Arctic Methane Cycle Study Warns of Potential Climate Change Impact

The Arctic Ocean, historically known as a significant greenhouse gas contributor, may once again play a pivotal role in climate change dynamics, researchers caution. The potential resurgence of methane emissions from the region is a growing concern as global temperatures continue to rise.

Methane (CH₄), a potent greenhouse gas, is surpassed only by carbon dioxide (CO₂) in its ability to trap heat in Earth’s atmosphere. Since 2020, human activities have increased atmospheric methane levels by approximately 10 parts per billion annually, more than doubling the increase rate of CO₂. However, the response of the methane cycle to ongoing global warming remains uncertain.

In a study published on September 25 in Nature Geoscience, researchers explored historical methane cycling to predict future trends. The focus was the Paleocene-Eocene Thermal Maximum (PETM), a period of rapid warming and ocean acidification around 56 million years ago, marked by disruptions in Earth’s carbon cycle, similar to today’s climate changes.

Evidence suggests the PETM was accompanied by significant releases of CO₂ and CH₄ into the seas and atmosphere, leaving distinct geochemical markers in ancient sediments. Despite decades of research, the origin of these gases remains elusive.

To investigate the carbon cycle during the PETM, researchers examined a 50-foot core of marine sediments from the central Arctic Ocean, obtained through the Integrated Ocean Drilling Program’s Arctic Coring Expedition. These sediments, dating back 66 million years, capture the PETM warming event and the following recovery period.

Researchers isolated organic molecules, known as biomarkers, from the sediments to identify the seafloor’s microbial life at the time of deposition. These biomarkers, characterized by lighter carbon isotopes, indicated the presence of methane-consuming microbes, whose activities shifted during the PETM.

Initially, methane formed deep beneath the seafloor, consumed by microbes through anaerobic oxidation of methane (AOM) using sulfate. However, biomarkers from AOM microbes dwindled during the PETM. Today’s AOM activity predominates due to abundant sulfate in modern oceans, but during the PETM, lower sulfate levels limited this process, potentially leading to a substantial methane release into seawater.

Upon entering the water column, methane was consumed by a different set of microbes via aerobic oxidation of methane (AeOM). This process likely transformed the Arctic into a CO₂ source post-PETM warming, as AeOM releases CO₂, contributing to ocean acidification and warming. Additionally, AeOM consumes oxygen, promoting the proliferation of oxygen-intolerant organisms and further depleting sulfate, impacting AOM microbes.

The possibility of a similar methane shift occurring today is likely, according to lead author Bumsoo Kim of NASA Johnson Space Center. As the Arctic Ocean warms and freshens, oxygen consumption could induce similar changes in the methane cycle, Kim noted, reflecting on his time at Texas A&M University.

Nonetheless, some scientists remain skeptical. Sandra Kirtland Turner from the University of California, Riverside, notes that past factors may not directly parallel future scenarios, given historical oceanic and chemical differences. She emphasizes the importance of understanding carbon cycle feedbacks, which could amplify or extend warming, yet remain poorly constrained and often overlooked beyond 2100.