Hill Heat

The amount of methane in Earth’s atmosphere shot up in 2007, bringing to an end a period of about a decade in which atmospheric levels of the potent greenhouse gas were essentially stable, according to a team led by MIT researchers.

Methane levels in the atmosphere have more than tripled since pre-industrial times, accounting for around one-fifth of the human contribution to greenhouse gas-driven global warming. Until recently, the leveling off of methane levels had suggested that the rate of its emission from the Earth’s surface was approximately balanced by the rate of its destruction in the atmosphere.

However, since early 2007 the balance has been upset, according to a paper on the new findings being published this week in Geophysical Review Letters. The paper’s lead authors, postdoctoral researcher Matthew Rigby and Ronald Prinn, the TEPCO Professor of Atmospheric Chemistry in MIT’s Department of Earth, Atmospheric and Planetary Science, say this imbalance has resulted in several million metric tons of additional methane in the atmosphere. Methane is produced by wetlands, rice paddies, cattle, and the gas and coal industries, and is destroyed by reaction with the hydroxyl free radical (OH), often referred to as the atmosphere’s “cleanser.”

The cause of the surge is unclear, particularly as it appears methane levels are well-mixed across the globe, although most methane emissions occur in the northern hemisphere. A disturbing possibility is that OH levels are declining, which could set off a catastrophic vicious cycle of rising methane and declining OH.