Did the rate of global warming decline around year 2000 or not?

Update, 6-6-15: I’m revising this post based on an issue that Gavin Schmidt of NASA made me aware of on Twitter, involving the averaging of trends. It will change the results I’ve given below. Stay tuned.

I don’t like having to write posts like this one but I’m at loss right now. I’m going to keep it short, if for no other reason than the depressing quality of this so-called “debate”.

People following the climate change debate probably know that Karl et al came out with a paper in Science yesterday (Supplement here). Try to give it a read if you can. The lead author is a NOAA scientist and it was heavily promoted by that agency on Twitter and their agency blog. I counted at least 4 or 5 Twitter announcements, with bold headlines regarding the paper’s findings. Gavin Schmidt and Doug McNeall both wrote blog pieces that I thought did a decent to good job tempering the NOAA hoopla. Judith Curry also wrote one, more critical, but I haven’t read it yet. I’m going to take it a step further here.

The basic claim of the paper is that the muliply- and variously-named “hiatus”, “slowdown” etc, in global mean surface temperatures over the last 15 to 17 years or so, is basically non-existent statistically. The last sentence of the abstract states “These results do not support the notion of a “slowdown” in the increase of global surface temperature“, and the last paragraph states:

“In summary, newly corrected and updated global surface temperature data from NOAA’s NCEI do not support the notion of a global warming “hiatus.” As shown in Fig. 1, there is no discernable statistical or otherwise) decrease in the rate of warming between the second half of the 20th century and the first 15 years of the 21st century.”

That figure is not particularly helpful as far as trend differences go. If you look instead at their data table in the Supplement (Table S1), it doesn’t show what they claim there, at all. Look at the table–the trend from 1951 to 2012, based on their new data/method with ocean temperature biases accounted for (“New”), gives a decadal warming rate of 0.129 degrees C. The rate from 1998 to 2012 is given as 0.086. This means the rate from 1951 to 1997, which is what you need for the proper rate comparison, but which is not reported(!), must therefore be:

0.086*(15/62) + x*(47/62) = 0.129, or x = (0.129 – .028) * 62/47 = 0.133
(fractions of 62 are the time period weights).

So that’s 0.133 vs 0.086, which is a ratio of about 1.55. If one wants to use the year 2000 as the demarcation point between the two periods, then the comparison’s a little trickier because Karl et al don’t actually report an estimated rate from 2000 to 2012. But one can estimate it by assuming the rate difference between between the two periods 1998-2014, and 2000-2014, both of which they do report, is a decent approximation of the difference between the 1998-2012 and 2000-2012 periods. When I do so, I get a ratio of warming rates between the two periods (1951-1999 and 2000-2012) that’s very similar to the first: 1.47. Taking the inverses, the rate of warming between the slowdown period and the pre-slowdown period, is about 2/3. Given the large number of data points (grid boxes x time points) that make up each year’s mean value, it has to be essentially certain that, according to these very data, there has in fact been a very statistically significant difference in warming rates between these two periods, regardless of whether you use 1998 or 2000 as the breakpoint year.

Note that I”m excluding any data post-2012 here, because they really shouldn’t have been including those years’ data when analyzing the effect of data biases on trends going back to 1951–that’s just additional data for more up-to-date trend estimates relative to the AR5, which is another issue entirely. And, although the authors discuss the decadal warming rates between variously defined periods, it is confusing and not at all a direct apples-to-apples discussion. In fact, it’s misleading.

So what gives on this?

As always, bring a definite argument to the table, preferably with hard numbers.

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4 thoughts on “Did the rate of global warming decline around year 2000 or not?

  1. Worries me that the authors (and by implication NOAA) have fallen into the trap of trying to justify what may be a short-term slow-down in the rate of warming amongst a consistently long-term upward trend (see their Fig 2). Whichever way the data are “corrected” the trend looks the same. But by resorting to special pleading (and p=0.1 significance) they’ve set themselves up for ridicule by the sceptics.

    • Yeah, the p = 0.10 choice doesn’t look good. Nor the increased standard errors for the modern, corrected data. But to me the big problem is the way they overstate their finding–there has indeed been a slowdown and it’s pushing a decade and a half now.

      Having said that, I think a lot of climate scientists want to see more of a focus on the physics of what drives the variations being observed. That’s what’s really important.

  2. I completely agree with all your points.

    I also noticed that most of the “change” was in the ocean data and not so much in the land data.

    This “science” smells like politicized science – science with an agenda.

    • Thanks Rick. Yes, the changes were mostly in the ocean data, due to the increasing number of buoys, which run, on average about 0.12 deg C cooler than ship intake temps, and which have increased in relative proportion over time.

      I want to caution everyone from getting into issues of intent/agenda etc. Please stick strictly to technical arguments as you have done so well in the past.

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