Radiative forcing due to stratospheric ozone changes , using updated trend estimates

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D20, PAGES 24,395-24,399, OCTOBER 27, 1999 Radiative forcing due to stratospheric ozone changes , using updated trend estimates Piers M. de F. Forster Department of Meteorology, University of Reading, Reading, England, U.K. Abstract. Recent estimates of stratospheric ozone changes since 1979, both from satellites and ground-based instruments, show a smaller rate of ozone loss than previous measurements indicated. This paper calculates the radiative forcing resulting from these ozone changes. The trends are derived from a combination of Stratospheric Aerosol and Gas Experiment (SAGE), Total Ozone Mapping Spectrometer (TOMS), and ground-basedata. By combining these data in a variety of ways, several problems of determining an accurate forcing from available data are highlighted. Using only statistically significant changes, a "best guess" radiative forcing of W m -2 is estimated for the period which is about 50% less negative than previous estimates of this forcing, but provides an offset of about 10% to the well-mixed greenhouse gas forcing over the same period. 1. Introduction large tropical depletion [Forster and Shine, 1997] would It is important to quantify the consequences of stratotherefore obtain a negative forcing in the tropics, which spheric ozone change as anthropogenic decreases in stratcould be largely fictitious. ospheric ozone over the last 20 years may have partially This paper uses the WMO [1999, 1998] ozone trend offset the warming from well-mixed greenhouse gas indata to calculate updated values of the radiative forccreases alone. Over the same time period the observed ing. This paper does not address problems with the cooling of the lower stratosphere may well have been radiative forcing concept [see, e.g., Hansen et al., 1997; largely caused by ozone depletion and provides some of WMO, 1999], but rather uses the IPCC [1995] definition the best evidence for an anthropogenic induced climate of radiative forcing to examine the effect of the updated change [see, e.g., Inter Governmental Panel on Climate ozone trend estimates. At the very least the results can Change (IPCC), 1995]. be compared to previous calculations of stratospheric Estimates of the radiative forcing resulting from strat- ozone radiative forcing, and approximately to the raospheric ozone loss, based on observed ozone changes, diative forcing from well-mixed greenhouse gases. have ranged between-0.02 and-0.23 W m -2 with a central value around-0.15 W m -2 [Shine and Forster, ]. New estimates of stratospheric ozone changes Method are now available [World Meteorological Organization (WMO), 1999, 1998], giving both column and vertical profile trends, which are roughly 30% smaller than previous estimates. In the tropics, between The radiation scheme is described by Forster and Shine [1997] and the input climatology is described by Christidis et al. [1997]. The fixed dynamical heating approximation [Ramanathan and Dickinson, 1979] is km, Stratospheric Aerosol and Gas Experiment (SAGE) trends show uncorroborated large ozone decreases, and WMO [1998] recommends that SAGE trends ought to only be used above 20 kin. Studies which employ this used to adjust stratospheric temperatures. The method adjusts the stratosphere to an equilibrium state assuming constant dynamical heating rates in the stratosphere. A zonally and seasonally averaged 5 ø latitudi- x Now at Cooperative Research Centre for Southern Hemisphere Meteorology, Monash University, Clayton, Victoria, Australia. Copyright 1999 by the American Geophysical Union. Paper number 1999JD /99/1999JD ,395 nal resolution climatology was used as the basis of the forcing calculations. The main source of error in the radiative forcing calcluations arises from uncertainties in the ozone trends discussed below. Other sources of error such as the use of seasonal, rather than monthly, averages or assuming that the stratosphere is in equilibrium lead to errors that are generally smaller than 10% [Forster et al., 1997]. The climatological ozone profile is perturbed by seasonally averaged ozone changes, and stratospheric temperatures are adjusted to give

2 24,396 FORSTER: RADIATIVE FORCING OF STRATOSPHERIC OZONE CHANGES o -10 I, I, Latitude (degrees) Figure 1. The zonal and annual average trend in total column ozone (% decade - ) for the SA column ozone change above 20 km (solid line), the TO total column ozone change (dashed line), and an estimate of the troposphericolumn ozone changes from T. Berntsen (personal communication 1999) (dotted line). The lc uncertainties are plotted for the SA and TO data from WMO [1998]. _ a W m -2 irradiance change at the tropopause. The tropopause is taken as the 2 K km - lapse rate defini- tion. Several different scenarios of ozone change are used. Uncertainties in the radiative forcing are also calculated, using the uncertainty in the trend estimates. SA uses seasonal SAGE trends and their uncertainty [WMO, 1998] to perturb ozone between km at I km vertical resolution. The uncertainty is given as a standard error of the mean trend and arises from the natural variability of the ozone concentration. Poleward of 4-60 ø, where no SAGE trends are available, the percent per decade SAGE trends at 60 ø were used. This assumption is somewhat supported by inspection of SBUV trends [WMO, 1998], which are available 15 ø further poleward. The annually averaged column change (20-50 kin) is shown in Figure 1, and the vertical structure of the trends are plotted in Figure 2 above 20 kin. In TO, using a similar methodology to previous studies [see WMO, 1999] seasonal Total Ozone Mapping Spectrometer (TOMS) total column ozone changes and ground station total column trends poleward of 4.60 ø [WMO, 1999] were used to deplete ozone by a constant 50 Ozone trend (% per decade) OI I, I [ I I, I,,,, I,, I Latitude [degrees] Max = Min = Figure 2. The zonal and annual averaged ozone change (% decade - ), as a function of latitude and height, for the ST scenario, discussed in the text. The light and dark shading show the lc and 2c confidence levels, respectively. Negative values are shown as dashed lines. The tropopause rises from 6-7 km in the polar regions to 17 km in the tropics.

3 . FORSTER: RADIATIVE FORCING OF STRATOSPHERIC OZONE CHANGES 24,397 percentage in the 7 km directly above the tropopause. This scenario has the same total column ozone change between the TOMS and SAGE total column changes. This positive trend is not supported by SAGE data, as the ST scenarios described below. Figure I shows discussed earlier, and ozonesonde data, which show no these total column trends and their uncertainty. In ST, the SA trends are used above 20 km. Ozone changes are not applied in the troposphere, and the TO-SA column residual is applied as a constant percentage change between the tropopause and 20 km. This assumes that tropospheric increases in ozone have not been larger than the uncertainty in the TO data. overall trend [WMO, 1998]. In the northern midlatitudes the depletions of 5-7% decade -x in the lower stratosphere agree well with ozonesonde trends [WMO, 1998]. The large depletions in the Antarctic lower stratosphere are also supported by the ozonesonde data. ST-la uses the same data as ST, but only changes ozone in regions where the absolute ozone change is Both observations [Ziemke et al., 1998] and chemical larger than its let uncertainty (i.e., ozone values are modeling data, shown in Figure I (T. Berntsen, per- only changed in the shaded regions of Figure 2). Likesonal communication, 1999, updated from Berntsen et wise, ST-2a only perturbs ozone where the absolute al. [1997]), indicate total column tropospheric ozone ozone trend is larger than its 2er uncertainty (the darker trends which are smaller than the uncertainty in the shaded regions of Figure 2). These scenarios are nor- TOMS ozone trends, arising from natural variability of the total column ozone. The annual averaged ozone changes are shown in Figure 2; it can be seen that malized to give the same local total column depletion as the TO and ST scenarios, as the TOMS total column changes were deemed the most robust data set. this method produces large positive increases in ozone, which are not statistically significant, just above the tropical tropopause. Above 20 km the uncertanties are 3. Results taken from the SAGE data. Below 20 km the uncer- Figure 3 shows the shortwave (SW), longwave (LW), tainty is estimated from the uncertainty in the risidual and net annually averaged radiative forcings for four 0.2[ E ß -0.6 _ I SA I I I i I_ I I I I I I I... ""'%... I _,;-t' ' ' ' ' -,._ I I I I I I I I I I Latitude (degrees) Figure 3. The zonal and annual averaged radiative forcing (Wm -2 decade -x) for the SA, TO, ST, and ST-la scenarios. Shortwave (dotted line), longwave (dashed line), and net (solid line) radiative forcings are shown with their la uncertainties.

4 24,398 FORSTER: RADIATIVE FORCING OF STRATOSPHERIC OZONE CHANGES scenarios. For the SA scenario the SW and LW forc- The best estimate in this study would probably be a ings tend to cancel each other out, over most of the globe, making the net forcing close to zero. For the other scenarios the ozone changes occur closer to the tropopause; this causes the stratospheric temperature changes to have a larger effect on the forcing, making the LW component dominate the net forcing. The ST scenario gives positive net forcings near the equator which are not statistically significant. This is due to ozone increases just above the tropopause. The forcing that lies within the ST-la and ST-2a scenario estimates (-0.08 to W m-2); more data on the vertical structure of ozone changes are needed to confine this forcing further. However, a forcing of-0.1 W m -2 between is still non negligible as it would have offset the well-mixed greenhouse by roughly 10%. Although linear trends are presented for comparison with other work, the Earth Probe and TOMS instruments both show a leveling off of the ozone trend since 1994, latitudinal gradient of the forcing, in all but the SA sce- implying that their has been little or no change in the nario, arises from high-latitude, lower stratospheric, de- ozone forcing since pletion and agrees with earlier studies [see, e.g. Forster It is also interesting to note that the difference beand Shine, 1997; Hansen et al., 1997]. The ST-la sce- tween the ST and ST-ia scenario is larger than the nario show similar high-latitude forcings to the ST sce- uncertainty estimate of the ST scenario. This is due to nario, but has more negative forcings on the equator, the sign of the ozone forcing swapping around 25 km. due to little of the ozone increases above the equatorial tropopause being significant at the la level. The spikes arise from the patchy nature of the statistical significance. For some latitudes, and seasons the ozone increases above the tropopause will be significant, and at other latitudes and seasons they would not. The As the uncertainty in the forcing is calculated from a perturbation of the same sign everywhere, there is some degree of cancelation between the effects of the perturbation above and below 25 km, so the resulting uncertainty estimate is somewhat less than expected. This paper has also shown that including ozone trends ST-2a scenario (not shown) has a slightly more nega- which are not statistically significant in estimations of tive and smoother net forcing near the equator and a reduced negative forcing at mid to high latitudes, but overall the pattern is similar to the ST-la scenario. the effects of ozone may paint a misleading picture. In this example, including the insignificant ozone trends, of a large tropical lower stratospheric ozone increase, reduced the forcing by a factor of 5. Although the ST 4. Discussion and Conclusions forcings agree with one another to within their respective uncertainties, they give quite different interpretations to the data. The three ST scenarios highlight the point that the ozone changes we are most sure about lead to a negative forcing, but the "uncertain" component could be just as large as the "certain" component, and of either sign. In radiative forcing and general circulation model studies it is important to consider these uncertainties. The global and annual average radiative forcings and their uncertainties, due to ozone, are shown in Table 1. The radiative forcings are generally less negative than the range of forcings from previous literature, quoted by Shine and Forster [1999]. The SA and ST scenarios give the smallest forcings and can perhaps be discounted for misrepresenting the ozone changes: S A for not including lower stratospheric ozone changes and ST for having large and unsupported ozone increases in the tropics. The TO radiative forcing is smaller than the Forster and Shine [1997] estimate, using the same method, due to the smaller total column ozone trends. This scenario probably confines the ozone depletion to a region too dose to the tropopause and therefore probably has a forcing that is larger than the true forcing. Table Radiative Forcing Scenario SW LW NET la(net) SA TO ST ST-le ST-2a Units are in watt per square meter. Acknowledgments. Thanks to Lane Bishop for providing the TOMS and ground-based total column ozone trends, Ray Wang for providing the seasonal SAGE trends, and Terje Berntsen for providing the tropospheric ozone trends. Keith Shine, Ann Thompson, and David Stevenson are thanked for useful discussion. The author was funded by a NERC grant GR3/ References Berntsen, T., I. S. A. Isaksen, J. S. Fuglestvedt, G. Myhre, T. Alsvik Larsen, F. Stordal, R. S. Freckleton and K. P. Shine, Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing, J. Geophys. 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