… and refreshes … and refreshes … and refreshes.

I speak, of course, of the non-existent hiatus in global temperature increase vigorously but incorrectly pointed to by deniers of global warming. What happened was this. We had a really warm year, owing to an El Nino, in the late 1990s. Then, things settled down a bit, and due to normal variation of the Earth’s climate system, that year was followed by a series of years in which the global surface temperature continued to increase, but very slowly. Meanwhile, of course, the Earth’s ocean temperature was steadily increasing, no pause or hiatus there. Then, after a few years, the Earth’s surface temperature warmed very rapidly. The period between that El Nino year, and the rapid return of temperatures rising, is where climate science deniers shoehorn their hiatus.

You can look at any system where a measure of something is going in one direction (in this case, temperature going up over time) where there is internal variability, and see pauses, times when the upward trend slows. Then you can get all excited and point to the pauses and jump up and down holding a snowball in your hand and scoff at the scientists who claim the measure is moving along nicely. But it is not fair to do that without also noticing the surges, the periods when the opposite of a pause happens, the periods when the measure jumps up quickly. That is what happens with the Earth’s surface temperature, mainly because the surface (the top of the ocean and the bottom of the air over land) transfers heat back and forth with the deeper parts of the ocean. The total heat stored on the surface of the ocean and in the bottom of the atmosphere is a tiny fraction of the heat stored in the ocean, so the ocean hardly notices this exchange while the air really feels it.

It is like your bank account. Your bank balance can jump up and down with paychecks going in, payments of bills going out, and over time be uncomfortably low for a while, at other times a bit more flush. But that whole time your bank balance is going up and down, the bank’s reserves, many times larger than your account balance, sit there and pretty much don’t notice the variation in your account. And, if you are lucky, your bank account is like global warming. It goes up and down over the short term, but over the long term, up, up, and more up. A good thing for your savings, a bad thing for the planet.

Have a look at this graphic.

Here, I’ve plotted one of the widely used data sets of the Earth’s surface temperature, the one maintained by NASA, from 1960 to the present. The blue box shows the period sometimes called, incorrectly, the “hiatus” or “pause” in global warming. That high spike at the beginning of the box is the aforementioned El Nino year. You can see that temperatures do continue to climb, but the overall effect is somewhat anemic. Then, of course, temperatures start to climb again, in a surge that makes the pause seem puny.

I’ve also indicated the period of time during which climate science deniers talked most about the pause, based on the mentions in the Wikipedia article on it.

The reason I mention any of this now, is the publication of two new peer reviewed articles on the pause phenomenon. Here are the refs and abstracts:

James S Risbey , Stephan Lewandowsky, Kevin Cowtan, Naomi Oreskes, Stefan Rahmstorf, Ari Jokimäki, and Grant Foster. 2018. A fluctuation in surface temperature in historical context: reassessment and retrospective on the evidence.

This work reviews the literature on an alleged global warming ‘pause’ in global mean surface temperature (GMST) to determine how it has been defined, what time intervals are used to characterise it, what data are used to measure it, and what methods used to assess it. We test for ‘pauses’, both in the normally understood meaning of the term to mean no warming trend, as well as for a ‘pause’ defined as a substantially slower trend in GMST. The tests are carried out with the historical versions of GMST that existed for each pause-interval tested, and with current versions of each of the GMST datasets. The tests are conducted following the common (but questionable) practice of breaking the linear fit at the start of the trend interval (‘broken’ trends), and also with trends that are continuous with the data bordering the trend interval. We also compare results when appropriate allowance is made for the selection bias problem. The results show that there is little or no statistical evidence for a lack of trend or slower trend in GMST using either the historical data or the current data. The perception that there was a ‘pause’ in GMST was bolstered by earlier biases in the data in combination with incomplete statistical testing.

Stephan Lewandowsky, Kevin Cowtan, James S Risbey, Michael E Mann, Byron A Steinman,Naomi Oreskes and Stefan Rahmstorf. 2018. The ‘pause’ in global warming in historical context: (II). Comparing models to observations

We review the evidence for a putative early 21st-century divergence between global mean surface temperature (GMST) and Coupled Model Intercomparison Project Phase 5 (CMIP5) projections. We provide a systematic comparison between temperatures and projections using historical versions of GMST products and historical versions of model projections that existed at the times when claims about a divergence were made. The comparisons are conducted with a variety of statistical techniques that correct for problems in previous work, including using continuous trends and a Monte Carlo approach to simulate internal variability. The results show that there is no robust statistical evidence for a divergence between models and observations. The impression of a divergence early in the 21st century was caused by various biases in model interpretation and in the observations, and was unsupported by robust statistics.

Notice the panoply of amazing authors in these overlapping teams. I am pretty sure that among these authors and other colleagues, there will be much general audience writing about this work, which I’ll link to here. Also there is likely to be a controversy arising because there always is in academia, and I’ll keep you posted on that.

But for now, I want to leave you with one simple point. A simple point made with a thousand words. But not words, a picture, since that is how much a picture is worth:

See:

• Global warming ‘hiatus’ is the climate change myth that refuses to die
• There was no “pause” in global warming!
• 2018 – the hottest La Niña year ever recorded

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