The U.N.'s chief climate diplomat has urged the coal industry to diversify to cleaner energy sources and leave most of the world's remaining coal reserves in the ground.

On the sidelines of a U.N. climate conference, Christiana Figueres told CEOs of coal companies meeting Monday at Poland's Economy Ministry that their industry needs to change radically to curb emissions of heat-trapping gases that scientists say are warming the planet.

The coal event is seen as a provocation by climate activists, who climbed the roof of the Economy Ministry and unfurled a banner that read, "Who rules the world? Fossil industries or the people?"

Poland generates about 90 percent of its electricity from coal and Economy Minister Janusz Piechocinski said coal must be a part of talks on reducing CO2 emissions.