3.1 Evolution of Global Stratospheric Temperatures

Figure 1 shows time series of global average temperature anomalies in the three SSU and MSU4 channels. As reported by Seidel et al. (2016), the updated NOAA and Met Office SSU records show greater consistency than the previous versions described by Thompson et al. (2012). Differences do remain, however, with the Met Office data set showing ~0.5 K less cooling in SSU channel 2 between 1979 and 2005, and ~0.5 K more cooling in channel 3, but these differences are smaller than those in Thompson et al. (2012) where they exceeded 1 K in channels 1 and 2. The CCMI multimodel mean generally follows closely the NOAA/STAR data set in SSU channels 2 and 3, with the exception of the periods around the major volcanic eruptions (see section 2.2). In the refC1 experiment (see Figure S1), the CCMI multimodel mean temperature anomalies show better agreement with the satellite data sets in the periods around the major volcanic eruptions, though this reflects averaging across a large spread, with some models not simulating any stratospheric heating from volcanic eruptions and some models strongly overestimating the response. In Figure 1, the CCMI multimodel mean shows larger differences from the Met Office data set in SSU channels 2 and 3. In the low to middle stratosphere (SSU channel 1), the two SSU data sets are in better agreement, but the CCMI multimodel mean shows slightly weaker long‐term cooling, on average, compared to the satellite data sets. Of the individual models, ULAQ‐CCM shows a much stronger quasi‐decadal temperature variability than observed, particularly in the upper stratosphere, which suggests an over estimation of the effect of the 11‐year solar cycle on stratospheric temperatures.

Figure 1 Open in figure viewer PowerPoint Time series of global monthly mean temperature anomalies (K) for the period 1979–2016 for the data sets and altitude ranges stated in the figure. Anomalies are shown relative to a baseline of 1979–1981. The number of individual ensemble members plotted for each model is shown in the legend. The multimodel mean is shown in thick purple. Note that only the CESM1(WACCM), GEOSCCM, ULAQ‐CCM, and UMUKCA‐UCAM models include the radiative effects of volcanic aerosols over the hindcast period in the refC2 experiment. Note the UK Met Office SSU data set is shown as 6‐month averages. (a) SSU channel 3 (~40–50 km). (b) SSU channel 2 (~35–45 km). (c) SSU channel 1 (~25–35 km). (d) MSU channel 4 (~13–22 km). SSU = Stratospheric Sounding Unit.

Figure 2 shows the trends in global mean monthly temperatures over the period 1979–2005 in the satellite data sets and the CCMI models. This can be compared with Figure 2 of Thompson et al. (2012). Note that the estimated confidence intervals on the trends in the Met Office SSU data set are larger than in the NOAA SSU data set because there are less data points in the regression and because there is a higher autocorrelation in the regression residuals than found for the monthly mean NOAA time series. Given that the updated Met Office data set is available only as 6‐monthly averages, this provides poorer statistical constraints on the trends than the previous version analyzed by Thompson et al. (2012), which was provided as monthly mean data.

Figure 2 Open in figure viewer PowerPoint Trends in global‐mean stratospheric temperatures between 1979 and 2005 are shown for (a–c) SSU channels and (d) MSU channel 4, for the satellite data sets and CCMI models. Trends are calculated from monthly mean data except for the Met Office data set where 6‐month averages are used. Satellite‐observed trends are denoted by the colored vertical lines. For models with more than one ensemble member, the ensemble mean trend is plotted. The normalized probability distribution functions indicate the confidence ranges on the trend estimates, taking into account the effective number of degrees of freedom in the respective time series. Black bars show the histograms of the trends from the CCMI models. The bin size is 0.01 K/decade. The number of model runs is given in Table 1 . Data in the 2 years following the two major volcanic eruptions in the period (El Chichón and Mt Pinatubo) are excluded from the trend analysis. SSU = Stratospheric Sounding Unit; MSU = Microwave Sounding Unit; CCMI = Chemistry‐Climate Model Initiative.

In the upper stratosphere (SSU channel 3), the CCMI modeled temperature trends range from −0.7 to −1.1 K/decade. There is substantially better agreement between the modeled and observed SSU channel 3 trends than reported by Thompson et al. (2012), who found that both satellite data sets showed stronger upper stratospheric cooling than simulated in most of the models they analyzed. In SSU channel 2, the modeled temperature trends range from −0.6 to −0.8 K/decade, which are also within the uncertainty range of the observed trends. This is again in contrast to the findings of Thompson et al. (2012), who found poor consistency between modeled and observed temperature trends at these altitudes, with the modeled trends lying between the estimated trends from the previous versions of the NOAA and Met Office SSU data sets. In the low to middle stratosphere (SSU channel 1), the modeled trends range from −0.4 to −0.7 K/decade and although these fall within the uncertainty bounds of both SSU data sets, they cluster closer to the best estimate of the trend in the Met Office record and lie below the 50th percentile of the estimated confidence intervals for the NOAA data set. In the lower stratosphere (MSU4), the modeled temperature trends over 1979–2005 range from −0.2 to −0.4 K/decade, which are in good agreement with satellite‐observed estimates.

In summary, there are significant improvements in the consistency between modeled and observed trends in SSU layer temperatures compared to the findings of Thompson et al. (2012). These improvements are predominantly from the recent revisions to the SSU records, while the distributions of the modeled trends are similar to the results from the fifth Coupled Model Intercomparison Project and Chemistry‐Climate Model Validation project models presented in Thompson et al. (2012). The subsequent analyses in this study focus on the extended NOAA/STAR SSU data set since the Met Office data set is provided as global averages and ends in 2005, when the SSU stopped making measurements, meaning an assessment of temperature trends over the recent past and of the spatial pattern of trends is not possible. For the MSU4 channel, we also show for simplicity only the NOAA/STAR data set, which has similar long‐term trends to the RSS data set but somewhat weaker trends than the UAH record.

Figure 3 shows vertical profiles of global mean stratospheric temperature trends over the periods 1979–2016 (Figure 3a), 1979–1997 (Figure 3b), and 1998–2016 (Figure 3c) in the CCMI simulations and satellite data sets. The trends over these periods in the SSU and MSU4 channels for all the data sets along with their 95% confidence intervals are given in Table 2. Over 1979–2016 there is an increase in the magnitude of the cooling trend with height. This is understood to be due to two factors: (1) the effect on stratospheric temperatures from an increase in CO 2 increases with height (e.g., Manabe & Wetherald, 1975) and (2) the vertical profile of ozone depletion which causes a relative maximum in cooling in the lower stratosphere and a larger peak in the upper stratosphere (see Figure 1 of Shine et al., 2003). The latter effect is particularly evident in the modeled temperature trends over the period 1979–1997 (Figure 3b) when ozone depletion was increasing with time. Global stratospheric cooling has continued at levels above ~60 hPa since 1998, approximately when atmospheric concentrations of halogenated ODSs began to decline (e.g., WMO, 2014), but the magnitude of cooling is weaker and the vertical profile shows a more gradual increase with height (Figure 3c) compared to the earlier period, consistent with the dominant effect of GHGs on stratospheric temperatures since ~1998. This can also be seen in the CCMI experiments with GHG concentrations fixed at 1960 levels (senC2fGHG) and with ODS concentrations fixed at 1960 levels (senC2fODS; Figure 4). When GHG concentrations are fixed at 1960 levels, the models show substantially weaker temperature trends in the middle and upper stratosphere over 1998–2016 with a small warming in the upper stratosphere (Figure 4c), which is presumably related to increases in upper stratospheric ozone over this period (Harris et al., 2015). In contrast, when increases in GHGs are considered but ODSs are fixed at 1960 levels (Figure 4f), the models capture the observed cooling in the middle and upper stratosphere over 1998–2016. ULAQ‐CCM is the exception as it appears to show stronger stratospheric cooling over 1998–2016 when GHGs are fixed at 1960 levels (Figure 4c); this is likely the result of the model simulating strong quasi‐decadal variability in stratospheric temperatures likely related to the solar cycle, (see section 3.1) and because the start and end points of the trend period are close to a maximum and minimum phase of the 11‐year solar cycle, respectively. Near 1 hPa, the CCMI models show approximately equal contributions to cooling over 1979–1997 from ODSs (Figure 4b) and GHGs (Figure 4e), consistent with earlier attribution studies (e.g., Shine et al., 2003). In the middle stratosphere (7–30 hPa), cooling over 1979–1997 was dominated by the effects of GHGs (see also Shine et al., 2003, Figures 1 and 2). In summary, the CCMI models simulate significantly smaller stratospheric temperature trends over 1998–2016 compared to 1979–1997, which are consistent with estimates from satellite data sets (see also Randel et al., 2016, 2017; Zou and Qian, 2016).

Figure 3 Open in figure viewer PowerPoint Vertical profiles of global mean annual temperature trends (K/decade) in the SSU‐AMSU and MSU4‐AMSU NOAA/STAR satellite data set (black bars) and the CCMI models (colors). Trends are calculated for (a) 1979–2016, (b) 1979–1997, and (c) 1998–2016. The number of individual ensemble members plotted for each model is given in the legend. The horizontal whiskers denote 95% confidence intervals on the observed linear trends accounting for the effective number of degrees of freedom in the time series. The vertical whiskers denote the approximate altitude range of the satellite weighting functions. Data in the 2 years following the two major volcanic eruptions in the period (El Chichón and Mt Pinatubo) are excluded from the trend analysis. SSU = Stratospheric Sounding Unit; AMSU = Advanced Microwave Sounding Unit; MSU4 = Microwave Sounding Unit channel 4; NOAA/STAR = National Oceanographic and Atmospheric Administration Center for Satellite Applications and Research; CCMI = Chemistry‐Climate Model Initiative.

Table 2. Linear Temperature Trends (K/Decade) With 95% Confidence Intervals for Different Data Sets and Periods MSU4 (~13–22 km) SSU1 (~25–35 km) SSU2 (~35–45 km) SSU3 (~40–50 km) Model/dataset 1979–2005 1979–1997 1998–2016 1979–2005 1979–1997 1998–2016 1979–2005 1979–1997 1998–2016 1979–2005 1979–1997 1998–2016 CCMI −0.25 ± 0.12 −0.37 ± 0.14 −0.02 ± 0.07 −0.50 ± 0.12 −0.66 ± 0.14 −0.34 ± 0.14 −0.70 ± 0.16 −0.92 ± 0.12 −0.49 ± 0.16 −0.88 ± 0.23 −1.16 ± 0.17 −0.58 ± 0.21 Met Office — — — −0.55 ± 0.35 −0.82 ± 0.25 — −0.50 ± 0.50 −0.74 ± 0.77 — −1.00 ± 0.86 −1.61 ± 0.50 — NOAA −0.28 ± 0.14 −0.48 ± 0.17 −0.07 ± 0.14 −0.64 ± 0.17 −0.90 ± 0.14 −0.25 ± 0.10 −0.72 ± 0.25 −1.07 ± 0.26 −0.40 ± 0.11 −0.80 ± 0.29 −1.15 ± 0.53 −0.46 ± 0.11 RSS −0.28 ± 0.12 −0.45 ± 0.15 −0.08 ± 0.13 — — — — — — — — — UAH −0.34 ± 0.12 −0.51 ± 0.15 −0.09 ± 0.13 — — — — — — — — —