The last hundredyears 59Atmospheric temperature observed by satellites
Since 1979 meteorological satellites flown by the National Oceanic and
Atmospheric Administration (NOAA) of the United States have carried
a microwave instrument, the Microwave Sounding Unit (MSU), for the
remote observation of the average temperature of the lower part of the
atmosphere.
Figure. 4.2(a) shows the record of global average temperature de-
duced from the MSU and compares it with data from sounding instru-
ments carried on balloons for the same region of the atmosphere, showing
very good agreement for the period of overlap. The record of temperature
at the surface is also added for comparison over the period from 1960
to 2000.All three measurements showsimilar variability, the variations
at the surface tracking well with those in the lower troposphere. The
plots also illustrate the difficulty of deriving accurate trends from a short
period of record where there is also substantial variability. Since 1979 the
trends from the three measurement sources have been carefully studied
and compared.They are 0. 04 ± 0. 11 ◦C per decade and 0. 03 ± 0. 10 ◦C
per decade for the satellite and balloon data respectively compared with
0. 16 ± 0. 06 ◦C per decade for the surface data. The trend in the differ-
ence of the surface and lower troposphere of 0. 13 ± 0. 06 ◦C per decade
is statistically significant. This is in contrast to near-zero surface tem-
perature trends from 1958 to 1978 when the global lower troposphere
temperature warmed by about 0.03◦C per decade relative to the surface.
There are substantial regional variations in the differences between sur-
face and lower tropospheric temperature trends since 1979. For instance,
the differences are particularly apparent in many parts of the tropics and
sub-tropics where the surface has warmed faster than the lower tropo-
sphere, while over some other regions, e.g. North America, Europe and
Australia, the trends are very similar.
Why the surface and lower temperature trends show significant dif-
ferences, especially over the tropical and sub-tropical oceans, is not com-
pletely understood although there area number of possible reasons for
the differences.^2 It is of course well known that the presence of increased
concentrations of greenhouse gases leads to cooling of the atmosphere
at higher levels (see Chapter 2). In the stratosphere therefore the tem-
perature trends are reversed (Figure 4.2(b)) ranging froma decrease of
about 0.5◦C per decade in the lower stratosphere to 2.5◦C per decade in
the upper stratosphere.Pinatubo in the Philippines in 1991 (the low global average temperature
in 1992 and 1993, compared with neighbouring years, is almost certainly
due to the Pinatubo volcano). But there is no need to invoke volcanoes or
other external causes to explain all the variations in the record. Many of