A team from the US Department of Energy's Pacific Northwest National Laboratory (PNNL) has said previous methane estimates failed to properly account for emissions from abandoned mines and the higher methane content in deeper coal seams.

"As more coal mines close, the share of coal mines that have been abandoned but are still emitting methane will increase," Nazar Kholod, a scientist at the Joint Global Change Research Institute, a collaboration between the University of Maryland and the PNNL, said at an American Geophysical Union event.

Kholod was the lead author of a study published in December that analysed historical emissions data from active and abandoned mines.

His team found that in 2010, 103 billion cubic meters of methane were released from working underground and surface mines and an additional 22 billion cubic meters from abandoned mines - 50% higher than the estimate of 83 billion cubic meters for that year by the Community Emissions Data System.

Methane emissions from abandoned mines are expected to grow more quickly than those from established mines. The team cites deeper mines, more abandoned mines, and a greater percentage of surface mines from which methane escapes more freely as reasons.

The researchers estimate that by the end of the century, methane emissions will have grown almost eight times what they are today from abandoned mines, and four times what they are from working mines.

Kholod added that climate mitigation strategies alone won't limit emissions from unused mines.

"If you stop producing coal, that doesn't mean that methane will stop being emitted from coal mines," said Kholod. "We can't just take coal out of the equation."