Global climate change is accelerating and human-caused emissions of greenhouse gases are the overwhelming cause, according to a landmark report released Monday by the United Nations. There is still time to avoid catastrophic warming this century, but only if countries around the world stop burning fossil fuels as quickly as possible, the authors warn.

The message to world leaders is more dire, and more unequivocal, than ever before.

"It is indisputable that human activities are causing climate change," says Ko Barrett, the vice chair of the U.N. Intergovernmental Panel on Climate Change (IPCC) and the senior adviser for climate at the U.S. National Oceanic and Atmospheric Administration. "Each of the last four decades has been the warmest on record since preindustrial times."