For more than 30 years, scientists have tried to inform governments and the public on the urgent threat of global warming, working hard — as scientific norms demand — to acknowledge honestly the limits to what they know. It hasnt worked. As a recent report from the United Nations Environment Program shows, global carbon dioxide emissions have gone up, not down, even in the few years since the once-promising Paris Agreement. Nations own projections for fossil fuel use suggest that emissions will keep rising at least through 2030, making the climate problem worse. Much of this is the direct result, of course, of a decades-long disinformation campaign funded by fossil fuel interests and right-wing politicians. But could the way scientists communicate also be part of the problem? Psychology studies show that people tend to accept a message more readily if they hear it expressed with brimming confidence, uncertainties suppressed. And recent experiments suggest that scientists going the extra mile to be honest about their uncertainty may actually make people less likely to trust them. But this finding isnt wholly depressing. It also offers some ideas on how scientists can show their uncertainty in a less self-defeating way. The key, it seems, is to realize that people understand that climate is not perfectly predictable, and so they expect to hear about a range of possible outcomes. But emphasizing the vast complexity of the problem only seems to overwhelm them. Psychologists know that people are often less likely to accept a message if they hear it alongside qualifications and admissions of uncertainty. For example, studies find that judges follow the advice of expert advisers when they seem more confident and certain. Eyewitnesses tend to be more convincing the more certain they are about what they saw; when they are uncertain, it stirs doubt. But few studies have looked in detail at such situations as climate change, where significant uncertainties obviously cant be eliminated, and therefore no mention of uncertainty would clearly be suspect. If everyone accepts there is uncertainty, how should scientists talk about it? Social psychologist Lauren Howe and colleagues undertook an experiment with more than 1,000 Americans across the social and political spectrums, testing to see what they took away from messages about future sea-level rise expressed in different ways. Some were given just a single prediction of the most likely rise by year 2100; others were given a most likely prediction along with an upper and lower bound (“We expect sea levels by 2100 to rise between one and seven feet”). Finally, a third message stated a most likely value, added a “worst case” estimate of how high levels might conceivably go, and also expressed the further view that it is probably impossible to accurately foresee the full consequences of sea-level rise given so many prevailing uncertainties, such as how powerful and frequent future storms may be. An encouraging finding is that people responded positively to the “bounded uncertainty” expressed through the upper and lower bounds. People accepted that message even more readily than the message made with no uncertainty at all. Admitting to uncertainty helps spread the message, Howe suggested to me in an email, because it builds trust. People understand that climate is an issue that involves inherent uncertainty, so not admitting to it would seem dishonest. Less encouraging, however, is how people responded to what, scientifically, might be considered the most brutally honest message: admitting that the full costs of sea-level rise are extremely difficult to assess. Thats useful, additional information, and certainly true. Yet this expression of open-ended uncertainty tended to increase listeners skepticism. Admission of too much uncertainty seems to make people doubt the science, and get them wondering if the scientists even agree among themselves. So what are scientists to do? The take-home message is that they shouldnt banish uncertainty from their communications but discuss it clearly and, whenever possible, put quantitative bounds on it. Theyre likely to reach more non-experts by doing so. But theres a danger in going too far and hiding behind the awesome complexity of the climate system to avoid making strong statements. Thats a cautious and comfortable place for scientists, but when it comes to irreducible uncertainty — the “unknown unknowns” of the climate problem — talking about this just seems to confuse people. “The public seems to struggle to understand how anyone can act on such information,” as Howe put it. “They think: Wait a minute, if scientists just shared this precise range of possible outcomes with me, why are they now telling me that this precise range doesnt capture everything that could happen?” Bloomberg