSTRESS, which measures the surface temperature of areas of the planet from its vantage point on the International Space Station.
It can measure the temperature of plants from orbit, and is able to determine how hot an individual field is. This data can then be used to understand how plants are responding to changes in water availability.
Following its launch in 2018, it was important to validate ECOSTRESS measurements on the ground. This involved heading into a region of the Amazon and taping sensors to individual leaves, as well as taking wider measurements of the tree canopy temperature.
Measurements from other rainforests in Puerto Rico, Panama and Australia were added to the data, and all drew the same conclusion: that some leaves were getting extremely hot.
Dr Martijn Slot, a co-author of the paper from the Smithsonian Tropical Research Institute, says, ‘Leaf temperatures can be a lot higher than the surrounding air temperature. This leads to damaging impacts on their photosynthetic machinery when it gets too hot.’
‘When leaves get too hot, they can’t transpire, they can’t photosynthesise, and they turn brown and die.’
While scorched leaves are only a minor issue at the moment, there’s the potential for them to become a much bigger problem in the future.
As air temperature rises, the temperature of leaves increases by a larger amount. Image © Anwar Attar/Shutterstock.
A warning of things to come?
With greenhouse gas emissions still on the rise, the future of climate change remains in the balance. While it’s estimated that global warming can be limited to 1.5⁰C, it’s more likely global temperatures will increase by about 2.4⁰C if emissions continue as they are.
At the moment, the researchers estimate that the canopies of tropical forests are 34⁰C on average. However, some areas are much hotter than others.
Dr Greg Goldsmith, a co-author from Chapman University says, ‘There are some good reasons to believe that no two tropical forests have the same canopy temperatures, and no two trees, or any given leaf, may do either.’
‘Differences between different tree species, such as the shape of their canopy or the size and thickness of their leaves, may affect their temperature threshold.’
To investigate how future global warming might affect trees, the researchers heated the air around individual leaves by 2⁰C, 3⁰C and 4⁰C. This caused much larger increases in leaf temperature of up to 8⁰C, meaning that only relatively small temperature increases could cause many more leaves to die.
Even the death of just a few leaves can start to have knock-on effects, which can build up across an entire rainforest.
‘Tropical forests are important for their climate regulation properties, and once you start losing bits of the forest this can cause feedbacks elsewhere,’ Christopher explains. ‘The death of some leaves can reduce levels of transpirational cooling, raising the temperature of the rest of the branch and putting additional stress on other leaves.’
‘The Amazon has experienced greater tree mortality than average in recent years, and while there are many theories as to why this might be the case, it might relate to temperature.’
While the figures are a cause for concern, there are a number of caveats to the research. Though a temperature rise of around 3.9⁰C was calculated as the threshold for most forests, the figures ranged from 2⁰C to 8⁰C.
It was also assumed that trees have no capacity to adapt to rising temperatures, because there’s not yet enough evidence to prove this one way or the other.
‘The few studies looking into this haven’t found significant increases in trees acclimatising to their environment,’ Martijn explains. ‘The tropics are generally stable in terms of climate, so it’s uncertain whether or not the plants can dynamically adjust to rising temperatures.’
‘However, as there is not a lot of data it is important to find out more about this. At the moment, we have seen tropical trees seem to have some capacity to adjust, and we hope ongoing work will be able to confirm if this is the case.’
The team are optimistic that by raising the alarm now, the countries of the world will redouble their efforts to cut their emissions over the next decade.
Dr Josh Fisher, the Science Lead for the ECOSTRESS mission and co-author, adds, ‘While 0.01% of leaves scorching sounds small, it’s a canary in the coalmine for what could be about to come.’
