Arctic Ocean methane 'switch' that helped drive rapid global warming discovered

The Arctic Ocean was once an important source of greenhouse gases to the atmosphere — and it could become one again, researchers warn.

The Arctic Ocean was once an important source of greenhouse gases to the atmosphere — and it could become one again, researchers warn.

Methane (CH₄) is second only to carbon dioxide (CO₂) in trapping heat in Earth's atmosphere. Since 2020, human-driven greenhouse gas emissions have increased atmospheric methane by about 10 parts per billion per year, more than twice as much as CO₂. However, scientists don't yet know how the methane cycle will respond as our planet continues to warm.

In a new study, published Sept. 25 in the journal Nature Geoscience, researchers looked to methane cycling in Earth's past for clues about our future.

The team focused on a period of rapid warming and ocean acidification that occurred around 56 million years ago, known as the Paleocene-Eocene Thermal Maximum (PETM). The PETM is one of the best examples of a major climate shift driven by disruptions in Earth's carbon cycle, much like the global warming we're experiencing today.

Scientists have previously shown that the PETM was accompanied by the widespread release of CO₂ and CH₄ into the oceans and atmosphere, which left distinct geochemical fingerprints in sedimentary rocks from that time. But despite 30 years of research, scientists still can't pinpoint where these gases came from.

To explore how the carbon cycle operated during the PETM, the researchers behind the new study looked at a 50-foot (15 meters) core of marine sediments drilled from the central Arctic Ocean by the Integrated Ocean Drilling Program's Arctic Coring Expedition. The sediments date back to 66 million years, preserving the PETM warming event and the subsequent "recovery" period, during which the climate eventually restabilized.

Once methane reached the water column, the biomarkers indicated a different set of microbes took over. These microbes consumed methane while breathing oxygen, via a process known as aerobic oxidation of methane (AeOM).

The researchers propose that this switch could have transformed the Arctic into a significant source of CO₂ after the onset of PETM warming. They explained that AOM in the sediments produces the alkaline compound bicarbonate, which helps to buffer the ocean and stabilize its pH. But AeOM in the water column releases CO₂, which contributes to warming and ocean acidification. AeOM microbes also consume O₂, enabling other oxygen-intolerant organisms to spread and gobble up sulfate, which further starves the AOM microbes.

Could a similar Arctic methane switch accelerate climate change today? "We think it is possible and very likely," said study lead author Bumsoo Kim, an organic geochemist at NASA Johnson Space Center. The Arctic Ocean is becoming warmer and fresher, which would consume more oxygen, driving similar changes in the methane cycle, Kim, who was a researcher at Texas A&M University at the time of the study, told Live Science in an email.

Cover photo:  Arctic Ocean became a major source of greenhouse gases around 56 million years ago, during a period of rapid global warming. (Image credit: Paul Souders/Getty Images)