A simple way to think about uncertainty

Lukewarmers like to complain about the state of our scientific knowledge; they like to complain about uncertainty, especially in impacts, where there is in fact a lot of uncertainty.

Is there a way to make good policy despite this very real uncertainty? Can it be true the uncertainty reinforces the case for action, or is that a contradiction?

We have been enjoying a very stable climate over the past 8,000 years. The entirety of recorded history has taken place within a narrow range of long-term average temperature, +/- 0.5C. Now we are increasing the temperature far outside that range, to a level not seen on Earth for millions of years. Is that safe?

Forget about sea levels, extreme weather, drought, food production and all the rest of it for a moment. There is, indeed, significant uncertainty about how much of the above will happen and how fast, even if, in truth, the uncertainty is more between "expensive and destructive" versus "catastrophic" rather than between "good" or "bad." Forget about the specifics, and imagine a spaceship.

This ship includes the entire population of humanity -- one billion people, say (never mind what happened to the old Earth -- Death Star got it, or something). And it is going to travel to one and only one planet. That planet will now be humanity's new home. And you know literally nothing about it (scenario one).

Suppose the ship's computer informs the crew that the climate on New Earth is changing, and unless you expend significant resources to prevent it (deploying solar mirrors in advance of your arrival, or what have you) the temperature will be 3C warmer when you arrive.

What is your response? Clearly, you don't care. The planet may be just three degrees to cold to support life now. Or it might be an uninhabitable volcanic hell, and 3C more will be nothing. Warming could make things better, or worse, or make no difference. With no way to know, you aren't going to spend resources to try and control the climate.

This is the circumstance some pseudoskeptics and "lukewarmers" feel we are in. We don't know what the effects of warming will be, they could be good or bad, so it would be folly to decarbonize our economy on that basis.

But scenario one is unrealistic; we don't know nothing about this warming planet. We know a large number of us are living on it and have been for some time. So change the scenario a little bit. Suppose we know exactly one thing about New Earth; we know that it will support one billion human lives. Now, same question. Three degrees warmer, or expend the resources to prevent it.

This is a totally different calculation. You know the planet's climate will support human civilization, that the people on the spaceship can live there; you do not know if a new, warmer climate would. If you have a climate that will support human civilization on the one and only planet on which humans can live, you would be uncommonly stupid to do anything to significantly alter that climate. And the less you knew or could predict about the response of that climate, the more stupid it would be.

We know one important thing about the earth's climate system; we know that the temperatures of the last 8,000 years will allow billions of humans and complex industrial civilization to thrive. We do not know that about a world that is warmer than has existed since humans came down from the trees. Scientists are doing their useful work to increase our understand of the climate and to predict the effects of warming. But as citizens evaluating policy, we do not need to know what will happen to the sea level or the rainfall, to the methyl hydrate deposits or the thermohaline circulation. This climate allows our civilization to survive; a new climate may or may not. That's what you need to know.