Earthquakes may contribute to global warming by releasing greenhouse gas from the ocean floor, a study suggests.

Scientists uncovered evid-ence that an 8.1 magnitude earthquake in 1945 released more than seven million cubic metres of methane into the North Arabian Sea.

The discovery exposes a natural source of greenhouse gas emissions that has not been considered before.

As a greenhouse gas, methane is some 20 times more potent than carbon dioxide, but less abundant in the atmosphere.

Enormous quantities of methane are locked in icy structures called hydrates on the floors of the continental shelves surrounding the earth’s land masses.

An estimated 1,000 to 5,000 gigatonnes of carbon is trapped in methane hydrates – more than the total amount released by burning fossil fuels each year.

Analysis of sediment cores taken from the North Arabian Sea in 2007 revealed signs of large-scale methane release.

“Based on several indicat-ors, we postulated that the earthquake led to a fracturing of the sediments, releasing the gas that had been trapped below,” said lead scientist David Fischer, from the University of Bremen in Germany. The researchers were writing in the journal Nature Geoscience.