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The “Pause” Might End Soon, Or Possibly Not

Dr David Whitehouse

There are many fascinating things about the “pause” – the 18-year long period of no statistically significant global surface warming. One of those interesting things is the language used to describe it, as a new paper in Nature Climate Change demonstrates.

Roberts et al (2015) use the other term often used – “hiatus” – in the title of the paper, but in the paper itself the lack of any statistically significant global surface temperature rise is refereed to as “unexpectedly modest,” though there were some in the past, including the author’s own institution, who pointed out that in their opinion there was nothing unexpected about the pause. Modest though is too ambiguous a word to use in a scientific paper.

Then there is the question of which side of the statistics to emphasise.

What the paper has done is run lots of computer models – the say 15,000 years of simulated climate – to gain an insight into the relationship between internal climatic variability and external forcing. It’s something that has been done many times before, in essence even though the computer models are diverging from reality in the past 15-years or so, if you run the models often enough, or dare I say torture them enough, then you can convince yourself, and usually the researchers want to be convinced, that the models can explain the “pause” as a low probability event. The idea is that lengthy model simulations allow details to emerge that would be impossible to see in real-world data of about 150-years duration!

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From Roberts et al (2015)

The authors hitch their wagon to the idea that the missing energy is in the depth of the oceans and will cause enhanced warming when it comes out. However the greenhouse effect warms the atmosphere, which in turn warms the surface of the ocean that can allow the heat to travel to lower levels. Given the observations it’s a difficult position to hold despite the suspicion by some that sub-tropical gyres might be the way the heat gets into the ocean. A paper published in the same journal last year suggested that the deep ocean has not warmed in the past decade.

One could extract the finding from the paper that there is, possibly, a 75% chance – or at least somewhat greater than 50% chance – that there will be an end to the “pause” in the next 5 years. The authors put it the other way, however, by saying that there is a surprisingly high – say 25% – chance that the “pause” will continue for this period. So the paper has no real predictive value. If the “pause” breaks then that’s understandable, but if it doesn’t then that’s understandable too. So whatever happens we should not be surprised.

Whatever happens the paper’s use is limited, as all papers like this must be. It looked at the statistics of hypothetical pauses – i.e. those generated by computer models. However, the real slowdown might be caused by additional factors not in the models.

Such research does not mean that the “pause” has been explained. There is still no convincing explanation among many competing suggestions, often mutually inconsistent. Nobody knows what is causing the unexpectedly modest “pause” – and nobody knows how long it will continue.