Et Tu LT?

In previous posts we have stressed that discrepancies between models and observations force scientists to re-examine the foundations of both the modelling and the interpretation of the data. So it has been for the apparent discrepancies between the Microwave Sounding Unit (MSU) lower tropospheric temperature records (MSU 2LT), radiosonde records and the climate models that try to simulate the climate of the last few decades. Three papers this week in Science Express, Mears et al, Santer et al (on which I’m a co-author) and Sherwood et al show that the discrepancy has been mostly resolved – in favour of the models.

It is worth encapsulating exactly what the problems have been and why they have taken so long to resolve. The MSU records are derived from a series of satellites that have been in orbit since late 1978. Each satellite has had different calibration problems (due to orbital decay, sensor issues etc.) and stringing them together has been fraught with difficulty. Different groups have made different decisions about how to do this and this has lead to quite some differences in MSU products particularly between the UAH group (Spencer and Christy) and the RSS group (Wentz, Mears and colleagues) . The differences have been mostly seen in the trends, rather than the monthly or interannual variability, and so have been more difficult to validate. Incidentally, it is a clear sign of ‘cherry-picking’ when people only report their favorite one of the groups’ trends instead of the range.

There have been three principle MSU products: Channel 4, Channel 2 and the 2LT records. MSU-4 is a record of lower stratospheric temperatures, MSU-2 is mainly mid-troposphere combined with a significant chunk of the lower stratosphere, and MSU-2LT is an attempt to use more viewing angles to try remove the stratospheric influence from MSU-2 and leave a lower-tropospheric record. (Recent upgrades to newer satellite instruments with more channels have lead to the 2LT record being renamed the TLT record).

The disagreement with the models related mainly to the MSU 2LT record. Models do quite well at matching the history of MSU-4 (whose variability is a function mainly of ozone depletion and volcanic aerosol effects), and models also match the lack of significant trend in MSU-2 (which is affected by stratospheric cooling and tropospheric warming which cancel out to some degree) (i.e Hansen et al 2002). So the problem has been principally with MSU 2LT, which despite a strong surface temperature trend did not seem to have been warming very much – while models and basic physics predict that it should be warming at a slightly larger rate than the surface.

In the first Science Express paper, Mears et al produce a new assessment of the MSU 2LT record and show that one of the corrections applied to the UAH MSU 2LT record had been applied incorrectly, significantly underplaying the trend in the data. This mistake has been acknowledged by the UAH team who have already updated their data (version 5.2) so that it includes the fix. This correction (related to the drift in crossing times at the equator) mainly affects the tropics, and was most important for one particular satellite (NOAA-11). Interestingly, Fu and Johansen (2005) singled out this same satellite and this same correction as being the source of divergence between the different records, though without being able to say exactly what the problem was. The fix leads to an increase of about 50% in the UAH global mean trend (0.086 to 0.12 deg/decade). The new RSS version of the 2LT record still shows a higher trend (0.19 deg/decade), with the difference being due to the methodology used to splice the different satellites.

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