The global warming hiatus is one of the most important topics in climate science. The data shows us something very interesting is happening and many scientists are producing amazing research in pursuit of an explanation. It will not be explained or dismissed by this or that paper, and the latest one championed in the media is unlikely to last long.
Being certain in science is very often being a hostage to fortune. There is nothing any scientist who expresses an opinion can do about it, and that’s a good thing because science is all about the unexpected. It’s easy to look at the history books and be amused by comments that science is finished and that all that remains is measuring to finer precision – then came the quantum revolution, or that continental drift is “out of the question” (Prof Harold Jeffreys), and also that space travel is utter bilge (Astronomer Royal). But it is not just the big pronouncements that are interesting. Arguably more important are the lower level declarations of certainty that accompany normal scientific progress as they too are very often wrong, and thus at the same time useful and embarrassing. But in climate change being wrong is seen by some as being unscientific, and often unforgivable.
Reading the latest edition of US Climate Variation and Predictability reminds me of the sweeping pronouncements and hostages captured that are made about the “hiatus” observed in global surface temperatures over the past 15 years or so. If you look at much of the media you would be forgiven for thinking that the question of the hiatus had been settled because up-to-date analysis using new and better temperature databases have shown it was an illusion. ‘Science shows the hiatus never existed’, say the headlines – and anyone who disagrees is a denier. But these latest papers tell a different story.
There are six readable papers and five of them acknowledge the hiatus and go about seeking explanations for it. One paper whose lead author is an independent statistician says the hiatus does not exist and never has. It is clear that this particular interpretation of the surface temperature data, stridently and intolerantly expressed, has not gathered deep support in the climate change community.
The Hiatus Is Clear
It is Gerald Meehl of the US National Center for Atmospheric Research that produces in my view the most thoughtful paper in the collection.
He says that the many adjustments of the surface temperature data sets – adjustments that invariably eliminate the hiatus – have not been as definitive as some suggest. He says that the claims of no hiatus rest on questionable interpretations of forced climate change due to greenhouse gasses and its relationship with inter-decadal and decadal natural climate variability. The hiatus is clear he says, and not an artifact of the data.
Indeed, it has been said many times that the lesson of the hiatus is that of an increasing appreciation of natural climatic variability and its relationship to forced climate change. Consider this: A decade ago it was not uncommon to hear that forced climate change was the dominant factor and that natural variability was minor. Then came the hiatus and now it’s the other way around for decadal timescales.
Meehl, like many others, thinks the answer to the hiatus may reside in the decadal temperature cycles of the Pacific. It is an interesting hypothesis, but we should remember that such cycles were identified less than 20 years ago and so we have little to go on when considering decadal variations.
There are indications that the Pacific Decadal Oscillation (PDO) went from a cool phase to a warm one in 2014, and some suggest that it marks the end of the hiatus. Whilst we wait for the data to see if this is the case, it is worth remembering that in the past there have been many predictions of the end of the hiatus. Remember the Climategate comments by Phil Jones that if the surface temperature didn’t start increasing after 15 years we should be worried; or take Ben Santer’s proclamation that a 17-year standstill is the time to start worrying. These predictions have come and gone and been replaced by new timescales with the “worry point” always a few years ahead of the data!
The hiatus is one of the most important topics in climate science. The data shows us something very interesting is happening and many scientists, looking at many aspects of the environment, are producing amazing research in pursuit of an explanation. It will not be explained or dismissed by this or that paper, and the latest one championed in the media is unlikely to last long.
The hiatus is not only telling us something about the importance of natural climatic variations but also about the polarisation of science exemplified by questions like whose side are you on.