BERLIN (AP) — The cost of keeping global warming in check is “relatively modest,” but only if the world acts quickly to reverse the buildup of heat-trapping gases in the atmosphere, the head of the U.N.’s expert panel on climate change said Sunday.

Such gases, mainly CO2 from the burning of fossil fuels, rose on average by 2.2 percent a year in 2000-2010, driven by the use of coal in the power sector, officials said as they launched the Intergovernmental Panel of Climate Change’s report on measures to fight global warming.