2.2.4 How do Surface and Upper Air Temperature Variations
Compare?
In Figure 2.12 we display the surface, tropospheric
and strato-spheric temperature variations using representative data sets from
those described above. Trend values (°C/decade) are shown in Table
2.3 with 95% confidence intervals, which in part represent uncertainties
due to temporal sampling, not those due to measurement error (see below). The
effect of explosive volcanic events (Agung, 1963; El Chichon, 1982; and Mt.
Pinatubo, 1991) is evident in Figure 2.12, as is
a relative shift to warmer temperatures in the lower troposphere compared to
the surface in the late 1970s, followed by large variations in both due to ENSO
(particularly in 1998). After the shift in the late 1970s, the overall tropospheric
temperature trend is near zero but the surface has warmed (see Figure
2.12a and Table 2.3).
| Table 2.3: As Table 2.1
but for annual average surface and upper air temperature anomalies from
various data sets. The surface temperature trends are of combined land-surface
air temperature (LSAT) and sea surface temperature (SST) or sea ice and
sea surface temperature (ISST) anomalies. The upper air trends are of temperature
anomalies corresponding to or approximately corresponding to temperature
anomalies from MSU channels 2LT and 4. The tropical region is defined as
the latitude band 20°S to 20°N for all the data sets except for
the GISS LSAT + UKMO ISST data set where the region is defined as the latitude
band 23.6°S to 23.6°N. The last line of the table shows trends in
the differences between temperature anomalies for the surface, from the
UKMO LSAT + UKMO SST data set, and for the lower troposphere, taken as the
average of the UKMO 2LT and MSU 2LT anomalies for 1979 to 2000 and as the
UKMO 2LT anomalies alone before 1979. None of the estimates of trends and
errors account for uncertainties in the annual anomalies as these are not
available. All calculations use data to the end of 2000 except for those
for the NOAA data sets, which include data up to August 2000 only. |
 |
| |
1958 to 2000
|
1958 to 1978
|
1979 to 2000
|
| |
Globe
|
Trophics
|
Globe
|
Trophics
|
Globe
|
Trophics
|
|
| Surface |
|
UKMO LSAT + UKMO SST
(Jones et al., 2001) |
0.10
(0.05)
|
0.08
(0.06)
|
-0.05
(0.07)
|
-0.09
(0.12)
|
0.16
(0.06)
|
0.10
(0.10)
|
|
GISS LSAT + UKMO ISST
(Hansen et al., 1999; Rayner et al., 2000) |
0.09
(0.04)
|
0.09
(0.06)
|
-0.03
(0.07)
|
-0.09
(0.11)
|
0.13
(0.07)
|
0.09
(0.10)
|
|
NCDC LSAT + NCEP SST
(Quayle et al., 1999; Reynolds and Smith 1994) |
0.09
(0.05)
|
0.09
(0.06)
|
-0.05
(0.06)
|
-0.08
(0.11)
|
0.14
(0.06)
|
0.10
(0.11)
|
|
| Lower troposphere |
|
UKMO 2LT
(parker et al., 1997) |
0.11
(0.07)
|
0.13
(0.08)
|
-0.03
(0.12)
|
0.07
(0.16)
|
0.03
(0.10)
|
-0.08
(0.12)
|
|
MSU 2LT
(Christy et al., 2000) |
|
|
|
|
0.04
(0.11)
|
-0.06
(0.16)
|
|
NCEP 2LT
(Stendel et al., 2000) |
0.13
(0.07)
|
0.08
(0.08)
|
0.02
(0.18)
|
-0.5
(0.17)
|
0.01
(0.11)
|
-0.07
(0.14)
|
|
NOAA 850-300hPa
(Angell, 2000) |
0.07
(0.08)
|
0.07
(0.07)
|
-0.08
(0.15)
|
0.04
(0.20)
|
-0.03
(0.15)
|
-0.11
(0.19)
|
|
RIHMI 850-300hPa
(Sterin, 1999) |
0.04
(0.04)
|
0.07
(0.05)
|
-0.03
(0.06)
|
0.07
(0.08)
|
0.00
(0.07)
|
-0.06
(0.09)
|
|
| Lower stratosphere |
|
UKMO 4
(Parker et al., 1997) |
-0.39
(0.15)
|
-0.31
(0.19)
|
-0.37
(0.21)
|
-0.07
(0.51)
|
-0.64
(0.47)
|
-0.50
(0.54)
|
|
MSU 4
(Christy et al.,2000) |
|
|
|
|
-0.52
(0.48)
|
-0.29
(0.51)
|
|
NCEP 4
(Stendel et al., 2000) |
-0.25
(0.62)
|
-0.04
(0.32)
|
-0.36
(0.33)
|
-0.46
(0.29)
|
-0.61
(1.21)
|
-0.57
(0.77)
|
|
NOAA 100-50hPa
(Angell, 2000) |
-0.64
(0.30)
|
-0.58
(0.39)
|
-0.23
(0.22)
|
0.20
(0.43)
|
-1.10
(0.58)
|
-0.68
(2.08)
|
|
RIHMI 100-50hPa
(Sterin 1999) |
-0.25
(0.12)
|
-0.22
(0.12)
|
-0.20
(0.27)
|
-0.08
(0.10)
|
-0.43
(0.24)
|
-0.45
(0.28)
|
|
| Surface minus lower troposphere |
|
| |
-0.01
(0.05)
|
-0.05
(0.07)
|
-0.03
(0.08)
|
-0.16
(0.10)
|
0.13
(0.06)
|
0.17
(0.06)
|
Global variations and trends in the lower stratosphere are temporally more
coherent than in the troposphere (Figure 2.12b),
though the warming effects due to the volcanic eruptions are clearly evident.
For the period 1958 to 2000, all stratospheric data sets except NCEP 4, which
contains erroneous trends, show significant negative trends (Table
2.3). Note that MSU 4, and simulations of MSU 4 (UKMO 4 and NCEP 4), include
a portion of the upper troposphere below 100 hPa and so are expected to show
less negative trends than those measuring at higher altitudes (e.g., the 100
to 50 hPa layers in Table 2.3 and the SSU in Figure
2.12b).
Blended information for 5 km thick levels in the stratosphere at 45°N compiled
by Chanin and Ramaswamy (1999) show a negative trend in temperature increasing
with height: -0.5°C/decade at 15 km, -0.8°C/decade at 20 to 35 km, and
-2.5°C/decade at 50 km. These large, negative trends are consistent with
models of the combined effects of ozone depletion and increased concentrations
of infrared radiating gases, mainly water vapour and carbon dioxide (Chapters
6 and 12).
The vertical profile of temperature trends based on surface data and radiosondes
is consistent with the satellite temperatures. Global trends since 1979 are
most positive at the surface, though less positive for night marine air temperatures
in the Southern Hemisphere (see Section 2.2.2.2),
near zero for levels between 850 to 300 hPa (1.5 to 8 km) and negative at 200
hPa (11 km) and above. Thus during the past two decades, the surface, most of
the troposphere, and the stratosphere have responded differently to climate
forcings because different physical processes have dominated in each of these
regions during that time (Trenberth et al., 1992; Christy and McNider, 1994;
NRC, 2000 and Chapter 12). On a longer time-scale, the
tropospheric temperature trend since 1958, estimated from a sparser radiosonde
network, is closer to that of the surface, about +0.10°C/decade (Figure
2.12a and Table 2.3) (Angell, 1999, 2000; Brown et
al., 2000; Gaffen et al., 2000a). Gaffen et al. (2000a) and Brown et al. (2000)
noted a decreasing lower-tropospheric lapse rate from 1958 to 1980, and an increasing
lower-tropospheric lapse rate after 1980 (Figure 2.12a).
However, Folland et al. (1998) showed that global upper-tropospheric temperature
has changed little since the late 1960s because the observed stratospheric cooling
extended into the uppermost regions of the troposphere.
Between 1979 and 2000, the magnitude of trends between the surface and MSU
2LT is generally most similar in many parts of the Northern Hemisphere extra-tropics
(20°N to pole) where deep vertical mixing is often a characteristic of the
troposphere. For example, the northern extra-tropical trends for the surface
and MSU 2LT were 0.28 and 0.21°C/decade, respectively, and over the North
American continent trends were 0.27 ± 0.24 and 0.28 ± 0.23°C/decade,
respectively, with an annual correlation of 0.92. Over Europe the rates were
0.38 ± 0.36 and 0.38 ± 0.30°C/decade, respectively. Some additional
warming of the surface relative to the lower troposphere would be expected in
the winter half year over extra-tropical Asia (whole year warming rates of 0.35
± 0.20 and 0.18 ± 0.18°C/decade, respectively), consistent
with the vertical temperature structure of the increased positive phase of the
Arctic Oscillation (Thompson et al., 2000a, Figure
2.30). The vertical structure of the atmosphere in marine environments,
however, generally reveals a relatively shallow inversion layer (surface up
to 0.7 to 2 km) which is somewhat decoupled from the deep troposphere above
(Trenberth et al., 1992; Christy, 1995; Hurrell and Trenberth, 1996). Not only
are local surface versus tropospheric correlations often near zero in these
regions, but surface and tropospheric trends can be quite different (Chase et
al., 2000). This is seen in the different trends for the period 1979 to 2000
in the tropical band, 0.10 ± 0.10 and -0.06 ± 0.16°C/decade,
respectively (Table 2.3) and also in the southern extra-tropics where the trends
are 0.08 ± 0.06 and -0.05 ± 0.08°C/decade, respectively. Trends
calculated for the differences between the surface and the troposphere for 1979
to 2000 are statistically significant globally at 0.13 ± 0.06°C/decade,
and even more so in the tropics at 0.17 ± 0.06°C/decade. Statistical
significance arises because large interannual variations in the parent time-series
are strongly correlated and so largely disappear in the difference time-series
(Santer et al., 2000; Christy et al., 2001). However, as implied above, they
are not significant over many extra-tropical regions of the Northern Hemisphere
such as North America and Europe and they are also insignificant in some Southern
Hemisphere areas. The sequence of volcanic eruption, ENSO events, and the trends
in the Arctic Oscillation have all been linked to some of this difference in
warming rates (Michaels and Knappenburg, 2000; Santer et al., 2000; Thompson
et al., 2000a; Wigley, 2000) and do explain a part of the difference in the
rates of warming (see Chapter 12).
The linear trend is a simple measure of the overall tendency of a time-series
and has several types of uncertainty; temporal sampling uncertainty owing to
short data sets, spatial sampling errors owing to incomplete spatial sampling,
and various other forms of measurement error, such as instrument or calibration
errors. Temporal sampling uncertainties are present even when the data are perfectly
known because trends calculated for short periods are unrepresentative of other
short periods, or of the longer term, due to large interannual to decadal variations.
Thus confidence intervals for estimates of trend since 1979 due to temporal
sampling uncertainty can be relatively large, as high as ± 0.2°C/decade
below 300 hPa (Table 2.3, Santer et al., 2000). Accordingly,
the period from 1979 to 2000 provides limited information on long-term trends,
or trends for other 22-year periods.
Uncertainties arising from measurement errors due to the factors discussed
in Section 2.2.3, including incomplete spatial sampling,
can be substantial. One estimate of this uncertainty can be made from comparisons
between the various analyses in Table 2.3. For trends below
300 hPa, this uncertainty may be as large as ± 0.10°C/decade since
1979, though Christy et al. (2000) estimate the 95% confidence interval as ±
0.06°C for the MSU 2LT layer average. For example, Santer et al. (2000)
find that when the satellite observations from MSU 2LT are masked to match the
less than complete global coverage of the surface observations during the past
few decades, the differences in the trends between the surface and the troposphere
are reduced by about one third.
Summarising, it is very likely that the surface has warmed in the global average
relative to the troposphere, and the troposphere has warmed relative to the
stratosphere since 1979 (Figure 2.12a,b; Pielke
et al., 1998a,b; Angell, 1999, 2000; Brown et al., 2000; Christy et al., 2000;
Gaffen et al., 2000a; Hurrell et al., 2000; NRC, 2000; Stendel et al., 2000).
However, the relative warming is spatially very variable and most significant
in the tropics and sub-tropics. There is evidence that the troposphere warmed
relative to the surface in the pre-satellite era (1958 to 1979, see Brown et
al., 2000; Gaffen et al., 2000a), though confidence in this finding is lower.
Uncertainties due to limited temporal sampling prevent confident extrapolation
of these trends to other or longer time periods (Christy et al., 2000; Hurrell
et al., 2000; NRC, 2000; Santer et al., 2000). Some physical explanations for
changes in the vertical profile of global temperature trends are discussed in
Chapter 12 but a full explanation of the lower-tropospheric
lapse rate changes since 1958 requires further research.
