As CO2 Levels Rise, Plants Thicken Their Leaves and Absorb Less Carbon

According to a new research study, when carbon dioxide levels in the atmosphere go up, plants thicken their leaves and no longer absorb as much carbon dioxide as before. That’s bad news for climate change models.

Up until now, scientists have assumed the carbon "sink" nature of plants and their leaves stayed constant as carbon dioxide levels rise in the atmosphere. The new University of Washington study shows otherwise, with potentially disastrous consequences for global climate.

Though scientists don’t fully understand why the thickening happens, many kinds of plants show similar effects. Woody trees and crops like potatoes, rice, and wheat all demonstrate similar effects. Many plants which have what is known as C3 carbon fixation, a form of photosynthesis found in 95% of all photosynthesis on the planet, have shown similar effects.

When exposed to higher levels of CO2, leaves can thicken by as much as 33%. That changes the ratio of surface area to mass in the leaf, among other things. That in turn changes the internal structures within the leaf, affecting capabilities for photosynthesis, gas exchange, evaporate cooling and sugar storage.

Why this happens may therefore not be understood, but the consequences of thicker leaves are understood. With thicker leaves there is far less efficiency in a plant’s ability to pull in atmospheric carbon. Since this property of plants is important in slowing the rise of carbon dioxide accumulation in the atmosphere, that the leaves thicken with more CO2 becomes a vicious cycle. More CO2 means thicker leaves. Those thicker leaves absorb less CO2 so the accumulation rate of greenhouse gases in the atmosphere goes up. As that goes up, more plants can be affected in the same way.

That cycle unfortunately points to a significant in global warming calculations. Those analyses have relied on constant carbon absorption rates by plants even as CO2 levels rise.

As senior author Abigail Swann, a University of Washington Assistant Professor of Atmospheric Sciences and Biology said about her team’s study, “Plants are flexible and respond to different environmental conditions. But until now, no one had tried to quantify how this type of response to climate change will alter the impact that plants have on our planet.”

By effectively lowering the “carbon sink” capability of plants worldwide, plants creating thicker leaves in the presence of higher levels of CO2 will have a major impact on global warming rates. According to climate simulations run by Swann and paper lead author Marlies Kovenock, a UW doctoral student in biology, the Earth could heat up an additional 0.3 to 1. 4 degrees Celsius over what has been projected on its own from conventional warming models.

As Kovenock said in comments about this, “If this single trait — leaf thickness — in high carbon dioxide levels has such a significant impact on the course of future climate change, we believe that global climate models should take other aspects of plant physiology and plant behavior into account when trying to forecast what the climate will look like later this century.”

In the study the researchers simulated a world where carbon dioxide concentrations were 710 parts per million. That’s more than the current 410 ppm of carbon already present, but it also happens to be far less than the levels of as high as 900 ppm which could happen to the planet within a century.

In addition to simulation showing 0.3-1.4 degrees Celsius higher temperature rises than expected overall for the globe, the simulations also showed regions in Eurasia and the Amazon basin as having a higher minimum rise in temperature. When that happens, the thicker leaves on plants may hold back plants’ ability to produce evaporative cooling and have much of an impact on cloud formation. That in turn can contribute to another vicious cycle of thickening, then eventually raising global temperatures, then making temperatures go up even further as the natural capabilities of evaporative cooling from plants cut back.

The researchers plan more detailed analysis on precisely how plants respond to increased CO2 emissions, as well as the interactive nature of how those changes contribute to other atmospheric and climate effects. As UW senior author Swann said about the study, “We now know that even seemingly small alterations in plants such as this can have a global impact on climate, but we need more data on plant responses to simulate how plants will change with high accuracy.”

This project was funded by the National Science foundation and the University of Washington.