Researchers at the University of Arizona have found that plant canopy temperatures are predicted to increase 16% more than the surrounding air by the end of the 21st century. This disparity suggests that current climate models, which rely primarily on air temperature, are significantly underestimating the actual heat stress experienced by vegetation.
For decades, the primary metric for climate policy and mitigation has been near-surface air temperature. While this is the most direct measure of human impact and the easiest to track locally, a new study published in the journal Nature Communications reveals a critical gap in how scientists model the natural world.
The research focuses on canopy temperature—the actual heat plants experience on their leaves. This metric is a more precise indicator of vegetation-climate interactions because it directly influences the biological processes that sustain plant life.
The 16% Gap in Canopy Temperature Modeling
The University of Arizona team modeled the difference between ambient air temperature and the temperature of plant leaves. They discovered a reliable relationship between the two, but found that leaves will heat up faster than the air surrounding them.
“So however much air temperatures increase by the end of the century, the temperature of plant leaves is going to increase more, by around 0.11 degrees Celsius, or around 16%.”
Julia K. Green, an assistant professor in the U of A Department of Environmental Science
This discrepancy occurs because leaves receive direct radiation from sunlight, causing them to heat up more than the air. Green compares the phenomenon to touching pavement on a hot day; the surface is significantly hotter than the air you feel. This underestimation in current models could have broad ramifications for predicting animal biodiversity, vegetation distribution, and the overall speed of climate change.
Transpiration Failure and the Arid Feedback Loop
Plants typically regulate their temperature through transpiration, a process where water is brought to the leaf surface and evaporated to provide cooling. Green describes this as being similar to what humans do when we sweat.
However, this cooling mechanism fails in extreme conditions. When air becomes too hot and dry, plants lose water faster than they can replace it. To survive, many plants shut down both transpiration and photosynthesis. Once these processes stop, the leaf surface heats up much faster than the surrounding air.
This creates a dangerous feedback loop. As plants stop transpiring to conserve water, the lack of evaporated moisture makes the surrounding air even drier, which in turn further increases the heat of the leaves.
High-Risk Zones: Arid Regions and Tropical Ecosystems
The disparity between canopy and air temperatures is not uniform across the globe. The researchers found that the most significant increases occur in regions where the air is predicted to become substantially drier. As the air dries, plants are forced to shut down photosynthesis earlier, leading to accelerated leaf heating.
While arid regions show the largest projected increases, the study highlights a specific vulnerability in tropical ecosystems. Unlike plants in drier climates, tropical vegetation is less accustomed to large temperature fluctuations.
“Tropical plants are less accustomed to large temperature fluctuations, so they aren’t adapted for the increase in temperature. Even relatively small changes to air and canopy temperatures can have a significant effect in those ecosystems.”
Julia K. Green, an assistant professor in the U of A Department of Environmental Science
The Biological Stakes of Surface Temperature
The reason this distinction matters is that air temperature is a proxy, while canopy temperature is the actual driver of plant health. According to Green, Plant surface temperature has a first-order impact on plant photosynthesis, transpiration, respiration and other important processes.
Green further warns that “Many of the researchers studying the impact of temperature on plants are using air temperature in their modeling, but our study shows that if you’re using air temperature alone, you’re going to be underestimating the temperature effects on plants.” Increased canopy temperatures could also impact ecological function and weather patterns.
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