The surest, least costly way to avert the worst impacts of climate change would be for global carbon pollution to peak by 2020, then fall to zero by 2100, the Intergovernmental Panel on Climate Change reported yesterday.

“We have the means to limit climate change,” said IPCC Chair Rajendra Pachauri. “The solutions are many and allow for continued economic and human development. All we need is the will to change.”

The IPCC said climate change has already increased the risk of severe weather and heat waves, food shortages, and violent conflict, and will produce “severe, widespread, and irreversible impacts” without deep, rapid reductions in carbon pollution.

That language, coming from scientists who are conservative by nature, “falls just short of announcing that climate change will produce a zombie apocalypse plus random beheadings plus Ebola,” wrote 350.org founder Bill McKibben.

But the report “almost certainly underestimates the actual severity of the situation,” since the authors err on the side of understatement and the most recent research came in too late to be included in the IPCC assessment.