HUMANS ARE having a hard enough time coping with the natural variability in our environment, which causes disasters such as heat waves, wildfires and floods. Just wait until climate change makes all three of those problems — and many more — worse.

That was the stern warning from the world’s scientific community last week, in the latest report from the Intergovernmental Panel on Climate Change (IPCC). The U.N.-chartered body produced its first comprehensive report since 2007 on the changes that might accompany a rising global temperature and on humanity’s potential to cope with them. It isn’t encouraging. A more rational Washington wouldn’t have needed this document to formulate a better plan for handling the many risks; that would have happened long ago. It’s a measure of the country’s dysfunctional debate on global warming — primarily the fault of Republican cynicism or senselessness — that many lawmakers want no such plan and will ignore this document, as they have many before it.

There are some things the scientists are very confident will happen in a warming world over the next century. Sea levels will rise, threatening coastal cities and low-lying nations. Island states could get hit very hard. There are probably going to be relatively more extremely hot days than extremely cold ones, and water- and food-borne illnesses will probably increase. Various sections of North America will probably be drier and more prone to wildfires, while cities will likely see more urban floods.

But part of the peril of climate change is that scientists don’t have a crystal ball to foretell how, exactly, the environment will respond to rising temperatures, and they are trying to predict effects over the course of decades. The experts anticipate that forests could die off and that wetland and rain forest ecosystems could crumble. They anticipate effects on water systems just about everywhere: more flooding in many places, less water in dry areas, more humidity inhibiting human labor in wetter climes.

There could also be some positive effects for some, the experts note. They predict fewer deaths due to cold, but more due to heat. Some areas of the planet, particularly in higher latitudes, might become more fertile. But lower crop yields elsewhere will outweigh those benefits. Indeed, the planet’s verdant lands along the equator might be much worse off. The scientists, for example, predict that fish and other aquatic life will flee elsewhere as ocean temperatures change. And, of course, there could be effects, good and bad, that the experts aren’t anticipating. That shouldn’t be comforting.

The experts leave little doubt about the right response: Cut pollution to head off the worst possible consequences and prepare for the risks the world is unlikely to avoid, given its inability to slash emissions quickly. Delaying action, they note, reduces the world’s options and affords vulnerable people less time to cope.

How much change should be headed off with emissions cuts now, and how much will we simply have to cope with? How much should we pay to prepare for the risks we are unwilling to avoid? How do we get the major countries of the world all moving in the right direction? These are essential questions that should be at the heart of our political debate. But answering them demands that both parties admit there is a dangerous problem that demands attention — now.