As nations repeatedly fail to make major cuts in their greenhouse gas production, scientists and others have begun to wonder if climate change might be halted not by emissions cuts but by technology that removes those gases from the atmosphere. The approach is called geoengineering. Unfortunately, a recent simulation of its effects on the oceans found that even extreme methods would not be able to completely rehabilitate the ocean environment. The work was published in Nature Climate Change on August 3. (Scientific American is part of Nature Publishing Group.)

The experiments focused on carbon dioxide removal (CDR), the process of extracting excess CO2 directly from the atmosphere. In theory this could help oceans because they become dangerously acidic when they absorb too much atmospheric CO2. One CDR idea is to plant trees that consume large amounts of CO2 and then burn the trees in facilities where the emissions can be captured and stored underground. But no one has ever tested this or similar carbon removal schemes on a large scale.

The next-best thing to large-scale testing is a large-scale simulation. In the new study researchers led by Sabine Mathesius, an environmental scientist at the Potsdam Institute for Climate Impact Research, used a computer model to investigate CDR’s effectiveness in rehabilitating seawater damaged by CO2 emissions.

A tale of two experiments
Mathesius’s team used their computer simulation in two experiments. In the first they wanted to see if carbon removal could reduce the atmosphere’s CO2 concentration to the level that was typical before the industrial revolution, thus restoring the marine environment its preindustrial state.

The researchers based their simulation on real-world carbon dioxide emissions from 1800 to 2005, and then projected those emissions into the year 2250. Then they simulated what would happen if technology could remove 18 billion tons of carbon dioxide per year—about half of present-day emission rates—from 2250 until 2700. The researchers found this did decrease atmospheric CO2 levels but could not restore the ocean’s preindustrial dissolved oxygen content or temperature. Even after 450 years of geoengineering, the model ocean was still almost as acidic as it would have been without any intervention at all.

But what might happen if someone invented a better, marvelous new carbon removal technique? The scientists wanted to know, so they raised the maximum removal rate to 90 billion tons of CO2 per year. That high performance is “frankly ridiculous,” says geophysicist Donald Penman from Yale University, who was not involved in the research. Yet even at this level, in the year 2700 the ocean was still warmer, more acidic and held less oxygen than the preindustrial marine environment.

In their second simulation the team modeled a world where, against all odds, present-day humans substantially reduced their greenhouse gas production. They wanted to learn whether emission cuts or after-the-fact carbon removal was better. And they did: When it came to preserving the ocean environment, removal was no match for decreased emissions. Even in the simulation where CDR pulled 90 billion tons of CO2 from the atmosphere per year, the deep ocean was far more acidic than it was in the reduced emissions model. Penman says, “It is clear that rather than trying to clean up a mess, it would be wiser to simply not create the mess in the first place.”

According to Mathesius, CDR is ineffective for combating ocean acidification because of the way seawater circulates. There are regions of the ocean where surface water submerges into the deep ocean and stays there for centuries or even millennia, cut off from the atmosphere. So even if CDR does manage to significantly decrease the atmospheric CO2 concentration, the deep ocean remains highly acidic.

This is not the only work indicating that carbon removal will come up short. A study published on August 3 in Nature Communications shows that capping global warming at 2 degrees Celsius requires removing 1.8 billion tons of carbon from the atmosphere yearly from now until 2100—a tall order that exceeds the capabilities of current technology.

The studies do not mean that carbon removal is completely useless, Mathesius notes. “We’re only saying that CDR would not be strong enough to counter a business-as-usual scenario,” she says. “But there’s still the possibility that you take CDR as a supplemental measure.” Removal, although it cannot undo all the damage of excess emissions, might still prove valuable if combined with actual carbon cuts.