“The Methane Switch”: Ancient Arctic Shift Offers a Chilling Warning for Today’s Warming Oceans 

By Yeavon Kim

About 56 million years ago, the fastest global warming event in Earth's history came to life. Scientists have discovered new evidence that the cause may be a sudden change in the methane cycle in the Arctic Ocean. This discovery is adding to concerns that the same process is happening again as oceans around the world warm up today.

The study behind the article, which was recently published in Nature Geoscience, analyzed a 15-m-long sediment core collected in the middle of the Arctic Ocean. The study analyzed molecular and isotope traces throughout the core showing how microorganisms treated methane in ancient times. This is the Paleocene-Eocene Thermal maximum warming period (PETM), when the global average temperature rose by up to 8°C in less than 20,000 years.

Before PETM, methane deep in the sea floor was mainly consumed by the anaerobic methane oxidation (AOM) process. In the AOM process, bacteria use sulfate instead of oxygen to 'burn' methane to convert it into hydrogen carbonate and hydrogen sulfide. Historically, this process has acted as a biological filter to trap methane in sedimentary layers and has prevented methane, a powerful greenhouse gas, from being released into the sea or atmosphere.

However, a recent study has found that these filters collapsed during the PETM period. The above study found clear evidence that a metabolic conversion from anaerobic methane oxidation (AOM) to aerobic methane oxidation (AeOM) took place. 

This conversion only occurs in shallow oxygen-rich water depths, which were dubbed a "methane switch." It is speculated that this change caused higher amounts of methane to be released from the seabed, which was oxidized at the sea surface and converted to carbon dioxide. The problem is that both methane (CH ₄) and carbon dioxide (CO₂) are greenhouse gases that intensify the greenhouse effect.

The researchers once said, "This change has completely changed the chemical composition of the Arctic Ocean. As methane began to escape from the sedimentary layers, global warming and ocean acidification accelerated, and this feedback loop likely lasted for hundreds of thousands of years."

The impact of this study on the world is significant. The modern Arctic Ocean is rapidly warming compared to the past as sea ice melts and freshwater flows in. It also interferes with the AOM process by reducing the oxygen concentration on the seabed. If the direction of AeOM is changed again, such as during the PETM period, methane trapped in frozen methane hydrates for thousands of years could be released, leading to much more severe global warming than current estimates.

Methane is a greenhouse gas that has heat absorption capacity about 28 times more powerful than carbon dioxide, so even small leaks from undersea storage in the Arctic Ocean can have a significant impact on the global climate system. Although this process is slow compared to industrial action, experts analyze that it will be a smarter choice in the long run.

At the same time, it is emphasized that it is careless to conclude that PETM will be repeated completely. This is because today's Arctic may have geographical and chemical differences from 56 million years ago. Nevertheless, it is a fortunate warning to have found this early 'methane switch' — an example of how rapidly the Earth's carbon cycle can change when it exceeds an important threshold.

In this era when the Earth is changing due to human behavior, the Arctic Ocean has once again become the center of the fate of ecology. How quickly we act to restore the climate — turn off the switch again — will determine whether the past will be reproduced.