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 21st century. The study's authors say that canopy temperature is a more accurate measure of vegetation-climate interactions.

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"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 U of A 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's the temperature that most directly affects humans. It's also easier to measure at the local scale than land surface temperature, which better reflects the canopy temperatures experienced by plants.

Even though it's 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're 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're 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 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 of 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."