Climate alarmists the world over heaved a sigh of relief a little over a month ago when a team of NOAA scientists led by Tom Karl published a piece in Science purporting to disprove “the pause” in global warming. The sigh was short lived, though, since quite a few able critiques of Karl et al. 2015 appeared quickly, a number of them linked in my earlier blog on the subject here.

Now the Global Warming Policy Foundation has published, together in a single post, two new critiques, by David Whitehouse and Gordon Hughes, that pretty well drive the nail into the Karl et al. coffin.

Whitehouse’s “Why Karl et al. 2015 Doesn’t Eliminate the ‘Hiatus'” begins:

Even accepting the statistical approach taken by Karl et al it is clear that their errors are larger than they realise, and that the trends they obtain depend upon cherry-picked start and end points that include abnormal conditions, i.e. the 1998-2000 El Nino/La Nina and the 2014 northeast Pacific Ocean “hot spot.”

Whitehouse then explains fairly simply the problems in Karl et al.’s handling of the data and statistics, and judges:

… the trends reported by Karl et al 2015 – which were only ever marginally significant at the 10% level – are much less significant. Comparing their trends – 0.086°C per decade for 1998-2012, 0.106°C per decade for 1998-2014 and 0.116°C per decade for 2000-2014 – with the outcome of the Monte Carlo simulation revealed positive trends between 0.08-0.12°C per decade 1,133 times out of the 10,000 simulations. We conclude that, irrespective of their quoted small errors in their trends, none of them are robust or provide evidence that the “hiatus” does not exist.

Super simplified, Whitehouse shows that the error margins admitted by Karl et al. in their data are so large as to make any trend claims statistically insignificant.

Hughes’s piece is more sophisticated but particularly valuable for non-statisticians by the opening illustration of how to obtain statistically significant results. Key point: You have to have lots of runs of your experiment before you can begin to disentangle various potential causal variables and start pinning causal role on a particular one. Karl et al. completely fails that criterion.

Here’s an excerpt from Hughes, including his graph, that sums up the case:

The study by Karl et al (Science Express, 4 June 2015) appears in a completely different light when scrutinised in this way.  It claims that for the 17 years from 1998 to 2014 their new data produces a trend increase in global temperature of 0.106°C per decade with a 90% confidence range of 0.048 to 0.164°C per decade.  Cross-checks show that the confidence range is calculated solely by using the variability in the period from 1998-2014.  But this does not accurately reflect the variability in their data for the full period from 1880 to 2014.  To demonstrate the point, the trend increase in global temperature can be computed for every 17-year period between 1880 and 2014 using the method followed by Karl et al.  This gives us the actual variability over all 17-year periods in the data, not an estimate based on a single period.  It turns out that the actual variability is more than 3 times the Karl et al estimate.  This analysis also shows that the distribution of 17-year trends is negatively skewed (the mean is much lower than the median), so that the empirical confidence range goes further into negative values for the trend than conventional calculations would suggest.

It is possible that there has been some change in the underlying variability of temperature has changed since the middle of the 20^th century.  Karl et al report trends for periods from 1950 and 1951, so the same exercise was repeated for all 17-year periods from 1950.  The variability of these trends is, indeed, lower than for the full period but it is still 2.4 times the estimate of variability based on the single period 1998-2014.  In fact, based on an analysis of 17-year periods since 1950 one cannot rule out the possibility of no trend in temperatures since the mean trend is 0.126°C per decade with a 90% confidence range of -0.017 to +0.217°C per decade.

The results of using the historical variability of the temperature data rather than variability estimated for relatively short periods is shown in Figure 1.  The estimated values are shown as hatched bars while the 90% confidence intervals are given by the vertical lines.  This demonstrates that the claim that the trend increase from 1998 to 2014 was “significant” rests on an erroneous estimate of the actual variability in estimates of the trend.  Indeed, even the large trend increase from 1951 to 2012 has a much wider confidence range when based on the variability of all 62 year periods since 1880.

It is important to be clear about the limitations of this kind of analysis.  The global temperature has increased since 1880.  It is probable but far from certain that the trend rate of increase accelerated after 1950.  However, given the variability in the trends estimated for relatively short periods, the hypothesis that there was a hiatus after 1998 cannot be rejected using the Karl et al data.  In fact, based on the full data series one would expect that a trend increase of at least 0.1 °C per decade would be observed in about 15% of all 17-year periods examined, even if the underlying trend in global temperatures is zero.

The lesson is that no study should rely upon trends over selected short periods of time to make claims about a series with as much variability over time as global temperatures.  That is as true for the relatively large increase from 1976 to 1998 as for the more recent period.  Even that trend has been exceeded in 10% of all 23-year periods since 1880.

There’s a good deal more wrong with Karl et al.’s use of statistics, which Hughes goes on to point out. Bottom line is that NOAA’s effort to eliminate “the pause” and pave the way to a global climate agreement in Paris in December fails scientifically.

But take heart, alarmists. That doesn’t mean it fails politically.

 

 

Featured image courtesy of Christopher Monckton.