There's several layers of uncertainty in making climate projections. The first one is just knowing how much greenhouse gases will be emitted. That’s why the IPCC considers a range of future emission scenarios. But then within each scenario, there’s an uncertainty that comes from physical processes.
I work on climate sensitivity, which is this basic question: how much global warming can we expect for a given increase in CO2. And to answer that question, we found that a lot of the uncertainty comes from clouds. Even fairly subtle changes in the amount, or type, or other properties of the clouds can have a pretty large knock-on effect on the amount of warming that you’d expect.
We can expect clouds to change due to global warming, and in turn those changes could accelerate warming further. There are two main changes that are important. One is that with global warming, we expect the amount of low clouds to decrease. Because clouds act a bit like a parasol and reflect sunlight back to space, if you have fewer of those parasols, then the surface ends up warming more as it absorbs more sunlight. And then the other change that we are quite confident about is that high clouds will rise to even higher altitudes. That will increase their greenhouse effect, because as well as reflecting sunlight, clouds also have a greenhouse effect of their own. It could be described as blanket. If clouds rise, this blanket effect actually increases. So we call both effects ‘positive feedbacks’, because they increase warming.
These effects were already in the models, just with large uncertainty. So on average, it seems that models were actually getting it right. We’re not suddenly finding that projections are going to be a lot worse, it’s more that we’re able now to reduce the range of uncertainty in these predictions.
There’s a lot of other uncertainties. We have to think about low-likelihood but high-risk scenarios. One really important one is the ice sheets. Ice sheets can melt of course, but they don’t just melt. With enough warming they can become unstable, so you could have a situation where ice sheets collapse, meaning they start flowing much faster and the rate of ice loss becomes much bigger. This would then have huge implications for how much sea level rise you can expect. It’s something that’s very hard to understand, because we can’t really model it reliably. So that’s a big question mark for future climate change.
For us as climate scientists, it’s a challenge sometimes to talk about these uncertainties. We want to be honest, but at the same time we don’t want appear as if we don’t know what we’re talking about. That’s why I think it’s also important to talk about the known facts, for example, that increasing greenhouse gases does cause global warming: there’s absolute consensus on that.