More in the saga of the BEST data and whether "there is no scientific basis for saying that warming hasn't stopped" (Judith Curry). Or in the
latest from Judith
"Has the rate of warming continued unabated, or has there been a pause in the warming?"
Judith has now offered a criterion:
"Here I define “pause” to mean a rate of increase of temperature that is less than 0.17 – 0.2 C/decade."
Stopped means below zero. Now you might think that, with the short periods involved, there would be some notion of significance involved. But no:
"Note that the short time scales considered here preclude determination of a statistically significant trend at the 95% confidence level, although lack of statistical signficance does not negate the existence of a pause as defined here."
Well, it occurred to me that if any drop, significant or not, below, say, 0.17C/decade is a pause, then we'd be seeing a lot of them over the years. So I thought I would check that out.
The period of time Judith and others is looking at is about ten years. So lets look at ten year periods in the recent past, and see how many "pauses" show up.
I'll start with the BEST land data. I've plotted each set to end 2009 to avoid the BEST problems with running out of data in early 2010. So here is a plot of decade gradients, plotted against the end of the ten year period. I'm using OLS regression. I've switched to units of °C/century, where the paused criterion is 1.7, shown with a horizontal line:
So a first surprise - BEST isn't currently showing a pause at all. Nowhere near. Muller was right - and Tamino
noted this. The BEST data shows a strong rise right through this decade.
The dataset that has been nearest to showing a pause in recent years is HADCRUT.
(Remember, it is Land/Ocean, BEST is Land). So lets see how it looks:
Quite a lot of pauses - in fact, if that's the criterion, more years showing a pause than not.
For completeness, here is Gistemp, which is somewhat in between:
Ten years is a popular period to examine at the moment. It's long enough to have some plausibility, doesn't tangle with the peak of 1998, and can be manoeuvred to show somewhat reduced warming. But I wondered what might have been the perspective in previous years. How hard would it have then been to show reduced trends? Is the present period really different?
So I made a plot of the various intervals that could have been tried, from six years to twenty, in two-year increments. The following plot shows which time intervals, looking back from each year, would have met the "paused" criterion. Again trying to see if there really is a pause now.
The length of trend period, looking back, is shown on the y-axis. The corresponding color shows whether that choice of trend interval, ending in the x-axis year, would have been declared "paused".
So first BEST again. The surprise is that it is quite hard, in this decade, to meet the paused criterion (slope ≤1.7C/century) with the BEST data. Just one or two 6-8 year periods satisfy it. It gets progressively easier going back, and of course in the 70's looking back, warming of 1.7C/century was rarely attained.
To explain further, if you look at the column above 2009, the bottom blue part says that the previous 6 years shoiwed a trend above 1.7 C/century - not "paused". The next red part says the previous 8 years (2002-9) showed a trend less than 1.7, but for 10,12,14,16,18, and 20 year periods, again, blue - no pause. The columns above other years can be read similarly.
Again Hadcrut shows a greater frequency of pauses:
But only very recently, and there's a lot more "pausing" in the '90s.With the longer periods pre-2000, red doesn't really mean a pause - it's getting back to the period before the current warming really started.
Again Gistemp is in between:
Finally, there is the "stop" criterion, where the gradient dips below zero. How often could such a period have been found in the past. Well, BEST again:
Not since 1995 has it been possible to find a period between 6 and 20 years with negative slope, at least in the (6,8,10,12,14,16,18,20)yrs examined.. And again it gets more common back in the 70's. And again Hadcrut does show a few more recently, and Gistemp is in between:
Conclusion
Two of Judith's criteria for slope without statistical test show no established region where they are met, and show no real tendency, independent of choice of interval, to occur in recent years.
A related post (from over a year ago) concerning trends in this decade is
here.