Anthony Watts noticed that the NOAA/NCDC global temperature for May 2013 was touted as the 3rd-hottest May on record. He didn’t like that, so he decided to call it into question. In fact he refers to the NCDC reported value as “irreconcilable.”



He compares the NCDC result to the UAH TLT (lower troposphere) temperature, mentioning that the UAH value is only 0.07 while the NCDC value is 0.66 — big difference! But this quite ignores the fact that the UAH TLT data and the NCDC data use a different baseline, i.e. a different zero point. You shouldn’t compare them without accounting for that. By the way, the UAH people may have revised their May 2013 figure ever so slightly, since the data now lists 0.08. No big deal really.

He also compares to the RSS TTT data set. It’s more customary to compare surface temperature to their “TLT” data set (lower troposphere) rather than “TTT” (total troposphere), since TLT is, well, closer to the surface. No big deal really, except that Watts seems unaware of what he’s doing. And of course, once again the data sets use a different baseline which must be accounted for when comparing.

He also compares the NCDC figure to the “WeatherBell 2 meter global temperature reanalysis,” but this is quite odd because not only is there a different baseline which Watts doesn’t account for, the graph he shows and the figure he quotes seem not to be a monthly average for May — they look like figures for 6:00 P.M. Greenwich Mean Time on May 31st. You really can’t compare a momentary value to a monthly average if you want to give an honest portrayal.

He even compares the NCDC figure to the report from NASA GISS, having this to say:



One thing is clear, since GISS almost always reads higher than other datasets, including NOAA, and in this case NCDC’s claim is higher than any comparable dataset, it doesn’t seem believable. Perhaps a correction will be forthcoming.



Besides not accounting for baselines differences and trying to compare a momentary value to a monthly average, Watts seems not to have investigated at all what kind of differences are typical for these data sets. Even if NCDC is higher than others this month, is the difference really notable or is it typical of differences that have occurred often? I suspect that Watts simply wanted to repudiate the NCDC report, and in his eagerness he jumped the gun.

It’s easy to check whether the latest NCDC figure is “out of whack” with other data. Here’s the difference between the NCDC data and the UAH TLT data for each month since 2000:

The May 2013 difference is on the high side, but so are a lot of other months. Certainly this month’s difference is not “irreconcilable.” And by the way, the average difference is 0.413 deg.C, so you really can’t ignore that baseline difference if you want to give an honest portrayal.

How about RSS? Let’s look at the difference between NCDC and RSS TLT:

Again, the May 2013 difference is on the high side but so are a lot of other months. Certainly this month’s difference is not “irreconcilable.” And by the way, the average difference is 0.341 deg.C, so you really can’t ignore that baseline difference if you want to give an honest portrayal.

What about the RSS TTT data?

Yet again, the May 2013 difference is on the high side but so are a lot of other months. Certainly this month’s difference is not “irreconcilable.” And by the way, the average difference is 0.365 deg.C, so you really can’t ignore that baseline difference if you want to give an honest portrayal.

As for GISS, here you go:

One more time: the May 2013 difference is on the high side, but so are a lot of other months. Certainly this month’s difference is not “irreconcilable.”

And by the way, the average difference is only 0.006 deg.C, but on average NCDC is higher than GISS, not the other way around as Watts claimed.

Perhaps a correction will be forthcoming.