The good news, in some twisted philosophical sense, is the confirmation of bad news. Climate science has progressed over the past couple decades to the point where it is now possible to make the definitive statement that it is extremely likely that global warming is caused by human activity.
But there’s an additional piece of bad news: the false sense of security conveyed by repeating how certain science is. We know enough to act. Ignoring that reality, by now, would amount to willful blindness. But what’s not known has the potential to make things significantly worse.
We know, for example, that last time concentrations of carbon dioxide were as high as they are today, sea levels were up to 66 feet higher than they are now. Camels lived in Canada. That was a bit over 3 million years ago, during the Pliocene epoch, and today’s world obviously looks much different.
The best available climate models come close in their temperature projections to what the world experienced during the Pliocene, but they aren’t predicting sea levels of up to 66 feet higher. Nor do they predict camels wandering around Canada. Not now, nor hundreds of years from now. Why? Donald Rumsfeld would chalk it up to “unknown unknowns.” Nassim Nicholas Taleb’s term is “Black Swans.”
Economists have a way of thinking about projections that can help clarify how this sort of issue should be treated. We typically like to point out how Frank Knight, in his dissertation in the 1920s, at the University of Chicago, gave us the ironclad definitions of "risk" and "uncertainty." The former describes what it’s like to play a game of cards and not knowing which card will be next. The latter describes when you don’t know the distribution from which your card will be drawn. Harvard’s Richard Zeckhauser has added a third category: "ignorance," when you don’t even know what game you are playing.
Talk of “more likely than not” implies that we know the distribution, but more often than not, we don’t.
Meanwhile, most climate models are unduly skewed toward the known, sometimes making them much too conservative. Until recently, most climate models predicted rising sea levels only based on thermal expansion of the oceans (and the melting of mountain glaciers), but they did not include the effects of melting ice sheets. Warmer waters take up more space, leading to higher sea levels. That mechanism alone has indeed contributed to over a third of sea-level rise in the past two decades. It’s also clear that melting glaciers in Greenland and Antarctica raise sea levels, but by how much is highly uncertain. Call it a "known unknown." Until recently, scientific understanding of melting polar ice caps had been so poor that most models simply left it out. The IPCC did not include it in its projections until 2013.
Even though climate models do get a lot of things right, there are still lots of uncertainties built in. They’ll always be there, at least until well after it is too late to do anything about the problem in the first place. Uncertainties exist around the amounts of global warming pollutants we emit, the link between emissions and atmospheric concentrations, the link between concentrations and temperatures, the link between temperatures and physical climate damages, and the link between physical damages and their consequences. Also, at least as important, there’s uncertainty about how society will respond: what coping measures will be undertaken, and how effective they will prove to be.