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earlier local times as it passes over a given region as it drifts. Thus, the natural
diurnal cycle of Earth’s temperature is aliased through time and can appear as an
artificial trend in the data. In version D, we apply an independently determined
diurnal trend correction to all satellites based on the differences of the individual
footprints across a scan line.
In Christy et al. (1998) we noted a newly discovered effect in which the tempera-
ture of the MSU instrument itself influences the temperature of the observations.
This effect is manifested as both intraannual and interannual variations, especially
for the satellites in the 0200=1400 orbit time node. The temperature of the instru-
ment fluctuates due to the east–west drift, which induces a changing solar shadowing
effect on the instrument itself. The net effect of these east–west drifts is to introduce
an artificial warming into the time series.
Wentz and Schabel (1998) have discovered yet another effect that for the lower
troposphere product has an important influenc e. During periods of high solar activ-
ity, the upper atmosphere expands and exerts greater drag on the satellites, causing
them to descend a few kilometers over the 2 or 3 years of increased solar activity
from their average altitude of about 850 km. The lower tropospheric temperature
utilizes a retrieval that is very sensitive to satellite altitude. The net effect of
the ‘‘falling’’ satellites is to introduce an artificial cooling to the lower tropospheric
time series.
To summarize , then, we know that the MSU data contain two artificial warming
effects and one artificial cooling effect. These have been quantified and removed
from the data set, and in fact they are almost exactly offsetting in their net effect.
However, we must not presume that this set of issues, combined with what we have
discovered previously, constitutes the complete body of knowledge regarding the
long-term stability of the MSUs and their peculiarities. These instruments are in
space, so we cannot examine them directly for anomalies we suspect may be devel-
oping. We are forced to diagnose problems based on the data they transmit, and this
is a difficult enterprise. However, this effort has been a largely successful endeavor
because we have the ability to examine the MSU data in light of completely
independent data from radiosonde observations. The results, in our view, are very
interesting.
In Figure 3 we show the annual anomalies of global tropospheric temperature
from Angell, UK Met. Office, RIHMI-WDC, and the MSU D (see Christy, 1995).
We must note that Angell studies the thickness temperature of the 850- to 300-hPa
layer, RIHMI looks at the average temperature of the 850- to 300-hPa layer, and
the UKMO at the raob-simulated MSU temperature. Geographical coverage of
the radiosonde data sets is limited, while the MSU essentially observes the entire
planet. However, the spatial coherence of the troposphere is such that fewer spatial
degrees of freedom exist than at the surface; thus fewer sites are necessary to define
the average global temperature.
For the period 1979–1997 all data sets indicate that the temperature of the tropo-
sphere has declined slightly. Before that period, all show a warming trend, principally
a sharp jump during 1976–1980, though the RIHMI anomalies are much less in
magnitude (the reasons are probably due to a stiff interpolation method). Many such
876 MEASURING GLOBAL TEMPERATURE
comparisons of multistation averages have been performed, and they indicate excel-
lent agreement between the radiosondes and MSU temperatures.
Some concer n has been raised regarding the lack of a tropical warming trend
observed independently by radiosondes and the MSU since 1979. It has been
proposed that the tropical troposphere should behave in concert with variations in
tropical SSTs as indicated by climate model results, which are forced with observed
SSTs. The observed atmospheric data seem to indicate the tropical troposphere is
somewhat more decoupled from the boundary layer than presumed. The issue was
put forth by Hurrell and Trenberth (1997) in which they suggested that when
compared with tropically averaged SSTs, there was evidence for two ‘‘spurious’’
downward jumps in the MSU record, one of 0.25
C in late 1981 and of 0.1
C in late
1991. Since these events seemed to be near the time when new satellites were
merged into the data set (NOAA-07 and NOAA-12, respectively), they suggested
that improper biases might have been applied to those two satellites.
It is possible to test these claims by producing the anomalies of each of the
satellites independently. A comparison of the anomalies indicate no such break
occurred in 1981 or in 1991, i.e., the anomalies of the time series remained the
same with or without the addition of the new satellites. In addition, comparisons
with radiosonde anomalies across both of the boundaries of alleged jumps verified
the MSU anomalies (Christy et al., 1997, 1998). Speculation cited by Hurrell and
Figure 3 Annual global tropospheric temperature anomalies 1958–1997 determined from
radiosonde data sets and the MSUs on polar orbiting satellites. The layer is roughly between 1
and 8 km altitude.
4 UPPER AIR TEMPERATURES 877
Trenberth (1997) for the causes of the ‘‘jumps’’ (i.e., surface emissivity effects, bias
errors, etc.) were investigated thoroughly and shown to be without foundation.
Mentioned, but not emphasized, by Hurrell and Trenberth was the possibility that
the SSTs and troposphere indeed do show evidence of slight independence in multi-
decadal trends (a phenom enon already verified in other regions, Ross et al., 1996).
The apparent jumps between SSTs and the troposphere are an intriguing observa-
tion. We examined this effect further and found the real source of the shifts appear to
be due to a relative warming of the SSTs in the tropical oceans from the Indian
eastward through the central Pacific. We compared the tropical SST time series with
that of the Night Marine Air Temperatures (NMATs), and in Figure 4 we see that,
remarkably, there are shifts between the water and air temperatures at the suspected
points in time (between 1981–1982 and 1991–1992). These shifts are evident with-
out any appeal to tropospheric temperatures and shows that the NMATs produce a
trend 0.11
C=decade cooler than the SSTs immediately underneath for this very
large region (Christy et al., 1998). These shifts are not seen in the tropical Atlantic
(Fig. 5). One possible factor being observed here is an apparent slight increase in the
instability of the tropical atmosphere on the order of 0.2 to 0.3
C over a vertical
extent of about 4 to 5 km (but mostly in the lowest 20 m) since 1979.
An additional possible factor for explaining the difference between surface and
tropospheric trends is related to the change in the SST observing system of the
tropical Indian and Pacific Oceans. Since the mid-1980s, moored and drifting
buoys have supplied a massive increase in observations. In some months in the
near-equatorial Pacific, over 90% of the observations now are derived from buoys.
It is possible, though not proven as of yet, that the buoy temperatures may be warmer
than traditional ship reports because their nominal depth of measurement is 1 m, a
Figure 4 Annual anomalies of tropical temperatures of seawater temperatures (SSTs) and
night marine air temperatures (NMATs) from the Indian eastward to the central Pacific oceans
(from UK Meteorology Office).
878
MEASURING GLOBAL TEMPERATURE
shallower and warmer depth than the typical ship-intake depth of 3 to 20 m. The
increase in buoy reports may have introduced a slight warming bias since the mid-
1980s. This is another issue that will be investigated to assure that the time series of
SSTs is as homogeneous as possible (C. Folland, personal communication).
It appears that notions of vertically fixed linkages between the tropical SSTs and
troposphere require some refinement in light of the comparisons shown above. In
any case, the evidence presented here shows that over the past 20 years (a woefully
brief period in terms of climate variations) the global troposphere has not experi-
enced any significant warming or cooling. What may be of most interest in terms of
climate is finding the explanation for the apparent difference in warming rates
between the tropical surface and the free atmosphere.
The lower-stratospheric temperatures reveal considerable interannual variations
(Fig. 6). Warming episodes related to eruption s of Agung (1963), El Chichon
(1982), and Mt. Pinatubo (1991) stand out as the largest features. Underlying this
punctuated time series is a general downward trend in the global stratospheric
temperature, with an apparent acceleration in the last years. The location and
degree of decline correlates well with similar decreases in the concentration of
stratospheric ozone (Christy and Drouilhet, 1994; McCormack and Hood, 1994).
With only a few decades available, however, it is difficult to know the character and
extent of natural variations for this layer.
5 CONCLUSION
It should be apparent that all of the data sets mentioned above contain uncertainties
in their ability to answer for us ‘‘what is the global temperature?’’ They provide
information for different parts of the global system and thus should be viewed as
pieces that imperfectly highlight specific components of the climate puzzle.
Figure 5 As in Fig. 4 but for the tropical Atlantic Ocean.
5 CONCLUSION 879
The global surface air temperature has surely risen in the past 150 years, while
between 1400 and 1850 it appears to have been relatively level. In the centuries
before 1400, the scant and murky evidence implies a period of relative warmth
unrelated to any human factor. The troposphere has shown warming since 1958,
which is due to a relatively rapid shift during only 5 years, 1976–1980. Without this
shift, there would be no warming observed (which points to the limitations of
discussing linear trends determined from a fluctuating time series). The global strato-
sphere clearly has experienced a decline in temperature, consistent with both ozone
depletion and enhanced concentrations of greenhouse gases, though the character of
natural variability is largely unknown for this layer.
The presence of so many types of uncertainty in our ability to determine the
global temperature should draw attention to the critical needs we currently face. Our
global observing system requires a more spatially and temporally complete set of
both in situ and remote observations made with instruments that are preci sely cali-
brated and consistent in quality. The current concerns of climate change have
brought the present inadequacies to light and hopefully will spur action to increase
our global network of observations so we will have the necessary informat ion to
make reasonable decisions about possible human impacts on climate.
Author’s note: Much of the material contained in this chapter was developed for
the Norwegian Academy of Technological Sciences and appears in Do We Under-
Figure 6 Annual global lower-stratospheric temperature anomalies 1958–1997 determined
from radiosonde data sets and the MSUs on polar orbiting satellites. The layer is centered near
20 km altitude.
880
MEASURING GLOBAL TEMPERATURE
stand Global Climate Change? (an international seminar at Oslo at Holmen
Fjordhotell, Asker, 11–12 June, 1998) Trondheim, June, 1998.
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CHAPTER 47
SATELLITE VERSUS IN SITU
MEASUREMENTS AT THE
AIR–SEA INTERFACE
KRISTINA B. KATSAROS
In this chapter we explore the trade-offs in selecting surface in situ versus satellite
platforms to measure properties near or at the air–sea interface. The most obvious
difference between the two observing platforms is sampling coverage in time and
space. A surface platform can obtain continuous measurements at a point, while a
polar-orbiting satellite instrument samples, at most, twice per day depending on the
swath width of the sensor. A geostationary satellite can sample the surface as
frequently as every 15 min (once per hour is typical), but the high altitude
(38,000 km) lim its the resolution that is achievable for some sensors. To focus the
discussion, we compare the following two variables commonly measured by both
in situ and satellite systems: the sea surface temperature (SST) and surface wind
speed, U, or wind vector, U
¯
.
1 SEA SURFACE TEMPERATURE
SSTs were traditionally measured from ships by the insertion of a merc ury thermo-
meter into water samples obtained by buckets lowered from deck. Currently, the
temperature of the water intake of ships is recorded and reported every 3 h via
satellite to the international World Meteorological Organization (WMO) weather
telecommunications network (Global Transmission System, or GTS) and distributed
to all weather services globally. Moored and drifting buoys have expanded the in situ
network to cover larger areas of the global ocean. In the case of moored buoys,
Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems,
and Measurements, Edited by Thomas D. Potter and Bradley R. Colman.
ISBN 0-471-21490-6 # 2003 John Wiley & Sons, Inc.
885