The image of more than a hundred thousand aircraft carriers floating through the air might sound like a scene from a Doctor Strange movie. But the weight of all those carriers equals the amount of carbon dioxide that humanity has pumped into the air every year over the past decade or so—11 billion tons per year. The carbon dioxide traps heat, warming the atmosphere.

The oceans help slow that process by absorbing about a quarter of the CO2 from the air, according to a recent report. More CO2 was being absorbed in parts of the North Atlantic and Southern Oceans.

Some of the carbon dioxide is dissolved into the water as winds blow across the surface. And some is taken in by microscopic organisms, which use sunlight to convert the CO2 into food.

The process is more efficient when the ocean surface is warmer. So more carbon is absorbed during El Niño years. But we’ve had several La Niña events in recent years, which bring cooler waters, reducing the carbon uptake.

Over time, a lot of the CO2 works its way into the deep ocean, allowing the surface to absorb even more. Some of it accumulates in the sediments on the ocean floor, where it can form rocks.

The extra carbon dioxide creates problems for the oceans as well. In the atmosphere it warms the water, and in the water it interferes with some creatures’ ability to make their shells, for example. So those floating “aircraft carriers” are a big problem—no matter where they dock.