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.

IPCC Homepage