Indirect radiative forcing of the ozone layer during the 21st century

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L02813, doi: /2006gl028252, 2007 Indirect radiative forcing of the ozone layer during the 21st century Robert W. Portmann 1 and Susan Solomon 1 Received 21 September 2006; revised 5 November 2006; accepted 8 December 2006; published 19 January [1] The response of a coupled two-dimensional radiativechemical-dynamical model to possible 21st century changes of the greenhouse gasses (GHGs) carbon dioxide, nitrous oxide and methane are explored using a range of IPCC marker scenarios of GHG emissions. The changes to the ozone layer caused by these GHGs are found to be relatively large (e.g., up to 5% global mean column ozone changes and 30% local changes for CO 2 using the IPCC A2 scenario between 2000 and 2100) and the mechanisms for these changes are discussed. The ozone changes are compared to the recovery of ozone due to expected decreases in chlorine containing compounds. Since carbon dioxide, nitrous oxide, and methane affect ozone they induce an indirect radiative forcing in addition to their direct radiative forcing. These indirect radiative forcings are computed using a combination of accurate line-by-line and band radiative transfer models and are compared to the radiative forcing of ozone during the time period. Although the changes in ozone are large at some altitudes over the time horizon, the range of associated future indirect radiative forcings from ozone over the range of IPCC scenarios are found to be 0.1 to 0.1 W m 2, which is small compared with the corresponding range of total direct radiative forcing of 2.2 to 6.2 W m 2 for these GHGs over this time horizon. Citation: Portmann, R. W., and S. Solomon (2007), Indirect radiative forcing of the ozone layer during the 21st century, Geophys. Res. Lett., 34, L02813, doi: /2006gl Introduction [2] The greenhouse gases (GHGs) CO 2,CH 4, and N 2 O are thought to be the most important gases that will affect changes in climate in the 21st century, through their direct radiative forcing [Intergovernmental Panel on Climate Change (IPCC), 2001]. During the years the indirect radiative forcing caused by ozone depletion due largely to CFC increases has been important as well, and reduced the total radiative forcing during this period. This indirect radiative forcing should reverse itself during the 21st century, mostly by 2050 as CFCs decrease [World Meteorological Organization (WMO), 2003]. However, it has long been known that the GHGs CO 2,CH 4, and N 2 O will also affect O 3 through their thermal and chemical roles in the stratosphere (see discussion below). In this paper, we consider whether the indirect radiative forcing from these O 3 changes, caused by GHG changes, could be significant 1 Chemical Science Division, Earth System Research Laboratory, NOAA, Boulder, Colorado, USA. This paper is not subject to U.S. copyright. Published in 2007 by the American Geophysical Union. in the 21st century. An estimate of this radiative forcing will be presented in this paper. 2. Modeled Future Ozone [3] There is a long history of model estimates of the effects of GHGs and CFCs on ozone, including Haigh and Pyle [1982], Brasseur et al. [1985], and Wuebbles et al. [1983]. These papers show that local ozone changes of the order 10 50% locally are possible due to feasible perturbations, and that the effect of increasing CO 2 could cancel the effects of increasing CFCs and N 2 O under some circumstances. However, the changes expected were in the middle and upper stratosphere and thus the impact on the ozone column was not large. Note that these studies were all carried out prior to the discovery of the ozone hole and thus do not contain heterogeneous chemistry, which greatly increases the potency of the CFCs at high latitudes and causes many other chemical changes in the lower stratosphere. More recently, studies by Randeniya et al. [2002], Chipperfield and Feng [2003] and Rosenfield et al. [2002] have modeled the future behavior of ozone due to several of these GHGs. Their results are qualitatively consistent with the earlier studies, with the exception of the Randeniya study that did not consider coupling to the future temperature changes. [4] We use the NOCAR two-dimensional chemicalradiative-dynamical model in this study [Garcia et al., 1992; Solomon et al., 1998; Portmann et al., 1999] to compute ozone distributions. This model has been used extensively to study past and future changes in ozone and includes comprehensive chemistry, detailed radiative transfer, and innovative dynamics including planetary and gravity wave breaking schemes (see references above for details). We force the model with the WMO scenarios for halogens [WMO, 2003] and use the IPCC A2 scenario for future concentrations of CO 2, N 2 O, and CH 4. The IPCC A2 scenario is generally considered to be a relatively high estimate for these gases compared to other scenarios. We choose it to maximize the effects of the GHGs on the ozone layer. This study focuses on the stratospheric ozone response and does not include emission scenarios for tropospheric ozone precursors such as NO x or CO. [5] Figure 1 shows the time evolution of ozone global mean anomalies for the period using the WMO [2003] halogen gases and the IPCC A2 GHG scenarios. This calculation shows the percent changes relative to 1980 for the model and the combined satellite record [Fioletov et al., 2002]. The global mean changes in the time period are about 5% and are caused primarily by CFC increases during this time period. There is then a recovery from these decreases due to the reduction of CFCs in the stratosphere, which is nearly complete by However, there is a super recovery due to the temperature changes L of5

2 from the GHGs in the stratosphere [Brasseur and Hitchman, 1998; Pitari et al., 1992; Rosenfield et al., 2002], which is discussed below. The individual effects of the GHGs are separated out in Figure 1 by re-doing the calculation with each GHG held fixed at its year 2000 value. This shows that increasing CO 2 and CH 4 cause increases in future ozone, while increasing N 2 O causes ozone decreases, and that these changes are all on the order 2 4% by Thus the future changes due to the GHGs have the potential to be nearly as large as the CFCs on this long time-scale. It is important to note that this is only true in the global mean and at low latitudes. At high latitudes, the ozone changes by CFC on the time-scale are much larger than the changes expected for GHGs in [6] Figure 2 shows the profile changes in ozone due to increases in CO 2 between 1980 and 2100 for several latitude ranges. The ozone changes are largest in the upper stratosphere and are primarily caused by temperature decreases at these altitudes, which reduce key gas-phase chemical rate coefficients. The largest change occurs in the temperature dependence of the reaction O þ O 3! 2O 2 ð1þ Figure 1. (a) The IPCC A2 scenario used in the model calculations presented in Figures 1b, 2, 3 and 4. (b) The variation of global mean total ozone anomalies during the 21st century due to the combined effects of Halogens and GHGs computed using the NOCAR 2-D model. The anomalies are computed with respect to years and have been smoothed with a 25 month smoothing function. The effect of varying GHGs has been separated out by individually keeping the gas at year 2000 levels compared with the full variation. The smoothed anomalies computed from the NIWA assimilated satellite record are also shown for the past [Fioletov et al., 2002]. which is a primary sink for odd-oxygen (i.e., O + O 3 )inthe upper stratosphere. In the middle and lower stratosphere at low and midlatitudes (and the global mean) there are ozone decreases that are caused by a reversed self healing (i.e., the reduced penetration of UV wavelengths due to increased ozone column overhead causes reduced ozone production from O 2 photolysis). The opposition of the upper and middle stratospheric changes reduces the effect of these changes on the ozone column, although the overall effect is still positive as shown in Figure 1. [7] Figure 3 shows the effect of increasing N 2 Oonthe ozone profile in the 21st century. The primary response is for increased N 2 O to cause increased NO x -induced ozone losses in the middle stratosphere where the NO x effect on ozone is the largest. These changes are reduced in the lower stratosphere due to the buffering effect of increased NO x on HO x (e.g., HO x is reduced by increases in HNO 3 ). In addition, NO x increases in the lower stratosphere can cause increased ozone production from chemical processes of the same character as tropospheric smog chemistry. The net effect of competing chemical processes in the lower statosphere leads to ozone increases at low latitudes and ozone decreases at high latitudes [see Nevison et al., 1999]. Figure 2. Profile change in ozone due to increases in CO 2 during the 21st century (the percent change from 2000 to 2100) for six 30 wide latitude bands computed with the NOCAR 2-D model. Ozone in the upper stratosphere increases due to changes in gas-phase rate coefficients caused by temperature decreases. The temperature decreases due to increased long-wave radiative cooling from CO 2. The ozone decreases in the lower stratosphere at low and midlatitudes from reversed self healing. 2of5

3 Figure 3. Same as Figure 2, except for N 2 O. The primary response is for increased N 2 O to cause increased NO x - induced ozone losses in the middle stratosphere where the NO x effect on ozone is the largest. These changes are reduced in the lower stratosphere due to the buffering effect of HO x on NO x. Figure 4. Same as Figure 2, except for CH 4.CH 4 causes increased H 2 O and HO x in the stratosphere and thus increased HO x induced ozone losses, especially in the upper stratosphere/lower mesosphere. In the troposphere increased CH 4 causes increased ozone due to NO x induced ozone production (smog chemistry). [8] The profile changes in ozone due to increases in CH 4 during the 21st century are shown in Figure 4. CH 4 increases cause increased H 2 O in the stratosphere and thus increases in HO x (especially in the middle and upper stratosphere). The increased HO x causes enhanced HO x - induced ozone losses in the upper stratosphere/lower mesosphere. These effects are quite large near and above the stratopause (10%). In the troposphere increased CH 4 causes increased ozone due to NO x induced ozone production (smog chemistry [see, e.g., Finlayson-Pitts and Pitts, 2000]). The magnitude of this effect is strongly dependent on the amount of tropospheric NO x, which the present model, like other two-dimensional models, simulates only crudely. For this reason, the stratospheric component of this effect is separated out below when calculating the radiative forcing. 3. Indirect Radiative Forcing From GHG Change [9] The ozone changes shown above were input into a radiative transfer model in order to compute the radiative forcing due to the changes. These are an indirect radiative forcing because the changes in ozone can be attributed to the change in the GHGs (of course these add to the direct radiative forcings of the GHGs themselves, which are large as discussed below). Radiative forcing is a measure of the extra heat (in W m 2 ) input into the troposphere due to the change and is found by computing the negative of the change in flux at the tropopause after stratospheric temperatures have relaxed to the perturbation. The stratospheric adjustment is usually computed using the fixed dynamical heating approximation. If stratospheric temperatures are not adjusted then the forcing is called the instantaneous radiative forcing. A line-by-line (LBL) radiative transfer (RT) model is used [Portmann et al., 1997] to compute the instantaneous radiative forcing. Then the stratospheric temperatures are relaxed using a straightforward time-marching relaxation technique using the longwave heating rate changes output from the LBL calculation. The radiative forcing model used in the FDH adjustment calculation is the CCM-IR2 code [Briegleb, 1992]. The calculations were done using six 30 wide latitude bins and temperature and cloud parameters obtained from the ISCCP data set [Rossow and Schiffer, 1999]. The line parameters were obtained from the HITRAN 2000 compilation [Rothman et al., 2003], including later corrections. [10] The radiative forcing caused by changes in O 3 is strongly influenced by the altitude at which the change occurs [Lacis et al., 1990; de F. Forster and Shine, 1997]. Also, unlike most other gases that induce radiative forcing, the radiative forcing from O 3 changes is strongly influenced by the stratospheric adjustment. The change due to adjustment is on the same order as the instantaneous change for O 3 (for changes in the lower stratosphere), while for most gases it is less than a 20% change. In general, the radiative forcing is largest for changes near the tropopause and in the lower statosphere and much smaller for high and low altitudes. For stratospheric O 3 changes, the shortwave Table 1. Global Mean Indirect Radiative Forcing for Ozone Instantaneous, W m 2 Adjusted, W m 2 Stratosphere Only Full Profile Stratosphere Only Full Profile CO N 2 O CH of5

4 Table 2. Global Mean Adjusted Radiative Forcing, CO 2 +N 2 O+ CH 4, IPCC Direct RF b, Indirect RF b via O 3, Scenario a Wm 2 Wm 2 A2 5.6 a 0.09 B1 2.3 a 0.03 Range c 2.2 to 6.2 a 0.1 to 0.1 a IPCC [2001]. b RF, radiative forcing. c Range across all SRES marker scenarios (A1B, A1T, A1FI, A2, B1, B2). radiative forcing is generally larger than the longwave radiative forcing for the instantaneous forcing, while the stratospheric adjustment causes a comparable offsetting change in the longwave forcing. [11] The instantaneous and adjusted radiative forcing for the ozone changes induced by the GHG changes over the time period are shown in Table 1. All of these indirect radiative forcings are less than 0.15 W m 2, which can be compared with the total direct radiative forcing of 5.6 W m 2 from these GHGs over this time horizon [IPCC, 2001]. The reason for the small indirect radiative forcing is due to the profiles of the ozone changes shown in Figures 2 4. While the absolute values of the local and total column O 3 changes are reasonably large, nearly none of the changes occur near the tropopause. In addition, there are both negative and positive changes in the stratosphere, especially in the case of CO 2, which compete with each other in the overall effect. 4. Conclusion [12] The radiative forcing of stratospheric ozone during the period was estimated by IPCC [2001] to be 0.15 ± 0.1 W m 2. This is important in attribution studies for recent decades since it is competing against a much smaller direct radiative forcing from the GHGs compared to that expected based upon scenarios for the 21st century. The changes in the profile and column ozone due to increases in CO 2,N 2 O, and CH 4 are likely to be substantial during the 21st century. This is especially true of the CO 2 induced changes near the stratopause, which can lead to as much as a 30% increase in local ozone abundances. These numbers are in general agreement with other calculations [Rosenfield et al., 2002; Chipperfield and Feng, 2003]. However, despite these large local changes, the radiative forcing of ozone in the 21st century due to changes of CO 2,N 2 O, and CH 4 is likely negligibly small compared with the direct radiative forcing of these gases. Table 2 compares the total direct forcing of these gases against their indirect forcing over the 21st century. It shows that the indirect forcing from ozone changes is less than a tenth of a W m 2 compared with about 5.6 W m 2 for the A2 scenario, or 2.3 W m 2 for the B1 scenario. These scenarios are at the high and low end of the range of the marker scenarios considered by IPCC [2001], which is also shown on Table 2. The small size of the ozone indirect radiative forcing estimates is due to competing changes in the ozone profile and because the largest changes are in the middle and upper stratosphere where they do not strongly influence the radiative changes at the tropopause. [13] The GHGs may cause larger changes to the stratosphere than can be modeled using a two-dimensional model. For example, there could be large changes in wave driving which could alter the stratospheric circulation, the polar vortices could change dramatically, or tropopause heights could change. In addition, other constituent changes not considered in this study such as water vapor increases or changes in stratospheric aerosol could alter ozone distributions. Note that all of these changes are highly uncertain and no reliable model estimates of their future magnitude exist. If these changes caused large ozone changes in the lower stratosphere then they could cause an indirect radiative forcing larger than that estimated in this paper. However, given the small radiative forcings found in this study for relatively large ozone changes it is unlikely that any of these effects will cause the indirect radiative forcing of ozone to be significant during the 21st century. References Brasseur, G., and M. Hitchman (1998), Stratospheric response to trace gas perturbations Changes in ozone and temperature distributions, Science, 240, Brasseur, G., A. De Rudder, and C. Tricot (1985), Stratospheric response to chemical perturbations, J. Atmos. Chem., 3, Briegleb, B. (1992), Longwave band model for thermal radiation in climate studies, J. Geophys. Res., 97, 11,475 11,485. Chipperfield, M. P., and W. 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