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 mid-century.

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 is not so clear. The group examined about 900 emissions mitigation scenarios and figured that the median estimated cost of meeting the 2-degree goal would total about 0.06 percentage points of global gross domestic product per year over the course of the century. That huge amount of money could represent the difference between abject poverty and decent conditions for people in developing countries, depending on who sacrifices to bring emissions down. On the other hand, the report noted that reducing greenhouse emissions also would result in a variety of positive side effects, including significant reductions in illnesses and deaths associated with air pollution. It is just very hard to quantify the full cost picture.

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. World governments have made promises to reduce their reliance on carbon dioxide. But if they make good on current pledges — which have not been the best indicators of what they actually accomplish — they can only be confident of keeping warming below 3 degrees Celcius.

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. In practice, that system would have to develop over time as many countries, led by the United States, individually establish carbon taxes or equivalent policy, then pressure others to join in.

But, given the many political barriers to achieving that soon, it is important to ask: What is the backup plan if the world overshoots its emissions goals? Typically, that has not been a question environmentalists have wanted to entertain. But, given the world’s current trajectory, they must.