A new paper by Karl et al. in Science makes a spirited argument that there really is no “hiatus” or “slowdown” in global surface temperature warming. The paper focuses on some of the more technical details of bias correction in the temperature data record, rather than on the dynamics of the climate. It is from a respected group, and from my limited knowledge of the subject seems well put together. I’ll let my more knowledgeable colleagues comment on the bias correction side of things.
The conclusion is a great hook of course, which is why I guess a paper on some small changes to the temperature record is in Science. I have a few thoughts.
1) The changes to the global mean temperature record are, in the grand scheme of things, really quite small. Of course adding two extra years of data in a world that is warming will, probably, increase the mean trend a little bit, bringing it closer to the expected trend from the CMIP5 models. However, the “hiatus” period of 1998-2012 is also estimated by the team to have warmed a little bit more more than previously thought, mostly in the oceans.
2) It doesn’t take much to remove a hiatus. This is what happens if your hiatus definition relies on oversimplified binary statistical thinking, rather than with reference to a deeper description of the climate processes that are happening, in the context of natural variability. I’m not singling out the NOAA group here, this has happened a lot.
My colleague Chris Roberts points out that the necessary comparison is with the expected warming (and associated distribution) from the models, not from the somewhat arbitrary threshold of “zero” warming. Here’s a comparison from our paper on the future of the hiatus, from earlier this year:
It it also worth noting that, even in this new analysis that the 90% CI for the hiatus period (1998 – 2012) still includes zero, when “extra uncertainty associated with our corrections” is factored in. Check out figure 1, box 1.
3) Hiatus communication can be a proxy war. Climate skeptics have long overemphasised the hiatus – they’re unlikely to miss a good opportunity to claim that climate change isn’t happening, or that the models are wrong. There seems to have been a counter effort to demonstrate that the hiatus never existed in the first place. I stand by our comments from last year – the thing branded a “hiatus” is an interesting feature of a complex system; a useful hook for exploring how to make the models better, and how to talk with people about uncertainty. Does it prove that global warming isn’t happening? No. Was it unexpected? Yes, a bit. Are we able to explain it with our current understanding? Yes, very probably. So let’s do that.
What’s more, the last decade-or-so is interesting, and we can learn from it. There were for example, winds in the tropical Pacific that were unprecedented in the historical record (and model run library) and worth explaining. Energy continues to be accumualted by the planet, and lots of it has been buried in the oceans. The pattern of surface temperature change looks like an unusually long La Nina. The hiatus has lead to good work on the uncertainty in recent anthropogenic and volcanic forcing, and it’s interaction with natural processes; the importance (and relative lack) of observations in the deep ocean; the uncertainty in estimates of natural variability in the Earth system.
4) The best way to ensure that a hiatus ends is to publish a paper pointing out that it could well continue for some while. I guess it was inevitable that one group or other would would get to announce the end of the hiatus, but I wasn’t expecting it until next year, after this year’s warming plays out.