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New coal mines are leaking methane gases that are in some cases just as destructive to the environment as the pollution released from burning the coal itself, according to a new study.

Methane, a greenhouse gas 80 times more potent than carbon dioxide in its first two decades, leaking out of some mines could be having as much of an impact on global warming as burning the coal they produce, researchers with Global Energy Monitor, a San Francisco-based non-profit group, said in the study. The amount of methane that would leak from new coal mines currently being proposed globally would do as much damage as all of the coal power plants in the U.S. combined.

“Methane emissions from coal have received much less attention from researchers and governments than methane emissions from oil and gas,” researchers said in the report. “The gap in attention has resulted in a deficit in implementation of measures to mitigate what by all estimates is one of the most significant sources of greenhouse gases.”

Methane, the simple hydrocarbon that makes up most of natural gas, is the second-biggest contributor to global warming after carbon dioxide. While releases are smaller in volume and it stays in the atmosphere less time, it can trap far more heat than CO2 on a relative basis. Coal mining accounts for about 9% of human-related methane emissions, according to the Global Methane Initiative.

Methane on averages increases total greenhouse gas emissions at major operating mines by about 20% when measured on a 20-year horizon, Global Energy Monitor found, with that level rising up to 50% in the gassiest mines.

Global Energy Monitor examined potential methane releases at 432 proposed new or expanded coal mines, and found that at the gassiest of them the leaks could account for half of their total greenhouse gas impact. If all were built, about 13.5 million tons of methane would be released annually, doing the damage of an equivalent of more than 1 billion tons of CO2.

By far the largest potential source is China, where 140 new mines are under development. But the biggest single project is Valiant Resources’ Hutton development in Australia, which would produce the annual equivalent of 60 million tons of CO2 if built out to full ambitions.