Why sucking CO2 out of the atmosphere can’t undo all the effects of climate change

A man walks with his donkey in the Tafilalet oasis on October 27th, 2016, near Morocco’s southeastern oasis town of Erfoud, north of Er-Rissani in the Sahara Desert. The oasis is at risk of disappearing as the area is drying up due to global warming. | Photo by Fadel Senna / AFP via Getty Images

Sucking planet-heating carbon dioxide of the atmosphere doesn’t reverse all the effects of climate change, new research tells us. Carbon dioxide removal, as it’s called, can sound like science fiction — but many companies are already counting on it to undo some of the damage caused by their pollution.

Companies might try to prevent their greenhouse gas emissions altogether, or they might try to clean it up after the fact. That’s why we’re seeing all kinds of brands, from Microsoft to the Houston Texans, saying that they’ll plant trees or invest in new technologies that are supposed to filter CO2 out of the air. But even if they’re successful in trapping CO2, does it reverse the consequences of creating that pollution in the first place?

Not completely — at least not on a reasonable timeline, suggests a study published today in the journal Science Advances. It looks at the consequences carbon dioxide emissions have on an atmospheric circulation pattern called the Hadley cell that has a big impact on weather across much of the world. The study shows what might happen to the world if people keep polluting willy-nilly before finding a way to take those CO2 emissions out of the atmosphere.

Surprise, surprise — things don’t just snap back to normal, the study finds. That shows how important it is to limit pollution now rather than waiting to clean it up later. “It’s easy to think that if we reduce the CO2 concentration, the atmosphere will recover to its original state,” says Seo-Yeon Kim, lead author of the study and a postdoctoral researcher at Seoul National University. “Nature is not that simple.”

The Hadley cell is a defining factor for many of the world’s deserts and rainforests. It moves moisture and heat between the tropics (regions closest to the equator) to the subtropics (adjacent areas further from the equator). Warm air rises in the tropics, bringing on lots of rain. The air then moves poleward to the subtropics, around 20 to 30 degrees north and south of the equator, before it comes back down as very dry air — which is why most of Earth’s hot deserts lie in these subtropical regions.

Greenhouse gas emissions from burning fossil fuels are messing up that system. The Hadley cell is expanding, intensifying droughts in subtropical regions.

“You’re going to have more regions that will have become more desert-like,” says Rei Chemke, an assistant professor at Weizmann Institute of Science focused on atmospheric, oceanic, and climate dynamics. “Even modest changes could have large climate impact there on people.”

Droughts in expanding dry zones could exacerbate food and water shortages and displace people from their homes, the new paper notes. The Hadley cell has already intensified severe droughts in Chile and South Africa and affected agriculture in Southern Australia, says Richard Seager, a climate scientist at Columbia University’s Lamont Doherty Earth Observatory.

Kim and her colleagues modeled what would happen if atmospheric carbon dioxide concentrations continue to rise by 1 percent every year for the next 140 years — similar to a “high-emissions” scenario that’s often described as “business as usual” within climate research. Then, after that concentration had quadrupled, they modeled what would happen if the amount of C02 in the atmosphere dropped at the same rate over the same time period. In theory, that should cause the Hadley cell to contract — bringing its borders away from the poles back to where they were before carbon dioxide pollution shifted its boundaries.

But that’s where things get a little wacky. The impact of taking carbon dioxide out of the atmosphere was different in the Northern Hemisphere compared to the Southern Hemisphere of the planet. In the study, the northern edge of the Hadley cell moved even further south from where it used to be historically, overshooting the desired target. But the southern edge of the Hadley cell shifted much more slowly — so slow that it didn’t return to its starting point. Given a couple more centuries, things could eventually shift back to normal. But in the meantime, that can make life particularly hard for people and wildlife dealing with the consequences.

Both Seager and Chemke, who were not involved in the study, say more work is needed to understand what might happen in a more realistic scenario than the one laid out in this study. Waiting 140 years before reducing carbon dioxide emissions leaves the world in a perilous place, with weather disasters already becoming more frequent and extreme. It might be more helpful to policymakers to see what might happen to the Hadley cell on a shorter timeline with action taken more immediately.

There are a lot of variables to keep in mind that can lead to unexpected outcomes with efforts to take CO2 out of the atmosphere. The differences between the Northern and Southern Hemispheres’ responses in the study were linked to ocean temperatures being more slow to respond to changes in CO2 concentrations, for instance. And ozone depletion in the Southern Hemisphere also makes the Hadley cell’s influence over shifting desert regions more pronounced there.

“There will be surprises in what happens [when we draw CO2 down],” Seager says. “It’s not going to be equal and opposite to what happened when we started warming up the planet.”

To avoid the worst effects of climate change, any efforts to take CO2 out of the atmosphere are really only supplementary at best. A transition to clean energy is what can stop planet-heating pollution from building up in the first place. “Reducing emissions right now is the most important thing,” Kim says.

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