| Details1: | Much Ado About Fu: The Satellite Saga Continues
By Roy Spencer (12/3/04)
The results of two research studies announced this week address the
infamous discrepancy between satellite and surface thermometer trends over
the last 25 years.
The original satellite dataset produced by the University of Alabama in
Huntsville (UAH) now has a warming trend of 0.08 degC/decade since 1979,
while the surface thermometer trend is two to three times this
value. Climate models, in contrast, claim that any surface warming as a
result of greenhouse warming should be amplified with height, not
reduced. This has led to varying levels of concern in the climate
community that the theory contained in the climate models might be in error.
As background, a study published earlier this year by Fu et al. (1)
attempted to estimate the amount of tropospheric warming by a simple linear
combination of the stratospheric and tropospheric channels of the Microwave
Sounding Units (MSUs) flying on NOAA polar-orbiting weather
satellites. (The troposphere exists from the surface up to a height of
around 8-12 miles, the stratosphere overlays it.) Since the tropospheric
channel has about 15% influence from the stratosphere -- which has cooled
strongly since 1979 -- the tropospheric temperature can only be estimated
through removal of the stratospheric component. Fu et al. used radiosonde
(weather balloon) data to arrive at an optimum combination of the two
channels that, when applied to the satellite-observed temperature trends,
resulted in a tropospheric warming trend that was larger than that
estimated by UAH with a different technique.
In the first article announced this week, Fu & Johanson (2) estimate the
stratospheric contribution to the satellite instrument's tropospheric
channel through a slightly different method than in their original
article. They used previously published radiosonde estimates of
temperature trends through the lower and middle stratosphere to estimate
the error in their method, as well as the amount of stratospheric cooling
contained in the tropospheric channel. While we would prefer to leave
detailed comments for a journal article, a couple of general points can be
made. For the period they examined (1979-2001), our (UAH)
lower-tropospheric temperature trend is +0.06 deg. C/decade, while their
estimate of the (whole) tropospheric trend is +0.09 deg C/decade. You
might notice that the difference between these two trends is small,
considering the probable error bounds on these estimates and the fact that
the two techniques measure somewhat different layers. Also, their method
depends on belief in the radiosonde-measured trends in the lower
stratosphere, even though we know there are larger errors at those
altitudes than in the troposphere -- and most published radiosonde trends
for the troposphere show little or no global warming (!).
As is often the case, the press release that described their new study made
claims that were, in my view, exaggerated. Nevertheless, given the
importance of the global warming issue, this line of research is probably
worthwhile as it provides an alternative way of interpreting the satellite
data.
The other study (3), published by Simon Tett and Peter Thorne at the UK's
Hadley Centre, takes issue with the original Fu et al. method. Tett and
Thorne claim that when the technique is applied to variety of radiosonde,
reanalysis, and global model simulation datasets in the tropics, it leads
to results that are more variable than the UAH technique produces. It also
mentions the dependence of the method on the characteristics of the
radiosonde data that are assumed.
What all this means in terms of observed and predicted global temperature
trends remains to be seen. As part of the requirements of the Bush
administration's Climate Change Science Plan, a variety of scientists are
now sifting through the satellite, surface thermometer, and radiosonde
data, and will report in the coming year on their findings.
References
1. Fu, Q., C.M. Johanson, S.G. Warren, and D.J. Seidel, 2004: Contribution
of stratospheric cooling to satellite inferred tropospheric temperature
trends. Nature, Vol. 429, p. 55-58.
2. Fu, Q., and C.M. Johanson, 2004. Stratospheric influences on MSU-derived
tropospheric temperature trends: A direct error analysis. Journal of
Climate, to be published December 15, 2004
3. Tett, S., and P. Thorne, 2004: Tropospheric temperature series from
satellites. December 2, 2004, at Nature online (subscription required).
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