If necessary, it might be possible to prevent the centuries-long effects of global warming by removing carbon dioxide directly from the atmosphere. But even if that works, don’t count on avoiding another, often overlooked consequence of burning fossil fuels: the acidification of the oceans.

A significant portion of the carbon dioxide we add to the atmosphere ends up dissolving in the ocean, which causes the water to become more acidic. The environmental ramifications of this are wide-ranging and still not fully understood, but it’s clear that acidification harms organisms whose skeletons or shells contain calcium carbonate, like coral, shellfish, and certain kinds of plankton.

In recent years, the idea that we could combat global warming by developing technologies capable of removing large volumes of carbon dioxide directly from atmosphere has been gaining momentum (see “Can Sucking CO2 Out of the Atmosphere Really Work?”). The Intergovernmental Panel on Climate Change noted in its latest major assessment that carbon dioxide removal schemes may be necessary to avoid dangerous climate change (see “Averting Disastrous Climate Change Could Depend on Unproven Technologies”). Besides building machines that capture the gas from the air, another approach involves growing biomass for fuel and then capturing and storing the emissions from burning it.

The authors of a new study examined various scenarios that assume several more decades of high global emission rates, after which the world begins removing huge amounts of carbon dioxide (at least half of today’s annual emissions) from the atmosphere. They found that in all scenarios, past emissions “leave a substantial legacy in the marine environment,” especially in the deep ocean. These effects last for centuries. In other words, as far as the oceans are concerned, carbon dioxide removal should not be considered a viable alternative to rapidly switching to a low-carbon energy system.

“A lot of people have been acting as if emitting CO2 now and taking it out later is equivalent to not putting it into the atmosphere at all,” says Ken Caldeira, a senior scientist at Stanford University’s Carnegie Institute for Science. Of course, it is better to remove it from the atmosphere than to leave it there, he says. Water near the ocean’s surface would indeed become less acidic in that case. “But it’s better not to put it in to begin with, because some of the carbon is going to get into the deep ocean, and that’s going to take a long time to get back out,” he says.

Some researchers are beginning to discuss potential geoengineering schemes aimed directly at reversing ocean acidification: for example, silicate or carbonate minerals could be added to the water to chemically neutralize its acidity. According to a recent report from the National Research Council, there is no evidence that adding alkalinity to ocean waters would have “deleterious effects.”

At a global scale this approach doesn’t look all that feasible, though, since it would involve mining and crushing volumes of rock per year that would be many times greater than all of the coal the world produces annually. Resorting to such a scheme at a small scale—for example, in a bay that contains a coral reef—could be sensible and even critical for protecting certain species of marine organisms, says Caldeira. “But the idea that you’re going to do the whole ocean is unrealistic.”