The mystery of the methane decline at the end of the twentieth century

It started in the 1980s: a sudden levelling of methane release into the atmosphere. It was a mystery as to why it happened. Now scientists have found the answer to the mystery, or in fact they have found two different reasons. An increased commercial use of natural gas on one hand and heavier commercial fertiliser use in Asia on the other hand.

Both explanations were found by researchers of the Department of Earth System Science at the University of California and published yesterday in the journal Nature.

Twenty times as potent as CO2

Methane is a highly potent greenhouse gas with a heat trapping potential over 20 times as high as CO2, but since atmospheric levels are lower it is considered the second-biggest contributor to global warming after CO2. Finding out what causes a large fluctuation in atmospheric methane such as the one occurring during the last two decades is therefore vital in understanding global warming.

Explanation number one

The first paper suggests that the change in atmospheric methane is primarily caused by changes in fossil fuel emissions. The involved researchers came to this conclusion by studying trapped air in ice from Greenland and the South Pole going back about a century. Instead of looking at methane they analysed the samples for ethane levels, a hydrocarbon with similar sources, but more easily traced. A rise in levels was found from the start of the industrial revolution early last century to the 80s when a sharp drop was discovered. This was around the same time that we started harvesting methane as a natural gas energy source, instead of it gradually escaping into the atmosphere. These findings led them to conclude that atmospheric methane levels are strongly connected to fossil fuel use.

The other explanation

The other research team found a completely different explanation for the methane decline. By looking at atmospheric methane from the late 1980s to 2005 they found the sharpest trend was connected to the change from organic manure to artificial fertilisers and a decrease in water use by Asian farmers. Rice paddies are traditionally big methane producers, but some bacteria in the soil are able to consume methane. It is believed that fertilisers increase that ability. According to the team about half of the decline in methane can be explained by a emissions reduction from Asian rice agriculture.

Who’s right?

It is surprising to find two answers to one mystery. The question that arises is: which of the two is the right one? It would seem both studies are scientifically sound, although the researchers of the second study may have dismissed the idea that methane harvesting has played a role a bit too readily.

What is most important is that both studies show that atmospheric methane levels are strongly driven by human activities. With signs that methane levels are on the rise again it is vital that we discover the most important contributors of which two may have already been found.

© Jorn van Dooren |

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