In a recent post, I examined seasonal divergence in troposphere temperature (LT) trends produced by teams at RSS (Remote Sensing Systems) and UAH (University of Alabama at Huntsville). The strong annual cycle in the recent UAH data set has led to a wide divergence of temperature trends depending on time of year.
Since then, the blogosphere has been atwitter over the marked divergence between UAH and RSS for the month of February, which showed the UAH estimated anomaly (0.35) more than a full tenth of a degree higher than RSS (0.23).
Update (April 9, 2008): The March LT global temperature anomalies are out and they show both RSS (0.17) and UAH (0.21) down from February. The divergence has narrowed from 0.12 deg C in February to 0.04 deg C in March, which is exactly the divergence seen for that month in 2004-2008 (see below). So far in 2009, the UAH annual cycle is alive and well.
However, a detailed look at the divergence month-by-month shows that this latest discrepancy is not so surprising. I’ll also take a look at the effect on UAH of the recent switch to the newer AQUA satellite, which has actually resulted in an enhancement of the UAH annual cycle. AQUA has a self-correcting propulsion system, so the UAH annual cycle, and the cyclic component of UAH-RSS divergence, can not be the result of differing methods of correction for diurnal drift resulting from orbital decay.
I start by showing the global temperature trends for each month for the 1979-2008 period for both LT and surface temperature sets.
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When we last left LG, he’d managed to cram four or five howlers into the first paragraph of his latest National Post screed. Fearing the onset of RSI, I decided to take a break. But let’s resume …
Indeed, the drop in temperatures since late-2007 has been so precipitous –nearly a full degree Celsius– that almost all of the global warming that has occurred since the late-1970s has disappeared.
In fact, late 2007 looks cooler than the last month (January, 2009).
In the wake of the kerfuffle concerning George Will’s column on sea ice in the Washington Post, Lorne Gunter has upped the ante in the Canadian daily newspaper National Post with a particularly error-ridden and nonsensical column on a favourite theme – the supposed “intellectual dishonesty” of climate scientists who refuse to accept the “fact” of “global cooling”.
A key issue in the Will case was the apparent failure of fact checkers and editors to catch what should have been obvious errors. Like Will, Gunter appears to lean on tidbits from such reliable sources as U.S. senator James Inhofe’s climate disinformation clearing house (run by aide Marc Morano), or blogs like Stephen Mcintyre’s Climateaudit.org or Anthony Watt’s Wattsupwiththat.com. Gunter then proceeds to somehow mangle even those dubious assertions.
The two most commonly cited estimates of temperature in the troposphere based on satellite data are provided on an ongoing basis by Remote Sensing Systems (RSS) and University of Alabama at Huntsville (UAH). Last year, a strong annual cycle was identified in the UAH data by Atmoz and was examined in detail by Tamino at Open Mind here and here. In UAH, the annual cycle results in significantly higher anomalies at the beginning of the year (northern hemisphere winter) than in the middle of the year. And this pattern appears to be present in all latitude zones.
It seems reasonable that such a cycle would affect the observed trends over the 30-year period of the satellite temperature record. And indeed, it turns out there is a strong seasonal divergence; UAH trends for June-July-August (NH summer or JJA) are markedly lower than for December-January-February (NH winter or DJF). This strong seasonal divergence can be seen not only globally, but even in the tropics, as shown in detail below. This suggests that UAH trends lack physical realism, and should not be relied upon until the source of annual cycle problem is identified and fixed.