The authors found that tackling the problem in a serious way would carry large costs, shaving a few hundredths of a percentage point off global economic growth each year.

By the end of the century, societies would most likely be far richer than today, but almost 5 percent poorer than they would have been if they had not spent the money to protect the climate, according to the study.

“Climate policy is not a free lunch,” Dr. Edenhofer said at a news conference Sunday in Berlin.

Against those costs, the economic benefits of acting are essentially impossible to calculate, the report found.

The biggest reason is that scientists do not know how likely it is that unchecked global warming could cause some sort of wildly expensive calamity, such as a rapid melting of ice sheets that would drown the world’s major coastal cities. This and other disasters are distinctly possible, the authors found.

In essence, the committee described money spent fighting climate change as a form of insurance against the most severe potential consequences. “It is up to the public and up to decision makers to decide if it is affordable or not,” Dr. Edenhofer said.


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