There is a recent post on Watts Up With That (WUWT) called in retrospect we predicted global warming would slow. It’s about a recent paper by Guemas, Doblas-Reyes, Andreu-Burillo & Asif (Retrospective prediction of the global warming slowdown in the past decade, Nature, 3, 2013). I have a feeling that WUWT has already discussed this paper, so maybe they’re running out of things to say.
Anyway, the basic implication of the post and the gist of most comments is that the term “retrospective prediction” is ridiculous. In other words, it’s easy to predict something if you already know the answer. Let’s think about this a little more and consider it from a scientific perspective. I develop a model that I use to predict the future evolution of something. However, I later find that my prediction isn’t quite what happened. What do I do? Do I throw up my hands and say, “Oh well, I was wrong. I give up. Someone else’s turn.” No, I go back and try to work out what was wrong with my model. Let me stop for a moment though. The issue in this case is that the observed global surface temperatures have risen more slowly than would have been expected based on the model predictions. They are, however, within the range of most models. The models aren’t absolutely wrong. It’s just that the model predictions of the most likely trend is higher than that observed.
Okay, let’s say I start to consider why my model results differ from what’s observed. Normally one would add things that previous models ignored. What do you do then? Well, you ask the question “what would my model have predicted had I included this originally?”. What would I call such a test? Well, I might consider calling it a “retrospective prediction”. Seems like a reasonable term to me; what – in retrospect – would my model have predicted. To me, mocking papers such as this makes those on WUWT seem infantile and ignorant. To go back and work out why the prediction of a set of models differed from what was observed is precisely what science is about. Not doing so would be unscientific.
Let me, however, highlight one paragraph from the abstract of the paper
Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m−2 interval during the past decade, which is successfully captured by our predictions.
This is a very important paragraph. It makes the claim that the most reasonable explanation for the slowdown is an increase in ocean heat uptake. Furthermore, their model correctly determines the top-of-atmosphere net energy input. This is crucial. Global warming is fundamentally about the increase in energy in the climate system and is not only about the increase in global surface temperatures. If their model is getting this correct, then that means that it is correctly determining the level of global warming as well as correctly “retrospectively” predicting the rise in global surface temperatures.
As a consequence of what I said above, I was going to be critical of the WUWT post for simplistically associating a slower rise in surface temperatures with a slowdown in global warming. Why? Because there is evidence that global warming has not slowed down; it is simply that a larger fraction of the excess energy is going into the oceans and hence less is available to heat the surface. However, the title of the Guedas et al. (2013) paper is “Retrospective prediction of the global warming slowdown in the past decade”. I think that this is a very unfortunate choice of title and actually adds to the confusion. In a sense its contradictory. The slower than expected rise in global surface temperatures is not – in itself – an indicator of a slowdown in global warming and the main result of this paper is to show exactly that. The slower than expected rise in global surface temperatures is not because there has been a slowdown in global warming, it’s because more energy than expected has gone into the deep ocean. My main criticism of the paper is, therefore, that they could have been more careful in their choice of title.