Radiocarbon innovation reveals methane emission sources
Greenhouse gas emissions are the primary cause of global climate change, and methane is the second most important contributor after carbon dioxide (CO2). “Methane is both naturally present in the atmosphere and human-induced,” explains METHID project coordinator Heather Graven from Imperial College London in the United Kingdom. “Major anthropogenic sources of methane emissions include agriculture, landfills and the fossil fuel industry.” The concentration of methane emissions has more than doubled since the start of the industrial revolution. To reduce warming caused by methane, the EU – and over 100 countries – have signed up to the Global Methane Pledge. This pledge promises to reduce global methane emissions by at least 30 % by 2030.
Measuring methane emissions
To assess such targets however, scientists need to be able to accurately measure the amount of methane in the atmosphere. And to take targeted action, we need to be able to attribute methane emissions to specific sources. “There are a number of ways of estimating methane emissions from different sources,” says Graven. “We can count the number of cows and multiply by the amount of methane we think each cow emits to estimate the total emissions from cows. We can do this for all kinds of sources. The problem is that when we count up all these estimates, we tend to end up with too much.” Scientists also routinely take atmospheric measurements to measure methane concentrations. Just measuring methane concentration however does not necessarily give us a clear picture of where these emissions have come from.
Identifying emission sources
The METHID project, which was funded by the European Research Council, sought to develop new ways of sampling atmospheric methane, and of determining the specific source of emissions using measurements of radiocarbon. “Normally you would need to collect hundreds of litres of air to get enough methane for radiocarbon measurements,” adds Graven. “But it’s difficult to collect and transport such large volumes of air, so there haven’t been many measurements made.” Instead, Graven and her team developed a new way of extracting methane directly from air. The methane is oxidised and trapped onto a molecular sieve material, and then easily transported to the radiocarbon lab. “Radiocarbon helps us to distinguish different kinds of methane sources,” Graven explains. “Fossil fuels are so old that all the radiocarbon – which is the radioactive form of carbon with a half-life of 5 700 years – has long ago decayed away.” What this means is that measuring higher amounts of radiocarbon indicates more biogenic sources such as landfills and agriculture, whereas measuring lower amounts of radiocarbon indicates fossil fuel sources, such as natural gas leaks.
Effective mitigation strategies
Graven and her team found more fossil-based methane in samples collected atop their laboratory roof than they had been expecting. This suggests that more natural gas is leaking from London than previously thought. “In order to develop effective mitigation strategies, we need a better idea of where to focus our efforts,” notes Graven. This is where the METHID project can really make a valuable contribution to future climate strategies. As a next step, Graven and her team will switch focus from source to sink – they plan to use their pioneering method to measure radiocarbon in carbon monoxide, which helps to understand the chemical reactions removing methane from the atmosphere.
METHID, methane, emissions, greenhouse gas, atmosphere, radiocarbon, carbon dioxide