Of all the reports that the U.N.-chartered Intergovernmental Panel on Climate Change (IPCC) has released over the last several months, this week’s might be the most distressing. The authoritative body of scientists, economists and other experts previously warned that the planet is warming, that humans are primarily responsible and that uncontrolled climate change would have a range of unwelcome effects. Those conclusions would not be too worrisome if the world were well on the way to heading off major problems. But the IPCC found the opposite.

World governments face a massive task. Scientists have recommended keeping the cumulative rise in global temperatures below 2 degrees Celsius or 3.6 degrees Fahrenheit. Getting there, the IPCC reckoned, would require a decrease in planet-warming greenhouse-gas emissions, particularly carbon dioxide, by 40 to 70 percent from 2010 levels by 2050. Greenhouse emissions must be near or below zero by 2100. That sort of transition requires low-carbon energy production to at least triple by midcentury.

Some observers argue that the IPCC’s latest report shows that such a revolution in how we fuel the global economy would not actually cost that much. But it isn’t so clear.

That uncertainty partly explains why the world is heading in the wrong direction. Humans released more carbon dioxide between 2000 and 2010 than in any decade before. The Great Recession resulted in an emissions dip, but the trend resumed its upward course as the world economy recovered. The smartest policy scenario is one that hedges against the possibility of very bad consequences as cheaply as possible, one in which “all countries of the world begin mitigation immediately” and for which “there is a single global carbon price,” in the IPCC’s words.