Methane occurs in smaller volumes than carbon dioxide and doesn't persist in the atmosphere for as long, but it is more effective at trapping radiation than carbon dioxide and has a greater warming impact. Kilogram for kilogram, methane's impact on climate change is around 30 times greater than carbon-dioxide's over a 100-year period.

Methane is emitted during oil and gas production partly due to flaring and venting – to prevent dangerous gas build-up underground – and partly due to unintended leakage. Until now global emissions of methane associated with oil and gas extraction have been calculated using measurements from oil and gas fields in the US and Canada. But other oil and gas fields don't always follow the same procedures.

Lena Höglund-Isaksson from the International Institute for Applied Systems Analysis in Austria developed a novel emissions model, taking into account the global variation in the generation of gas from oil and gas fields and how the gas is handled, to see if this might explain the observations of excess methane from fossil-fuel sources in the atmosphere.

By combining country-specific information on gas flows with inter-annual variations in observed flaring of gas, from satellite images between 1994 and 2010, Höglund-Isaksson was able to calculate each country's likely contribution to methane emissions from its oil and gas extraction. Her results show that venting of gas associated with oil production can explain most of the mismatch.

"Both the US and Canada recover almost all the associated gas generated, but in other parts of the world recovery rates are significantly lower," said Höglund-Isaksson. For example, in the year 2005 her estimate of methane from oil and gas systems is 34% higher than that produced by the EDGAR database (from the Joint Research Centre of the European Commission). The findings are published in Environmental Research Letters (ERL).

Many Western oil-producing countries have gas recovery rates of at least 95%, but in Russia and several oil-producing countries in Africa, Asia, Latin America and the Middle East, recovery rates are more likely in the range 50 to 80%. "Large quantities of unrecovered associated gas is a terrible waste of a potentially valuable resource, which in poor countries could have been used as a source of energy if only the infrastructure had been there," said Höglund-Isaksson.

But now that it is clear where the excess methane is coming from, it should be possible to stem some of the leaks. "Routine monitoring of emissions and leaks should be a standard requirement by governments when granting companies drilling rights," said Höglund-Isaksson.

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