Eli and Tamino have posted about the obvious long time deviation of the lower troposphere satellite temperature records often called TLT. Eli and Steve Mosher, with friends and cynics have been doing a dosy do at ATTP and Rank Exploits.
It looks like either a) there is some aging effect in the AMSU receivers or b) the atmospheric/surface composition has shifted in the last 15 years or so (e.g. less ice/ more water vapor) in a way that biases the returns.
It started with Nick Stokes looking at the trends of various temperature anomaly records
In the midst of the usual ill tempered fro and to about trends at Rank Exploints, it struck Eli that there were really two questions, the long term trends about which much had been said, and the actual measurements which take place over a day or less and about much less has been said, at least in blogs and Congressional hearing, or even on the radio.
To get at this Eli compared the monthly variation in CRUTEM4 and RSS, showing that they were a pretty good overlay. Tamino showed both that on the short term (months) there was a perfect match between the UAH and RATPAC balloon sonde records but that they deviated starting in about 2000
Eli's original POV was that the drift is most likely in the AMSU satellites or the processing of the AMSU data. Unanticipated aging of the receiver or the internal hot calibration target seems to Eli most likely, although there might be something involving orbital decay (less likely now because this caused a lot of trouble early on) or even changing land/sea/ice patterns which affect the AMSU response.
However, upon reflection it appears equally likely that there has been some change in the atmosphere (humidity was suggested) or surface emissivity (Mosher's idea) that has befuddled the atmospheric model used by RSS and UAH. That the same effect is seen in RSS and UAH 6.0 indicates that the atmospheric models are idempotent or close.
That the break between the RSS and UAH records and the balloon sonde/surface temperature anomaly records come at the same time as the change over from the MSU to the AMSU, A standing for advanced, makes it hard to choose.
It is well known that the (A)MSU sensors have trouble with ice and snow as well as measuring over high land, so Eli, in his naivety, thinks that point by point comparison with temperatures measured by the (A)MSUs at specific locations might be useful and Mosher has a project moving in this direction
also the weighting function relies on an assumption of a constant emissivity for earth. gimme a few days and I may be able to tell you if gridded delta’s between RSS and BE are correlated with changes in emissivity.But, rather than go much further into this, Eli would like to point out how this discussion has been mind bending.
A principal attack on RSS and UAH has been that the measurements are five km or so up there and we live on the surface. The short term global comparisons that Eli and Tamino have done shows that the precision of the (A)MSU measurements as estimates of immediate global temperature are pretty good.
The question now is whether the long term drift is engineering or science
Extending the comparisons between surface, balloon sonde and satellite measurements to specific regions and a daily basis will really nail the precision and maybe the source of drift. Opportunity exists for using aircraft platforms (research and commercial) to even improve on this and, of course there is the Taiwanese/US COSMIC GPS occultation program.
Closing the loop on temperature anomaly measurements is within our reach. At that point each of the methods will support confidence in the other and the strengths of each will allow a deeper understanding of the climate system.
ADDED: In the comments Eric Swanson posted a table which Blogger could not handle really well, of trends. Here it is prettified.