Science journalist Mallory Pickett sat down with Schneider to find out how his research will do this, and why it matters.

How much uncertainty is there in current climate models?

There is a measurement called “climate sensitivity.” It’s the global mean surface temperature increase that you get after doubling CO2 concentrations and letting the system equilibrate. With current climate models, the climate sensitivity for doubling CO2 ranges somewhere between two degrees [Celsius] warming up to five degrees warming.

What are the implications?

Take the two-degree target of the Paris agreement. We’ve had about one degree of warming already, so it’s one more degree to go. How much more CO2 can we put into the atmosphere before we have warmed Earth another degree?

For a model that has a climate sensitivity of around two degrees, you can get to CO2 concentrations of close to 600 parts per million. We’re at 410 parts per million, so even if we continue emitting a lot, we won’t reach 600 before 2060 or so. In a model that has a climate sensitivity closer to five degrees, [one more degree requires] about 480 ppm, so that’s only about 70 to go. That’s something we’ll reach in the next two decades or so.

Why the uncertainty?

The single biggest contributor is uncertainties about clouds, and specifically about low clouds in the tropics. Low clouds over tropical oceans reflect sunlight because they are white, and this cools the Earth. We don’t know if we’ll get more or fewer of them as it warms, and that’s the key uncertainty in climate predictions.

One other important piece is how much carbon is being taken out by the biosphere. Right now only about half the carbon that humans emit ends up in the atmosphere. The rest is taken up by oceans and the land biosphere, and we don’t quite know where it goes.

Ryan Young

If there’s so much uncertainty, do we really even know that things will get bad with a lot of CO2?

When you put more CO2 or other greenhouse gases in the atmosphere, they absorb thermal radiation. What happens if you put more of these greenhouse gases in the atmosphere is that everything else being equal, you ought to warm the surface. The physics of that’s completely clear, undisputed by any serious scientist.