That is not the same as claiming that all questions about climate change have been answered. In fact, enormous questions remain, and the science of global warming entails a robust, evolving discussion.

The new report walks through a series of potential consequences of planetary warming, without asserting that any is sure to happen. They are possibilities, not certainties, and the distinction is crucial for an intelligent public debate about what to do. The worst-case forecasts include severe food shortages as warming makes it harder to grow crops; an accelerating rise of the sea that would inundate coastlines too rapidly for humanity to adjust; extreme heat waves, droughts and floods; and a large-scale extinction of plants and animals.

“What’s extremely clear is that there’s a risk, a very significant risk,” Dr. Molina said by telephone from Mexico, where he spends part of his time. “You don’t need 100 percent certainty for society to act.”

Some of the scientists on Dr. Molina’s committee like to point out that people can be pretty intelligent about managing risk in their personal lives. It is unlikely that your house will burn down, yet you spend hundreds of dollars a year on insurance. When you drive to work in the morning, the odds are low that some careless driver will slam into you, but it is possible, so we have spent tens of billions of dollars putting seatbelts and air bags in our cars.

The issue of how much to spend on lowering greenhouse gases is, in essence, a question about how much insurance we want to buy against worst-case outcomes. Scientists cannot decide that for us — and the report recognizes that by avoiding any specific recommendations about what to do. But it makes clear that lowering emissions, by some means, is the only way to lower the risks. Because so many people are confused about the science, the nation has never really had a frank political discussion about the options.

Only a few decades ago, the world confronted a similar question regarding chemicals called chlorofluorocarbons, then common in refrigerators, air-conditioners, cans of hair spray and deodorant.