18.104.22.168 Synthesis and Comparison with the Surface Temperatures
Figure 3.17 presents the radiosonde and satellite global time series and Figure 3.18 gives a summary of the linear trends for 1979 to 2004 for global and tropical (20°N to 20°S) averages. Values at the surface are from NOAA (NCDC), NASA (GISS), UKMO/CRU (HadCRUT2v) and the NRA and ERA-40 reanalyses. Trends aloft are for the lower troposphere corresponding to T2LT, T2, T4 and also the linear combination of T2 and T4 to better depict the entire troposphere as given by Fu et al. (2004a). In addition to the reanalyses, the results from the satellite-based methods from UAH, RSS and VG2 are given along with radiosonde estimates from HadAT2 and RATPAC. The ERA-40 trends only extend through August 2002, and VG2 is available only for T2. The error bars plotted here are 5 to 95% confidence limits associated with sampling a finite record where an allowance has been made for temporal autocorrelation in computing degrees of freedom (Appendix 3.A). However, the error bars do not include spatial sampling uncertainty, which increases the noise variance. Noise typically cuts down on temporal autocorrelation and reduces the temporal sampling error bars, which is why the RATPAC error bars are often smaller than the rest. Other sources of ‘structural’ and ‘internal’ errors of order 0.08°C for 5 to 95% levels (Mears and Wentz, 2005; see Appendix 3.B.5) are also not explicitly accounted for here. Structural uncertainties and parametric errors (Thorne et al., 2005b) reflect divergence between different data sets after the common climate variability has been accounted for and are better illustrated by use of difference time series, as seen for instance in T2 for RSS vs. UAH in Fu and Johanson (2005; see also Karl et al., 2006).
From Figure 3.17 the first dominant impression is that overall, the records agree remarkably well, especially in the timing and amplitude of interannual variations. This is especially true at the surface, and even the tropospheric records from the two radiosonde data sets agree reasonably well, although HadAT2 has lower values in the 1970s. In the lower stratosphere, all records replicate the dominant variations and the pulses of warming following the volcanic eruptions indicated in the figure. The sonde records differ prior to 1963 in the lower stratosphere when fewer observations were available, and differences also emerge among all data sets after about 1992, with the sonde values lower than the satellite temperatures. The focus on linear trends tends to emphasize these relatively small differences.
A linear trend over the long term is often not a very good approximation of what has occurred (Seidel and Lanzante, 2004; Thorne et al., 2005a,b); alternative interpretations are to factor in the abrupt 1976–1977 climate regime shift (Trenberth, 1990) and episodic stratospheric warming and tropospheric cooling for the two years following major volcanic eruptions. Hence, the confidence limits for linear trends (Figure 3.18) are very large in the lower stratosphere owing to the presence of the large warming perturbations from volcanic eruptions. In the troposphere, the confidence limits are much wider in the tropics than globally, reflecting the strong interannual variability associated with ENSO.
Radiosonde, satellite observations and reanalyses agree that there has been global stratospheric cooling since 1979 (Figures 3.17 and 3.18), although radiosondes almost certainly still overestimate the cooling owing to residual effects of changes in instruments and processing (such as for radiation corrections; Lanzante et al., 2003b; Sherwood et al., 2005; Randel and Wu, 2006) and possibly increased sampling of cold conditions owing to stronger balloons (Parker and Cox, 1995). As the stratosphere is cooling and T2 has a 15% signal from there, it is virtually certain that the troposphere must be warming at a significantly greater rate than indicated by T2 alone. Thus, the tropospheric record adjusted for the stratospheric contribution to T2 has warmed more than T2 in every case. The differences range from 0.06°C per decade for ERA-40 to 0.09°C per decade for both radiosonde and NRA data sets. For UAH and RSS the difference is 0.07°C per decade.
The weakest tropospheric trends occur for NRA. However, unlike ERA-40, the NRA did not allow for changes in greenhouse gas increases over the record (Trenberth, 2004), resulting in errors in radiative forcing and in satellite retrievals in the infrared and making trends unreliable (Randel et al., 2000); indeed, upward trends at high surface mountain stations are stronger than NRA free-atmosphere temperatures at nearby locations (Pepin and Seidel, 2005). The records suggest that since 1979, the global and tropical tropospheric trends are similar to those at the surface although RSS, and by inference VG2, indicate greater tropospheric than surface warming. The reverse is indicated by the UAH and the radiosonde record although these data are subject to significant imperfections discussed above. Amplification occurs in the tropics for the RSS fields, especially after 1987 when there are increasing trends with altitude throughout the troposphere based on T2, T3 and T4 (Fu and Johanson, 2005). In the tropics, the theoretically expected amplification of temperature perturbations with height is borne out by interannual fluctuations (ENSO) in radiosonde, RSS, UAH and model data (Santer et al., 2005), but it is not borne out in the trends of the radiosonde records and UAH data.
The global mean trends since 1979 disguise many regional differences. In particular, in winter much larger temperature trends are present at the surface over northern continents than at higher levels (Karl et al., 2006) (see Figures 3.9 and 3.10; FAQ 3.1, Figure 1). These are associated with weakening of shallow winter temperature inversions and the strong stable surface layers that have little signature in the main troposphere. Such changes are related to changes in surface winds and atmospheric circulation (see Section 3.6.4).
In summary, for the period since 1958, overall global and tropical tropospheric warming estimated from radiosondes has slightly exceeded surface warming (Figure 3.17 and Karl et al., 2006). The climate shift of 1976 appeared to yield greater tropospheric than surface warming (Figure 3.17); such climate variations make differences between the surface and tropospheric temperature trends since 1979 unsurprising. After 1979, there has also been global and tropical tropospheric warming; however, it is uncertain whether tropospheric warming has exceeded that at the surface because the spread of trends among tropospheric data sets encompasses the surface warming trend. The range (due to different data sets, but not including the reanalyses) of global surface warming since 1979 from Figure 3.18 is 0.16°C to 0.18°C per decade compared to 0.12°C to 0.19°C per decade for MSU estimates of tropospheric temperatures. A further complexity is that surface trends have been greater over land than over ocean. Substantial cooling has occurred in the lower stratosphere. Compensation for the effects of stratospheric cooling trends on the T2 record (a cooling of about 0.08°C per decade) has been an important development. However, a linear trend is a poor fit to the data in the stratosphere and the tropics at all levels. The overall global variability is well replicated by all records, although small relative trends exacerbate the differences between records. Inadequacies in the observations and analytical methods result in structural uncertainties that still contribute to the differences between surface and tropospheric temperature trends, and revisions continue to be made. Changes in the height of the tropopause since 1979 are consistent with overall tropospheric warming as well as stratospheric cooling.