Higher CO2 levels leading plants to contribute to warmer temperatures

One of the elevated dangers of global climate change is discoveries outside the boundaries of expected changes – whether temperature, sea level and other predicted results of higher atmospheric carbon concentrations. UGA scientists now have added plants to net contributors to rising global temperatures.

The scientists detail the findings in a study published in the Nature journal Climate and Atmospheric Science documenting the impact of coincidental plant physiological responses to increasing extreme heat and humidity.

“When my major professor first proposed the idea, I originally thought ‘aren’t more trees supposed to be good for the planet?” said Ashley Cornish, doctoral candidate in the Franklin College of Arts and Sciences department of geography and lead author on the study. “But it turns out their response to rising CO₂ might actually be contributing to warming.”

While the majority of projected 21st century warming comes from the enhanced greenhouse effect, the study demonstrates that warming by plant physiological forcing also makes a notable contribution. An important and relatively under-explored control on atmospheric moisture and temperature over land is evapotranspiration, the process that moves water from the Earth’s surface into the atmosphere as vapor, via plant physiological responses to higher CO₂.

Heat stress results from high temperature and humidity. Decreases in evapotranspiration and increases in temperature can lead offsetting influences on heat metrics like the heat index. The team analyzed plant physiological forcing in idealized simulations that isolate plant physiological from radiative impacts of rising CO₂. The results demonstrate that increasing temperature has a larger influence than declining moisture in affecting heat stress.

This study utilizes multi-model output from the Coupled Climate-Carbon Cycle Model Intercomparison Project experiment, which is a part of the larger Coupled Model Intercomparison Project. The C4MIP experiments include four simulations, including an 1850 pre-industrial control and simulations that isolate plant physiological forcing and radiative warming associated with the greenhouse effect.

“Our recent research has demonstrated that plants, in response to rising CO₂ concentrations on a global scale, can influence many aspects of the climate system, including rainfall, flooding, drought, and temperature,” said Gabriel Kooperman, associate professor of geography at UGA and co-author. “In carrying out this project, we analyzed very large datasets that included results from many different Earth system models and multiple specialized simulations. Overall, the work shows that plant responses to rising CO₂ can contribute to increases in extreme heat events, and motivates future work needed to better constrain the influence of these processes in the climate system.”

Canopy transpiration refers to the process by which the collective leaves, branches, and stems of plants lose water that comes through roots; canopy evaporation refers to water that collections on leaves, branches and stems from rain, which re-evaporates without reaching the ground. Canopy transpiration tends to decline with rising CO₂ while canopy evaporation tends to increase due to increases in leaf area with rising CO₂ by collecting more rainwater. The billions of trees in the Amazon rainforest, for example, regularly churn as much as 20 billion tons of moisture into the atmosphere on a typical day.

While the pre-industrial levels of canopy evaporation are nearly identical between different C4MIP model versions, the changes resulting in plant physiology simulations are vastly different.

These results signify that physiological forcing can amplify the effects of global warming, adding to the risk of heat-related human health impacts. These results also demonstrate the critical importance of reducing uncertainty in not only atmospheric processes but also land-surface processes for better constraining future changes in extreme heat events in Earth system models.

"This study shows how important it is to capture the role of vegetation — and its impact on heat stress factors like temperature and humidity — in climate change modeling," said Andrew Grundstein, professor of geography and co-author on the study.

The research team, which includes Christopher Skinner of the University of Massachusetts, Lowell, Abigail Swann of the University of Washington, began their work in Fall 2020.

As the team proposed the idea of incorporating a humidity metric into the analysis, the process of disentangling the role plants play in extreme heat events became even more nuanced through competing temperature and moisture related processes. 

“While overall more vegetation uptake of CO₂ is a net benefit for the climate, this research highlights some important contrasts and competing effects in a changing global climate system,” said Cornish.

“The impacts of plant physiological responses to rising CO2 on humidity-based extreme heat” was published May 3.

Image: photo of "sunlight through the trees" made available through a creative commons license.