Europe’s use of the fossil fuel spiked last year after a long decline, powered by a surge of cheap U.S. coal on global markets and by the unintended consequences of ambitious climate policies that capped emissions and reduced reliance on nuclear energy.

The new dependence on one of the dirtiest fuels shows just how challenging it is to maintain the momentum needed to go green, analysts and officials say, and demonstrates the far-reaching effects of America’s natural gas boom…

“It’s been very welcome that U.S. greenhouse gas emissions have been going down because of the switch to gas,” said David Baldock, executive director of the Institute for European Environmental Policy in London. “But if we’re simply diverting the coal somewhere else, particularly to Europe, a lot of those benefits are draining away.”