Plants Are “Leaking” – And It Could Make Global Warming Even Worse

Rising temperatures cause plants to leak more water, reducing their ability to absorb carbon and potentially turning them into carbon sources, worsening climate change.

Plants are essential in regulating Earth’s climate, but new research suggests rising temperatures may disrupt this balance. Scientists have discovered that plants release more water into the atmosphere than previously believed, which could have significant implications for climate predictions.

Dr. Sean Michaletz, an assistant professor at the University of British Columbia and a recent Sloan Research Fellow in botany, studies how plants respond to heat. His research challenges a long-held assumption about plant water loss, potentially reshaping how climate models forecast future warming.

What do “leaky” plants have to do with climate change?

Our entire biosphere depends on plants. During photosynthesis, plants absorb carbon dioxide through tiny pores in their leaves and, using light, ‘breathe out’ water vapor and oxygen in an exchange. Since carbon dioxide is the main driver of global warming, understanding how temperature affects this process is crucial for predicting climate change.

It was previously thought that plants lose most of their water through their pores, which close in extreme heat to conserve water. But our research found that as temperatures rise, plants lose more water through their cuticle—the waxy layer on their leaves, which cannot close—than through their pores. The thinner the cuticle, the greater the water loss.

This means that in extreme heat, plants continue losing water but cannot take in carbon dioxide, limiting photosynthesis and reducing their role as a carbon sink. In extreme temperatures, they could even become carbon sources, accelerating climate change.

My back-of-the-envelope calculation suggests that a medium-sized leaf exposed to 50 °C could lose about one-third of a teaspoon of water per day through the cuticle. Scaled up to entire forests, this could alter global water and carbon cycles—an impact that our current climate change models may underestimate.

How hot is too hot?

In another study of 200 plant species in Vancouver, we found that photosynthesis starts to break down between 40 and 51 °C. During the 2021 heat dome, temperatures soared to 49.6 °C, pushing plants to their limits.

Our ongoing research suggests that 60 °C may be the highest temperature plants can survive—beyond this point, proteins break down, leading to cell injury and death. Only a few desert and tropical species have ever been observed surviving at such extreme temperatures.

Globally, researchers are working to determine the “tipping point” where Earth’s vegetation releases more carbon dioxide than it absorbs, switching from a carbon sink to a carbon source. Our estimates suggest this could happen around 30 °C, though key uncertainties remain—especially how microclimates and water availability affect photosynthesis under extreme heat.

With global temperatures already averaging 16°C, understanding these limits is critical for predicting climate feedback loops and the future of Earth’s ecosystems in a warming world.

What can we learn from human-made biospheres?

As a postdoctoral fellow, I worked at Biosphere 2, a research facility originally designed as a self-sustaining, closed ecological system. Researchers, called biospherians, were sealed inside for a planned two-year experiment to test whether humans could survive without external oxygen or supplies. The goal was to test this concept on Earth, with the idea of sending such domes into space someday. However, the experiment faced unexpected challenges: concrete curing led to a carbon dioxide buildup, while prolonged isolation triggered social and psychological stress among the biospherians.

Biosphere 2 later transitioned into a research and public education center, where I studied how high temperatures affect plants in its experimental rainforest.

Plants have survived climate shifts for hundreds of millions of years, but all species face upper limits set by the laws of physics. While some plants can tolerate higher temperatures better than others, the precise breaking point – and how soon plants might reach it – remains uncertain. But based on recent measurements, we may be closer than we think.

Reference: “Temperature governs the relative contributions of cuticle and stomata to leaf minimum conductance” by Josef C. Garen and Sean T. Michaletz, 14 December 2024, New Phytologist.
DOI: 10.1111/nph.20346

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