Specificity can help reduce the numbing complexity of climate change to something that we can all understand — and fear. And perhaps that is the first step in mobilizing to fix the problem. But scientists speak in probabilities. They can measure where we are and venture predictions of where we are going; they cannot tell us precisely where temperatures will end up, what the impacts will be, and where important tipping points lie along the way.

Global atmospheric levels of carbon dioxide passed 400 parts per million earlier this year — higher than at any time in the last three million years. Even at these concentrations, we are facing enormous uncertainties.

Roughly three million years ago, global sea levels were 50 to 80 feet higher than today, and camels lived in Canada, which just goes to show how large the uncertainties truly are. We aren’t anywhere close to turning this around. The atmospheric concentrations of greenhouse gases are still going up, and that increase is still accelerating.

What this will mean for future temperatures is hard to pinpoint with precision, but we estimate that without further action to reduce emissions, the planet is on track to see the eventual global average rise by at least 3 degrees Celsius (5.4 degrees Fahrenheit) above preindustrial levels. This is most likely past the point when we will see the melting of the ice sheets in Greenland and West Antarctica, raising sea levels by dozens of feet. But putting too much emphasis on one particular temperature figure is like zeroing in on the year 2047.

What is scarier still is the uncertainty about the truly extreme outcomes. Our own calculations estimate that there is a roughly 5 percent to 10 percent chance that the eventual average temperature could be 6 degrees Celsius higher, rather than 3. What this would mean is outside anyone’s imagination, perhaps even Dante’s. We can obsess about all of these scenarios. A rise of three degrees would be bad enough. But when you factor in the uncertainty, there is even more reason to put global warming on an even more sharply decreasing path.