Dot Earth Blog: A Closer Look at a Shift in Britain's Near-Term Global Warming Forecast

7:22 p.m. | Updates below |
Quite a few professional climate skeptics have been crowing in the last few days about a 20-percent downward shift in the short-term forecast for global temperature (through 2017) from Britain?s weather and climate agency, best know as the Met Office.?Here are the graphs showing the change, with the first published in 2011 (see the related release here, thanks to?Leo Hickman of the Guardian) and the next one from late last month:

The focus of those questioning the importance of greenhouse-driven warming is not so much the forecast downgrade itself, but how it would, if it holds up, create a statistically significant span in which global temperatures have not markedly risen. While global temperatures are the highest they?ve been since formal records began in the 19th century, warming has largely stalled since 1998.

Climate change occurs in fits and starts, and there?s plenty of research finding that pauses are normal, but if the current pause persists through 2015 or beyond, questions will build. In 2011, a team led by Ben Santer at Lawrence Livermore National Lab?found that a 17-year span would be long enough ?to separate human-caused global warming from the ?noise? of purely natural climate fluctuations.? [*?Santer weighs in below.]

Leo Hickman has helpfully rounded up an enormous range of views and voices on the Met Office move and has drawn this conclusion:

The rate of decadal rise in average global temperatures has clearly slowed over the past decade or so, compared to the previous couple of decades, but to say it has ?stopped? altogether seems to be a misleading statistical sleight of hand.

It also strikes me as complacent, or even reckless, to assume that any slowing is proof that global warming is nothing to worry about. As many scientists in the field point out, there are several likely causes ? both natural and anthropogenic variables ? that could be masking or absorbing the so-called ?missing heat?, not least the oceans and/or soot released by the burning of coal in fast-industrialising nations such as China. To assume global warming has been falsified is, in my view, a very cavalier, wrong-headed display of long-term risk analysis.

I?m not sure how many people are asserting that the Met Office shift falsifies greenhouse theory, so some of what Hickman says has a straw man feel. But over all, his points are sound.

In a release posted yesterday, the Met Office said the new forecast (which it stresses is experimental) was generated with a revised model including ?a comprehensive set of improvements.?

I have a query in to Ben Santer, the leader of last year?s Livermore analysis of the difference between trend and variation on short time scales, and will add his thoughts when they come in.

I also queried Richard Betts of the Met Office a couple of days ago (I?ve only now had time to run this; apologies all around). Here?s what he said:

There?s lots going on here, and the first thing to note is that decadal forecasts are still regarded as experimental, which is why this has first appeared on our research pages and not as a paper or a press release with a big fanfare ? it?s not a ?ta-daaaa? moment, more a ?OK here?s what the latest setup tells up, feel free to take a look while we continue developing the techniques?.

One factor is that it?s a new model (HadGEM3* instead of HadCM3* ? so entirely new atmospheric dynamics, amongst other things).

Another is that the initial data and observational constraints are different because of course time has moved on, so we have more data with which to constrain the models and a different set of starting conditions. It should be noted that this is of course only a five-year forecast so it?s within the timeframe which is still dominated by natural internal variability. Nevertheless it is interesting to consider this alongside, say, Stott and Jones (2012) who showed that constraining the models with recent observations makes the higher end of long-term projections look less likely, although long-term warming is still projected. However it?s still very early days, my colleagues in the seasonal-to-decadal area need to look at this a lot more. There will probably be a paper in due course.?[I added the links in Betts's statement to background on the models.]

In sum, there will come a time in this decade when the robustness of current thinking on the forces shaping climate will be reinforced or eroded.

That does not mean folks should sit around waiting to push toward less-polluting energy choices. To me, plenty of basic science still points to centuries of warming and coastal retreats, and a good chance of unwelcome surprises, if humans continue to rely primarily on burning coal and oil to generate the energy needed to drive 21st-century civilization.

7:22 p.m. |Update

Ben Santer sent two reactions by e-mail, one of which is here and the other ? far more technical ? is added as a comment:

The bottom line is that the identification of human effects on climate is a signal-to-noise problem. A human-caused warming signal is embedded in the rich, year-to-year and decade-to-decade noise of natural internal climate variability. Scientifically, we never had the expectation that there would be some monotonic warming signal in response to slow, human-caused changes in greenhouse gases, with each year inexorably warmer than the previous year. In detection and attribution studies, we beat down the large noise of year-to-year and decade-to-decade variability by looking at changes over longer sweeps of time. When you consider the entire satellite era (1979 to present), signal-to-noise ratios for global-scale changes in lower tropospheric temperature now exceed 5 ? even for UAH lower tropospheric temperature data (see??fact sheet?). This is what the discussion should focus on ? the signal rather than the noise.

Skeptical Science has posted a relevant piece, and video, tonight that strips away sources of known ?noise? and shows a steady underlying warming trend: ?16 (more) years of global warming.? Here?s the video explainer:

Source: http://dotearth.blogs.nytimes.com/2013/01/09/shift-in-british-climate-analysis-blunts-governments-short-term-warming-forecast/?partner=rss&emc=rss

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