Because of Rising CO2, Trees Might Be Warming the Arctic

Because of Rising CO₂, Trees Might Be Warming the Arctic

Less water loss from plants causes the surrounding air to warm, and currents can transport that heat poleward

The Arctic is one of the fastest-warming places on the planet —and scientists still aren’t completely sure why.

Melting snow and ice may be speeding up the warming. Changes in atmospheric circulation could be playing a role. Many factors could be influencing the region’s temperatures, which are rising at least twice as fast as the rest of the world.

Now, scientists think they may have discovered an additional piece of the puzzle. Plants, it turns out, may have an unexpected influence on global warming.

As carbon dioxide levels rise in the atmosphere, plants become more efficient at carrying out photosynthesis and other basic life functions. And they’re often able to save more water in the process.

Water that plants exchange with the air helps cool local temperatures. When they lose less water, their surroundings start to warm up.

A study published last month in Nature Communications suggests that this process is helping to warm the Arctic.

“The influence of plants has been overlooked before,” said study co-author Jin-Soo Kim, a scientist at the University of Edinburgh, in an email to E&E News. “This study highlights the vegetation impacts on Arctic warming under [an] elevated CO₂ world.”

The study used a suite of earth system models to arrive at its findings.

The models suggest that rising CO₂—the result of human greenhouse gas emissions—is causing plants to lose less water throughout the Northern Hemisphere, including densely vegetated regions in the tropics and the midlatitudes. This process causes temperatures in these places to warm even more than they would from climate change alone.

At the same time, large-scale atmospheric circulation patterns help transport heat between the tropics and the Arctic. The study suggests that this extra heat is warming the Arctic at an even faster rate.

In fact, the extra warming may actually contribute to other processes also speeding up Arctic climate change.

For instance, scientists believe that melting sea ice plays a big role in Arctic warming. Sea ice, with its bright, reflective surface, helps to beam sunlight away from the planet. As ice disappears, more sunlight —and more heat —is able to get through to the surface of the Earth.

The extra heat drifting up from the lower latitudes may be helping to melt sea ice at faster rates, the researchers suggest. And this, in turn, also contributes to faster Arctic warming.

Overall, the study estimates that the plant effect may account for nearly 10% of the Arctic’s warming each year. And it could explain as much as 28% of the warming across the Northern Hemisphere’s lower latitudes.

But there’s still a lot of uncertainty about those estimates.

The scientists used an ensemble of eight models in their study and considered all the model results together. But from one model to the next, there are pretty large differences in the size of the plant effect.

That may be partly because the response of sea ice is still uncertain and tends to vary among different models.

But there’s also been some debate among scientists about the exact effect of rising CO₂ on plants.

Plants take in CO₂, and also exchange water with the atmosphere, through tiny pores in their leaves called stomata. More CO₂ means plants don’t have to keep their stomata open so wide. They can still get enough carbon dioxide through smaller openings, and they can save water in the process.

On the other hand, more CO₂ can sometimes cause an increase in plant growth—and when there are more plants around, there’s more water being exchanged with the atmosphere.

These two effects—more plant growth, but also smaller stomata openings—can have conflicting effects on local temperatures.

For now, recent studies suggest that the stomata effect tends to win.

“I think it’s pretty clear that in many ecosystems, we actually don’t see as much plant growth as we sort of naively think we should by bumping up the CO₂,” said Leander Anderegg, a postdoctoral researcher at the University of California, Berkeley, and the Carnegie Institution for Science who commented on the new research for E&E News. “And there, the increase in these plants using water more efficiently and closing stomata definitely offsets the growth aspect.”

But, he added, the exact size of these effects is still uncertain and can vary from place to place.

“I think that it’s something that is pretty well-established that it’s sort of like an important unknown,” he said.

So scientists are still working to understand exactly how much influence plants have on the global climate. But other studies also suggest they may play an important role.

Previous research published in 2010 in the Proceedings of the National Academy of Sciences has found that the plant effect will increase global warming beyond what scientists would otherwise expect, based on climate change projections. Other studies, such as a 2018 analysis in Nature Communications, have suggested that the same effect will amplify extreme heat events, causing more frequent and more intense heat waves.

And still other studies have linked the plant effect to regional climate patterns in places outside the Arctic. For instance, one study published in Geophysical Research Letters in 2018 found that reduced water loss from plants may contribute to a drying pattern in the Amazon.

This is all an emerging area of research, with the exact magnitude of the effects still unclear. As a result, the effect is not well-represented —if at all —in most climate models.

According to Kim, that means there’s a chance that some model projections could be underestimating future climate change, particularly in the Arctic. More research may clarify whether that’s actually the case and exactly how much plants are contributing to the warming that’s happening all over the globe.

For now, the fact that many studies with many models all seem to be converging on the same basic idea gives scientists more confidence that they’re on the right track, Anderegg said.

“And even if we have some amazing breakthroughs in how we model plants ... I think what’s absolutely durable about the paper is how plants respond to CO₂ isn’t gonna save us,” he added.

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