You climate scientists and your models, part 1

Since my op-ed on hurricanes was published in the New York Times, I have gotten a number of emails and comments on this blog about it. Some of these can be characterized as climate-denying hate mail. This is inevitable any time one publishes virtually anything about climate in a prominent forum. It isn’t practical to respond to every one individually, and in some cases I don’t think it would be productive to do so. But I do spend some time thinking about what people write, if it seems that they’ve put any thought into it – even if that thought is expressed in a hostile way – and want to respond in a general way to some of it.

I should explain first that you won’t see the comments I am talking about here. This is a moderated site, moderated by me, and I don’t approve climate denialist comments. You can tell me I’m wrong about something and that’s fine – I’ll still approve it, if it’s written in a way that suggests you’re open to evidence and argument. But I don’t approve comments that parrot the standard tropes you hear on Fox News and other standard anti-science sources in order to justify complete rejection mainstream climate science. Typical indicators of such comments are conspiracy theories, mentions of Al Gore*, calling me or other climate scientists “liars” etc. There are plenty of places to vent this kind of stuff online; I’m not going to host it here for my little blog’s twelve readers.

That said, today I want to respond to one criticism that was common to a number of the responses I got, though. This is that my argument was based to a large extent on models and theory. Several critics wrote that this is not real “evidence”.

(One can also argue that the models are bad, having been disproved by observations, and so should be disregarded. In a few days, I’ll put up a second part to this post in which I’ll respond to that. Today I’m just writing about the general question of whether models and theory can constitute evidence for a scientific claim at all.)

When we are talking about the future, models and theory are essential, because there can be no prediction of the future without a model or theory of some kind. If you think that the only valid evidence in a scientific argument is an observation of something that has already happened, then you can’t talk about the future at all. It hasn’t happened yet, so there are no observations of it.

Science is about taking observations and integrating them within the context of ideas about how the world works – theory and models – using them to test those ideas, and then, to the extent the ideas hold up, using the ideas to make predictions of things that haven’t been observed yet. If you want to disqualify predictions based on models and theory from science, then you also have to argue that science can’t make any predictions about the future of anything. If you really believe that, then say that. But don’t say there’s something wrong with climate science in particular because it uses models.

(Of course, there are some predictions that are so trivially easy that they seem not to require a model. For example, I predict the sun will rise tomorrow. We could argue about whether I am still using an implicit model of some kind to make that prediction, but it doesn’t matter here because climate change is not a case of this kind. Let’s restrict ourselves to nontrivial predictions, where we can’t rely on very simple patterns from past experience to tell us what will happen.)

That said, there are models and there are models. I argued in the op-ed that “physics” agreed with “models” and that that showed consistency between two lines of evidence. One commenter argued that the models are also based on physics, so these are not independent. Depending on how you interpret the words “physics” and “models”, this can be read as a fair criticism – assuming you read only the op-ed, which had to skip over a lot of the argument in our paper in Science on which it was based. I used the word “physics” to mean the same thing as what I am calling “theory” here.

(I’m sorry that the Science paper itself is behind a paywall; I am told by the journal that I will soon get a link allowing me to legally provide the full paper for free from my academic publications web page, but I don’t have that yet.)

The Science paper focuses on a quantity called potential intensity. This derives from a theory in the tradition of physics. (This does not mean it is speculative, or just a hypothesis unsupported by evidence. It means it is a set of logically connected statements that explains and synthesizes many observations. Newton’s theory of gravity is a theory too.) Potential intensity theory contains an explicit set of statements about what a hurricane is and how it works. It then takes the equations of classical mechanics and thermodynamics and makes a set of approximations and simplifications based on those statements. (For example, the hurricane is assumed to be circularly symmetric, so the equations simplify from three dimensions to two, radius and height.) This leads to a prediction of how strong a hurricane can get within a given local climate. Most hurricanes don’t reach their potential intensities, but how strong a real hurricane gets, on average, is still related to the potential intensity. So if potential intensity increases, we predict that real hurricanes will get stronger. Knowing whether potential intensity will increase or not requires us to make a detailed prediction of how climate will change – not just the surface temperature, but the temperature and humidity throughout the atmosphere as well. We can do this with climate models.

The distinction between a “theory” like potential intensity theory and a “model” like the ones used to make weather forecasts or climate change projections is one of complexity. The models we use for real weather and climate prediction are very big, complex computer codes that solve the equations of physics as applied to the atmosphere. The solutions can’t be written down in simple form, but can only be represented approximately by huge numbers of numbers, which we typically visualize in maps and charts. The models make some approximations too, but not nearly as many or as severe as a theory like potential intensity does. They retain as much of the atmosphere’s full complexity as they can (quite a lot, on today’s powerful computers) and the results of their simulations can’t generally be predicted ahead of time.

The climate models used to make climate change projections generally don’t simulate hurricanes well, because they don’t have enough resolution – just like a digital camera image without enough pixels to make out something small. They do simulate the climate well, though. (Not perfectly, of course, but remarkably well; but defending that statement is not today’s topic.) We take those models’ predictions of the local climate all over the world in the future, and calculate the potential intensity from it. We compare that to the potential intensity that the same models produce in the present climate, and use the difference to predict how hurricanes’ intensities should change. That is one piece of evidence.

Then there are some climate models that have high enough resolution to simulate hurricanes well. With those models, we don’t need to use potential intensity theory. We can just run them in a warmer climate and see how their hurricanes change compared to a cooler climate. That is another piece of evidence. It’s different than the one in the previous paragraph, because it involves direct simulation of hurricanes and does not require us to accept potential intensity theory as valid. The two kinds of models are similar in how they predict the climate itself, but completely different in how they represent hurricanes. And my argument in the op-ed is not about climate change itself – I take for granted that we know how the climate as changing, at least in broad outline – but rather how that climate change affects hurricanes. So for this purpose, it’s two pieces of evidence, both of which indicate that warming due to greenhouse gases, acting alone – that is, without too much aerosol cooling to counteract it – should cause hurricanes to strengthen.

Yes, both pieces of evidence are from models. But again, if you want to predict the future there is nothing else.

 

*Al Gore’s movie, An Inconvenient Truth, was reasonably accurate, as I recall, given the climate science of its time. And while I haven’t watched it since then, I’d say it’s probably still mostly right, at least in broad outline. I do recall, however, that the material about hurricanes was overstated even given the science of the time, and the science on that topic has advanced a lot further since then. But in any case, Al Gore is a politician, not a scientist, and his movie was not a statement by the scientific community. When people say climate science is wrong because they don’t like Al Gore or his movie, it is an indicator – usually accompanied by others – that they are forming their opinions based on political affiliation rather than consideration of evidence or argument.