A new paper by Kevin Cowtan and Robert Way in QJRoyMetSoc is getting a lot of discussion. See Real Climate here, SkS here and here, Lucia here and here, Cliamte Etc here.
The authors say:
"A new paper published in The Quarterly Journal of the Royal Meteorological Society fills in the gaps in the UK Met Office HadCRUT4 surface temperature data set, and finds that the global surface warming since 1997 has happened more than twice as fast as the HadCRUT4 estimate."
Some eyebrows have been raised at the size of the trend change from improving a relatively small area. I was surprised, too. So I did some calculations to see.
Update: The R code for calc and plotting is here. The data is provided by the authors here.
The need for the changeMet station coverage of the Earth is uneven. When grid averages are taken, and then combined for a hemisphere or global average, quite a lot of cells have no data. What to do?
The default is to leave them out of the average. But that is not a neutral decision. In arithmetical effect, they have been replaced by the average value of the cells with observations. And this may be a poor approximation. It should be improved with whatever information is available.
The particular issue with Hadcrut is data at the poles. In computing trends, missing cells are given the global average trend, but the poles are warming much faster. This is a big bias.
Cowtan and Way used UAH satellite data to get that improvement. I won't go into detail here about how they did it, but I'll just look at the dataset results. They looked in particulat at a period from 1997-2012 (16 years) which is commonly discussed as a pause. They showed trends including their hybrid method, which uses UAH-based infill:
|Dataset||HADCRUT 4||UAH||C&W hybrid|
So the new trend isn't that much greater than UAH. But to see just how modifying polar trends made the difference, I'll show the latitude averages for the 5° ranges.
In computing these for the HAD 4 data, I replaced data-free cells with the global average for that month. So they aren't a good guide (for HAD 4) to actual trends, but, as described above, they do correspond to what effectively goes into the global average, so you can see the effect of changes. Here's the plot:
(An earlier version of this plot had the sign of latitude wrong)
As you'll see, the Antarctic and Arctic trends for the hybrid are large, but not so very much larger than UAH. I am still a little surprised that this is so, but it's not unbelievable.
AppendixHere are the numerical results as plotted:
|Latitude||HAD 4||UAH||C&W hybrid|