Methane responsible for 85% of natural gas emissions across London

Researchers analysing London’s atmosphere find that the city is releasing more methane than previously thought, due to natural gas infrastructure leaks

Previous assumptions about London’s gas emissions noted that landfill sites were polluting the most methane, now, new research indicates that it is actually the result of natural gas leaks.

Methane is a more potent greenhouse gas than carbon dioxide, producing stronger warming effects on the planet, though very dangerous, it stays in the atmosphere for less time.

However, rising methane emissions globally are a major cause of concern, so plans to continue to reduce these emissions could greatly help the fight against climate change.

Methane emissions were underestimated by around 30-35%

Published in Atmospheric Chemistry and Physics, researchers at Imperial College London find that the cumulation of lots of small leaks is adding up to considerable extra methane emissions across the city.

Other major sources of methane include agriculture, landfill and waste sites, natural gas infrastructure and natural sources such as wetlands.

Previous estimations of methane emissions are usually based on a ‘bottom-up’ approach – where emissions were calculated based on statistics – this can be seen with cows, as they produce some methane, calculating on average how much methane each cow produces and multiplying this by the number of cows in the UK gives an estimate of the emissions from cows across the country.

First author of the study and Science and Solutions for a Changing Planet Doctoral Training Partnership student Eric Saboya, from the Department of Physics at Imperial, said: “Our study shows that London is producing more methane than we thought, but because we’ve been able to pinpoint the source of much of this extra methane, we have a clear direction to reduce emissions.

“Previous estimates suggested that landfill sites in London were the biggest emitters of methane, but our study shows that natural gas leaks are a bigger problem. Mitigation strategies can now be directed where they are most needed, such as upgrading leaky old metal pipes with newer plastic versions.”

Using a ‘top-down’ approach on emission estimates

However, unlike other studies, the new study used a ‘top-down’ approach, where they sampled the actual atmosphere in London using equipment installed on Imperial’s South Kensington campus. This way they were able to check if the measurements agreed with measurements of the ‘bottom-up’ methane emissions.

From using the data from both approaches over continuous measuring from March 2018 to October 2020 – as well as models of atmospheric transport and the concentration of methane in the local atmosphere – they found the source of the methane using small but measurable differences between the properties of methane from different sources.

They then compared these figures to two emissions ‘inventories’ which were bottom-up estimates. While one inventory (EDGAR) correlated relatively well with the measurements in total methane concentration, the other – from the UK National Atmospheric Emissions Inventory, (UK NAEI) – appeared to underestimate methane emissions by 30-35%.

It concluded that both of the inventories had underestimated the fraction of emissions from natural gas. Where the NAEI inventory suggested natural gas accounted for around 25% of the methane measured in South Kensington, the real measurements showed that the actual amount is around 85%.

“Atmospheric measurements like these are key to tracking the UK’s progress on this pledge.”

Study co-author Dr Giulia Zazzeri, from the Department of Physics at Imperial, said: “This is not just a London problem – cities such as Paris and Boston have shown similar results – but the local make-up of methane sources is different for every city, showing the power of these measurements for determining where mitigation should be directed to help cities reduce their greenhouse gas emissions.”

Study co-author Dr Heather Graven, from the Department of Physics at Imperial, added: “The UK was one of over 100 countries who pledged to reduce methane emissions 30% by 2030 as part of the recent COP26 meeting in Glasgow. Since methane is emitted by various sources that are difficult to estimate, atmospheric measurements like these are key to tracking the UK’s progress on this pledge.”