Climate models have underestimated how bad heatwaves will become, particularly in northern Europe, a new study has found. The problem is that we have overestimated the cooling effect of many plants, something that matters most when temperatures are highest.

It is well known that plants can cool local environments, inspiring cities to grow urban forests. They release moisture through their pores, known as stomata, absorbing heat like an evaporative cooler.

“Heatwaves are also strongly modulated by the land surface,” a paper in Scientific Reports noted, particularly in the persistent anticyclonic conditions that lead to spells of multiple hot days.

Unfortunately, as carbon dioxide concentrations in the atmosphere rise, we will not only experience more frequent and intense heatwaves, but plants will do less cooling, just when we need them the most. The same stomata that release water also absorb carbon dioxide from the atmosphere, and the more CO₂ available, the less they need to open to get what they need. Partially closed stomata release less water, and therefore do less cooling.

Familiar as this is, the paper points to a problem: All plants that use the C3 photosynthesis pathway have been treated as responding equally to increased carbon dioxide.

In fact, different plants respond differently.

The paper reveals that a collaboration between plant biologists and climate scientists found something frightening. By studying 314 plant species, the authors concluded that some plants close their stomata much more than anticipated, releasing even less water and reducing their cooling effect drastically.

“We often underestimate the role of vegetation in extreme temperature events as it has not been included in enough detail in climate models up until this point,” said first author Dr. Jatin Kala of Murdoch University, Australia, in a statement.

Kala told IFLScience that northern needleleaf plants are particularly prone to stomata closure under high carbon dioxide conditions. When this information was fed into climate models, they revealed global temperatures are likely to rise slightly more than we have anticipated. Most of the effect will be felt during hot summer spells in places where these trees are common, such as northern Europe.

^{Small as these leaves are, they play a big part in cooling the environment during heatwaves, but are doing less than they used to. Studio_smile/Shutterstock}

The effect is large. Modeling suggests that by the end of the century, if carbon dioxide emissions continue as expected, European heatwaves will be 5°C (9°F) hotter than the already extreme temperatures foreseen without accounting for vegetation differences.

Kala said he is “not advocating” the mass replacement of pine forests with more evaporative species, but that the information needs to be considered when anticipating life in the Greenhouse. The only way we can address the problem, he suggested, is to “release less carbon dioxide.”

Kala also expressed concern to IFLScience about the viability of future research, noting the collaboration “would not have happened without access to CSIRO's [Commonwealth Scientific and Industrial Research Organization] climate models.” Recently announced cuts will almost eliminate climate modeling within CSIRO, arousing fears about Australia's capacity to maintain a substantial climate research effort, particularly for cross-discipline projects such as this one.