Earth’s Crust Absorbs Lots of Carbon Dioxide, But Not Enough to Save Humanity

New research finds that subduction along the seafloor absorbs carbon dioxide, which helps explain why the climate fluctuates over millions of years.

The carbon cycle of Earth's oceans | Adriana Dutkiewicz
The carbon cycle of Earth's oceans | Adriana Dutkiewicz

The seafloor is absorbing carbon dioxide, the greenhouse gas most associated with climate change, according to researchers at the University of Sydney in Australia. But while the ocean bottom might someday help reverse global warming, the process would take millions of years.

The authors of the study published recently in the journalScience Advances said their work provides a sense of scale for the damage humankind is wreaking on the planet today.

“We are starting to alter the environment — the surface conditions of the planet — a lot. It’s a fair question to ask, ‘Where will this ultimately lead?’” study co-author Dietmar Müller, a geophysicist. “In the very long run, we’re heating up the planet. Perhaps all ice will be melted one day. What safeguards has the planet built in? Ultimately, it will start cooling again. That will maintain habitability of the Earth in the very long run and make sure not all life will become extinct.”

With funding from the Australian Research Council and the Alfred P. Sloan Foundation, Müller and his colleague, sedimentologist Adriana Dutkiewicz, studied decades’ worth of drilling samples from the ocean floor. Using computer models that took into account changing water temperatures, they determined how much carbon the seabed absorbed.

They found that the ocean floor could absorb as much as 22 million tons of carbon annually.

That might sound like an enormous number, but in relative terms, it’s not. The planet’s atmosphere contains almost 950 billion tons of carbon. Humans dump around 37 billion tons of carbon into the atmosphere annually, according to the Global Carbon Project.

“This research is not about looking at short time scales,” said Müller.

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Carbon sinks through subduction, or the process of tectonic plates on the earth’s crust moving and buckling under one another into the Earth’s mantle, the zone of magma and rock that is between the planet’s outer layer and core. As the plates under the water move over the eons, they rupture the Earth’s crust under the sea, forming fissures that expose minerals like calcite that capture carbon and turn into concrete-like materials. The warmer the water, the more efficient this chemical reaction that occurs.

The carbon doesn’t necessarily stay in the ocean crust forever. Instead, it’s recycled. Volcanos that often form along the ridges of subduction zones — like the so-called Ring of Fire along the rim of the Pacific Ocean — later spew up some of that carbon.

The researchers found that the capacity of the ocean to handle carbon runs in cycles of 26 million years. The planet is currently in an expansionary cycle, said Müller. It’s not clear why the crust operates according to this schedule, but geological records of fossils, salt deposits and other phenomena also follow patterns along similar time periods, he said.

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But it’s clear the cycle helps explain why the planet has fluctuated between hotter temperatures and ice ages rather than heading in one direction permanently, said Müller. The process was like the planet’s life insurance, he added.

“We know that the Earth has gone through many cycles of hothouse climates and icehouse climates,” said Müller. “The question is: why doesn't it become a runaway fact? The answer is: Earth has these feedback mechanism, built in.”