The Hiatus In Global Temperature Explained?

The paper by Kosaka & Xie, “Recent global-warming hiatus tied to equatorial Pacific surface cooling,” published on 28 August in Nature, has attracted some comment, not least by the media, some of whom see it as an explanation for the flatness of global surface temperatures over the past decade and a half. The Guardian calls the hiatus the “greatest puzzle of climate change,” and they are right to do so. But explaining the hiatus with one factor, or even as a combination of several, throws up contradictions.Some media reports say that the Kosaka & Xie paper has solved this great puzzle and that it is down to a region of the tropical pacific which, thanks to a natural cycle of warming and cooling called the Pacific Decadal oscillation, has been moderating the underlying upward trend in global temperatures due to greenhouse gas forcing.Except that it isn’t a complete explanation but a possible factor. This is what some environmental journalists should realise. Very few single papers published represent a definite solution. Over-emphasising individual papers is called “single paper syndrome.” There are other explanations for the pause that this paper does not replace; it rather sits alongside these other explanations uneasily.“Accelerated Global Warming”One of the lessons from the pause in global surface temperatures is that natural climate variations can over periods of a decade or more be larger than the global warming “signal” which the IPCC says is 0.2 deg per decade. This means that we have 0.3 deg of warming waiting to 'get out' just as soon as whatever is keeping the Earth’s surface cool abates. I’ve said before that it is a strange situation that given the increasing climatic forcing due to the differences between the actual surface temperature of the Earth’s surface and what the amount of greenhouse gasses would like it to be, that the Earth’s surface temperature remains flat. Such a balance between warming and cooling seems all the stranger as each year passes.But an explanation for the warming hiatus has ramifications for the late 20th century warming as well. The Xie paper calls the period from about 1980 to the late 1990s one of  “accelerated global warming.” The global surface temperature increased about 0.4 deg in about 20 years, curiously obeying the IPCC’s average expected rate of increase, and also curiously looked at in retrospect as an “accelerated” rate of warming.Judy Curry is correct in her reading of the Xie paper in that it implies that about 50% of the warming seen between 1980 and the late 1990s can in their model be accounted for by natural variability. I don’t think, as Xie has reposted, it is an artefact of choosing El Nino and La Nina start and end dates as it is easy to choose other dates and see the same magnitude of effect. Strange then, that “natural variability” has reduced the increase due to greenhouse gasses (if the remainder is all die to greenhouse gasses) to 0.1 deg per decade, ie less than the IPCC’s warming rate, indicating that in the past 75 years we have never seen its rate of increase actually happen!The question of the rise becomes more interesting when one considers the other factors that may be at work, specifically stratospheric water vapour. This is a naturally variable factor that until recently was known about but had little quantification of its effects. It has been used to partially explain the recent hiatus in global temperature, but it might also be responsible for up to a third of the warming in the previous two decades.The calculations by Kosaka & Xie do not include stratospheric water vapour, so taking it into account with the natural climate variability they find we have two natural factors that taken together have the possibility to account for about 80% of the 1980-1997 warming, or over 0.3 of the 0.4 deg C increase of the period. Let’s say there is 0.1 deg left (over two decades) into which greenhouse gas forcing and other natural climate variability must be included. That’s 0.05 deg C per decade.It would be fair to say that the IPCC’s estimate of average global warming is looking a bit out and natural factors are looking considerable more important that estimated by the IPCC.Feedback: david.whitehouse@netzerowatch.com