A new University of Arizona study shows that rising temperatures are going to hit plants harder than most current models predict, which could have significant implications for everything from plant and animal biodiversity to future weather patterns.
Photo Credit: Rafael Rodrigues
Scientific Frontline: Extended "At a Glance" Summary: Plant Canopy Temperature Dynamics
The Core Concept: Plant canopy temperature—the actual heat experienced on leaf surfaces—is increasing at a significantly faster rate than ambient air temperature. Current climate models largely rely on air temperature, leading to a systematic underestimation of the thermal stress vegetation will face as global temperatures rise.
Key Distinction/Mechanism: While ambient air temperature is the standard metric for climate modeling, leaves absorb direct solar radiation and can become substantially hotter than the surrounding air. Plants normally mitigate this heat through transpiration, a biological cooling process analogous to sweating. However, under severely hot and dry conditions, plants shut down both transpiration and photosynthesis to conserve water. This cessation creates a localized feedback loop, causing the leaf surface to heat rapidly while simultaneously depriving the surrounding air of evaporative moisture.
Major Frameworks/Components:
- Transpiration and Feedback Loops: When rising temperatures and arid air force plants to halt transpiration to preserve internal water reserves, the loss of evaporative cooling drives rapid, localized spikes in leaf surface temperatures.
- Regional Vulnerabilities: Arid environments will experience the largest absolute discrepancies between canopy and air temperatures. Conversely, tropical ecosystems face critical risks because native flora lacks the evolutionary adaptation to withstand even minor thermal fluctuations.
- Carbon Sink Degradation: When temperatures exceed the optimal threshold for photosynthesis, metabolic shutdown occurs, drastically reducing the volume of atmospheric carbon dioxide sequestered by global vegetation.
- Hydrological Disruption: A global reduction in plant transpiration decreases atmospheric water vapor, fundamentally altering cloud formation, solar radiation shielding, and precipitation patterns.
Branch of Science: Climatology, Plant Physiology, Ecology, Environmental Science, and Hydrology.
Future Application: Integrating canopy temperature data into Earth system models will yield highly precise forecasting tools. This calibration will allow scientists and policymakers to develop robust climate mitigation plans, predict agricultural stability, and anticipate shifts in global biodiversity.
Why It Matters: Modeling climate change based solely on air temperature obscures the true physiological threat to the biosphere. If widespread thermal stress forces plants to halt photosynthesis, the resulting drop in carbon sequestration and atmospheric moisture will accelerate global warming and trigger cascading ecological failures.
For decades, climate scientists have warned that rising global temperatures pose a significant risk to human health. Climate models show that increases in average air temperatures could lead to more frequent extreme weather, rising sea levels, and more intense wildfires. The models also forecast major impacts on plant life—and according to a new study led by researchers at the University of Arizona, those impacts are likely to be far greater than expected.
The study, published in the journal Nature Communications, reveals that canopy temperature—the temperature plants experience on their leaves, which impacts plant health and productivity—is predicted to increase 16% more than the surrounding air by the end of the twenty-first century. The study's authors say that canopy temperature is a more accurate measure of vegetation-climate interactions.
"Plant surface temperature has a first-order impact on plant photosynthesis, transpiration, respiration, and other important processes," explained lead author Julia K. Green, an assistant professor in the University of Arizona Department of Environmental Science. "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."
That underestimation could have significant ramifications for climate change models. Increased canopy temperatures could affect plant and animal biodiversity, vegetation distribution, and ecological function, and even weather patterns and the speed of climate change itself.
Air Temperature vs. Canopy Temperature
Near-surface air temperature has long been the focal point of climate policy and mitigation plans. This makes sense—that is the temperature that most directly affects humans. It is also easier to measure at the local scale than land surface temperature, which better reflects the canopy temperatures experienced by plants.
Even though it is difficult to predict canopy temperature, Green's team was able to model the difference between canopy temperature and ambient air temperature.
"There's a reliable relationship between increases in air temperature and increases in plant canopy temperature," she said. "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%."
To understand why leaves heat more quickly than the surrounding air, Green suggests imagining you are in a parking lot on a hot, sunny day.
"If you put your hand on the pavement, it's going to be much hotter than the air temperature that you're feeling. That same thing happens with plants—their leaves are receiving direct radiation from the sunlight, so they can heat up much more than the air temperature around them," she said.
Plants have some mechanisms to regulate their temperature, including a process called transpiration. As plants photosynthesize on warm days, they are able to bring water to the leaf surface, where it evaporates and cools the leaf surface.
"It's similar to what humans do when we sweat," Green said. "But if it's really hot and it's really dry, plants can end up in a situation where they're losing all this water trying to cool their leaves, and then they can't replace it. In those situations, many plants will just shut down photosynthesis and transpiration, and that makes the leaf surface heat much faster than the air."
Areas of Concern: Arid and Tropical Regions
Green's team wanted to understand what was causing canopy temperatures to rise more than air temperatures. Their research suggests that the disparity is likely related to dry air conditions.
"We found that the areas where we saw the largest increases in the difference between canopy temperature and air temperatures were regions where the air is predicted to get a lot drier," Green said. "What's likely happening is that as the air becomes hotter and drier, plants will lose more water to the transpiration process, which means they'll probably shut down photosynthesis earlier, which will lead to more heating of the leaves."
She explained that as plants attempt to conserve water by shutting down transpiration, the air around them will become even drier from the lack of evaporated water from leaf surfaces, forming a feedback loop.
While the study predicts that arid regions will see the largest future increases in canopy temperature relative to air temperature, Green is also worried about this effect on tropical ecosystems.
"The places that were more concerning for me were tropical regions," she said. "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."
Systemic Significance
The study points to an urgent need to focus on canopy temperatures as well as air temperatures in Earth system and climate models. Beyond their foundational roles within their ecosystems, plants capture carbon dioxide from the atmosphere and have significant effects on weather patterns, all of which affect—and are affected by—climate change.
"Plants have an optimum temperature for photosynthesis. If temperatures continue to rise above that, photosynthesis decreases, which means the plants are taking in less carbon dioxide from the atmosphere," Green said. "If more carbon dioxide is staying in the atmosphere, that would lead to accelerated warming and effects of climate change. It also means there could be changes in vegetation distributions and mortality events."
Additionally, plant transpiration helps cool the ambient air temperature, and it even helps power the weather.
"When plants transpire, that water vapor enters the atmosphere, where it can become part of cloud formations and future rainfall events," Green said. "If you have plants that are struggling and shutting down photosynthesis and transpiration, you have less moisture evaporating into the atmosphere, which affects rainfall. Changes in cloud coverage will have an impact on how much radiation is getting through to our land surface, which will affect weather and climate as well."
Green expects that this study will allow scientists to adjust Earth system models to more accurately model canopy temperature, making them more accurate tools for informing policymakers and the public.
"The more accurate our climate change models are, the more informed our decisions can be," Green said. "Having a clearer picture of what could happen is necessary to create adaptation and mitigation plans that can actually be effective."
Published in journal: Nature Communications
Title: Vegetation responses to air dryness amplify future land surface warming
Authors: Julia K. Green, Trevor F. Keenan, Xu Lian, David J. P. Moore, and Philippe Ciais
Source/Credit: University of Arizona | Erin Schauer
Edited by: Scientific Frontline
Reference Number: as072126_01
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