The Natural Sphere


The Atmospheric Methane Mystery - by Joe Pitt

Methane in the atmosphere is the second largest contributor to human-induced climate change, behind only carbon dioxide in terms of importance. Whilst sources of carbon dioxide are well known, with the vast majority of human emissions resulting from fossil fuel burning, understanding of the relative strength of key methane sources is much poorer. Hence whilst global political efforts and mitigation strategies have focussed on reducing carbon dioxide emissions, changes in atmospheric methane remain one of the most interesting and hotly debated topics in atmospheric science today.

Rising atmospheric methane levels were measured throughout the 80s and 90s, but methane concentrations stabilised at a roughly constant value between 2000 and 2007. Since 2007 methane has been rising again, at a rate which has accelerated in recent years. The reasons for the 2000-2007 plateau and the subsequent rise are still unresolved, although many competing theories have been developed.

An obvious starting point to try to understand these changes is to look at known sources of atmospheric methane emissions. The largest global source is thought to be natural wetlands, but human-induced emissions from agricultural sources also contribute significantly, in particular due to cattle farming and rice paddies. Fossil fuel extraction is another important human source (natural gas is virtually entirely composed of methane), as is emission associated with landfill sites. Methane stored in hydrate form, sometimes known as fire ice, could represent a potential future source in a warming climate, although the impact of this source is highly contentious.

Working out what combination of emission changes in these source categories explains the current growth in atmospheric methane is tricky. However, it is possible to gain some understanding by looking at what other species each source is known to emit. For example, fossil fuel extraction is also a source of ethane, a gas which is not associated with most other key methane sources. Emissions of methane molecules containing a heavier carbon isotope (13-carbon) can also act as tracers for different source types.

Here, a summary is presented of ongoing work to improve understanding of methane emissions and methane removal from the atmosphere, changes in which could also explain the observed trend. It is of vital importance that we understand the mechanisms driving changes in the atmospheric methane budget if we are to take appropriate action to prevent further increases driving future climate change.

Joe studied physics at Oxford University before completing a PhD in atmospheric science at the University of Manchester. He now works on the MOYA project (Methane Observations & Yearly Assessments) which seeks to improve understanding of the global methane budget through a range of new measurement and modelling techniques.