Scientists work in a world of uncertainty and nuance. They live by caveats. They get ahead by grabbing conventional wisdom and picking it apart.

Politicians live in a world of black and white. They like bottom lines. They get ahead by grabbing conventional wisdom and running with it.

The clash between scientific uncertainty and political reductionism lies at the core of the fight over what to do about climate change. The question: How sure do people need to be about the kind of damage global warming might cause before they’ll accept big changes—some of them expensive—to try to keep the problems at bay?

This tension has massive economic and environmental stakes. And it’s the unspoken subtext of a report the Obama administration released to much fanfare this week detailing ways it says global warming already is hurting the United States and is likely to hurt it more in the future. The report, called the National Climate Assessment, has generated headlines for the the scary climate-change impacts it lays out: more-severe storms, declines in some crop yields, increases in certain diseases.

What has gotten little attention—but what history suggests could prove a big, unwelcome surprise to the public—are the scientific equivocations that underlie many of those frightening projections.

Administration officials stressed the bottom line in releasing the report. “The science is clear. We have to act,” Dan Utech, President Obama’s special assistant for energy and climate change, told an audience at a New Republic climate-change forum in Manchester, N.H., on Tuesday, the day the report was released. The report “is really the latest, greatest, most authoritative assessment of what the impacts will be in the U.S.,” he said.

The report—the result of several years of work by hundreds of scientists from dozens of institutions—may well be the most authoritative assessment to date of the impacts of climate change in the U.S. But that doesn’t mean it erases all scientific doubt about global warming’s effects. To be sure, the presence of doubt needn’t change the report’s fundamental conclusions. Some will say the doubt does deflate the report’s importance; others will say it doesn’t. The point is that whatever doubt exists needs, in the interest of transparency, to be explained and understood.

One of the report’s most striking features, for instance, is a series of chapters each focused on climate-change impacts in one region of the U.S. “Evidence of climate change appears in every region and impacts are visible in every state. Explore how climate is already affecting and will continue to affect your region,” says a teaser on an impressively user-friendly website that breaks the report down into easy-to-digest pieces.

If you live in, say, Atlanta, and you click onto the report’s section on the Southeast U.S., you read this as an opening: “Sea level rise poses widespread and continuing threats to the region’s economy and environment. Extreme heat will affect health, energy, agriculture, and more. Decreased water availability will have economic and environmental impacts.”

But if you click further, and read the full chapter on the Southeast, things quickly get murky. Take the issue of precipitation. “Because the Southeast is located in the transition zone between projected wetter conditions to the north and drier conditions to the southwest, many of the model projections show only small changes relative to natural variations,” the report says. Translation: Across the Southeast, climate change may not significantly alter precipitation after all. The report notes that “many models do project drier conditions in the far southwest of the region and wetter conditions in the far northeast of the region, consistent with the larger continental-scale pattern of wetness and dryness.” Yet questions loom. “Projections of future precipitation patterns,” the report notes in several places, “are less certain than projections for temperature increases.”

The uncertainty about potential changes in the rain-and-snow patterns extends to other possible regional effects of climate change. Consider public-health impacts. “Increasing temperatures and the associated increase in frequency, intensity, and duration of extreme heat events will affect public health” and other parts of life, says a “key message” in the report’s Southeast chapter. But how it will affect public health remains up in the air. “It is still uncertain how regional climate changes will affect vector-borne and zoonotic disease transmissions,” says the second paragraph under the “key message” headline.

This is wonky stuff. But the issue of scientific uncertainty about the potential regional effects of climate change is more than academic.  Carbon dioxide, the chief greenhouse gas, can’t be seen, smelled or, in any real way, touched. As the White House’s Utech noted on Tuesday, that makes climate change an “amorphous” environmental issue—less tangible than, say, rivers full of sewage or air yellow with smog. According to polls, Americans are likelier to support moves to curb greenhouse-gas emissions if they’ve actually seen what they believe to be tangible effects. Scientists and politicians know this, which is why those who want to spur action to curb global warming are increasingly preoccupied with providing proof of climate change’s effects on a scale that matters to John and Jane Doe.

The attempt to project climate-change effects on scale that’s local enough to resonate with most people has backfired before. The Intergovernmental Panel on Climate Change, a global body involving multitudes of scientists that publishes a compendium of the latest climate science every few years, was embarrassed following its 2007 report when it was forced to acknowledge that two predictions about how climate change might affect life in specific parts of the world were wrong. In one error, the report claimed that glaciers in the Himalayas could disappear by 2035. In another, the report warned of severe cuts in African agricultural yields as the result of climate change. After the report was published, IPCC officials conceded that the science didn’t support those claims. IPCC officials said, and subsequent reviews of the panel agreed, that neither error impugned the report’s basic conclusions about the causes or potential global impacts of climate change. But the errors stood as a black eye against the IPCC.

Uncertainty is anathema to politics, but it’s fundamental to science. The reality is that the computer models scientists use to project the effects of greenhouse-gas emissions are, at some level, imprecise. Like someone who has trouble seeing things up close, the models get less precise as scientists use them to try to project climate change’s effects on a finer scale. So projecting what might happen to the average global temperature is one thing. But projecting what might happen to the average temperature in a state or even a city—not to mention projecting how such a temperature increase might affect rainfall or disease transmission—is quite something else. The inconvenient truth for partisans pushing for tougher action on climate change is that that sort of perilously granular projection is precisely what’s likeliest to get people to sit up and take notice.