HADCRUT4 V HADCRUT3
By Paul Homewood
The Hadley Centre introduced their latest global temperature dataset, HADCRUT4, last January, but only provided revised historical temperature data up to 2010. In the meantime they have only been issuing ongoing data under the old HADCRUT3 set.
However they have now brought out monthly figures for HADCRUT4 which take us bang up to date for August 2012. Let’s take a look then at the differences between the old and new versions.
Figure 1 shows the 12-month running averages for the two sets, which helps to take out some of the noise produced by monthly variations. From 1980 to 1998, the two follow each other pretty closely. From around 2005, however, they start to diverge quite noticeably.
A look at the numbers bears this out.
|12 month average anomaly||HADCRUT3||HADCRUT4||Diff|
The difference has increased by 0.04C since 1980. This may seem relatively insignificant. but it scales up to 0.13C per century, which interestingly is exactly the same amount that the latest version of GHCN adjustments has added to the warming trend. Perhaps more significantly it altered the comparison with 1998, thus:-
|12 month average anomaly||HADCRUT3||HADCRUT4|
The new version increases warming (or rather decreases cooling) since 1998 by 0.09C, a significant amount for a 13 year time span. Whilst the changes should not affect the trend in future years, they will affect the debate as to whether temperatures have increased in the last decade or so.
So what is different in the new version? The paper by Morice et al (2012) explains that the major change is the addition of extra stations in and around the Arctic. As the Arctic has warmed in recent years, this filling in of gaps has tended to push up the global average temperature anomaly. However, the paper also makes clear that “Measurement coverage in the Southern Ocean and the Antarctic remains sparse”
We already know from UAH satellite data that the Antarctic has been cooling since 1980. It would therefore seem that HADCRUT4 has introduced a bias of its own, by increasing coverage of the warmer Arctic but not doing the same in the cooler Antarctic. The maps below, from the Morice paper, show just how significant this introduced bias is.
The maps on the right show the station coverage for HADCRUT3 and HADCRUT4, for January 2005. The extra coverage in the Arctic for the latter is clear. However the large gaps in the Antarctic and Southern Oceans in HADCRUT3, where the anomalies are largely negative, still appear in HADCRUT4.
If you were looking for a way to create an artificial warming trend, I cannot think of a better way than this.
There is an even better map below, from one of Phil Jones’ presentations that shows where the new coverage is. There is none at all anywhere near the Antarctic.