Radiative forcing due to stratospheric water vapour from CH 4 oxidation

Transcription

1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01807, doi: /2006gl027472, 2007 Radiative forcing due to stratospheric water vapour from CH 4 oxidation Gunnar Myhre, 1,2 Jørgen S. Nilsen, 1,3 Line Gulstad, 1 Keith P. Shine, 4 Bjørg Rognerud, 1 and Ivar S. A. Isaksen 1 Received 6 July 2006; revised 24 October 2006; accepted 20 November 2006; published 9 January [1] Here we report on estimates of the changes in stratospheric water vapour (SWV) due to methane oxidation based on observational data. Above the tropopause oxidation of methane results in a decrease in its mixing ratio with altitude and this is a major source for SWV. The vertical profile of SWV changes from methane oxidation is presented here using satellite observations of the vertical profile of methane. Trends in the SWV are shown to be small in the lower stratosphere, but can reach 0.7 ppbv at 30 km at high latitudes over the period The radiative forcing for this indirect effect of methane increase over the industrial era is estimated to be slightly weaker than 0.1 Wm 2 which implies a larger contribution of water vapour to the methane global warming potential than used in recent Intergovernmental Panel on Climate Change assessments. Our estimate considers only chemical changes and not SWV of dynamical causes. Importantly, we find substantial differences in the temperature change in the stratosphere for a homogeneous change in SWV and SWV change from methane oxidation. This has implications for trend analysis of SWV and understanding and attribution of the stratospheric temperature trend. Citation: Myhre, G., J. S. Nilsen, L. Gulstad, K. P. Shine, B. Rognerud, and I. S. A. Isaksen (2007), Radiative forcing due to stratospheric water vapour from CH 4 oxidation, Geophys. Res. Lett., 34, L01807, doi: / 2006GL Introduction [2] Measurements from a variety of sources show an increase in stratospheric water vapour (SWV) during the period from mid-1950s to 2000 [Oltmans et al., 2000; Randel et al., 2004; Rosenlof et al., 2001]. The change may have been as large as a doubling over this time period. However, the uncertainties are large since no long term global network of SWV observations exists. A decline in SWV has been observed since 2000 based on several types of measurements techniques, although the magnitude of the change differs [Randel et al., 2004]. Randel et al. [2006] links the recent decrease in SWV to a cold tropical tropopause and the Brewer-Dobson circulation. The apparent reversal in trend, differences in the trend between various 1 Department of Geosciences, University of Oslo, Oslo, Norway. 2 Center for International Climate and Environmental Research, Oslo, Norway. 3 Now at Ullevål University Hospital, Oslo, Norway. 4 Department of Meteorology, University of Reading, Reading, UK. Copyright 2007 by the American Geophysical Union /07/2006GL027472$05.00 instruments, and lack of a physical explanation raises questions of the significance of the reported long term trend. [3] Several studies have investigated the radiative effect and temperature change based on the observed trends in SWV [Forster and Shine, 1999, 2002; Oinas et al., 2001; Smith et al., 2001]. The potential for enhanced SWV to be a strong radiative forcing mechanism has been shown by Forster and Shine [2002]. They obtained a radiative forcing as large as 0.6 Wm 2 over the period 1960 to 2000 using a SWV trend of 0.05 ppmv/year. Most of the earlier studies used a vertically independent change in the SWV profile with cooling in the lower stratosphere of K/decade at high latitudes [Forster and Shine, 1999, 2002; Oinas et al., 2001; Shine et al., 2003] but more realistic changes in the SWV vertical profile have also been used [Shine et al., 2003; Smith et al., 2001]. A cooling in the lower stratosphere of more than 1 K/decade at mid to high latitudes is observed over recent decades. Observed temperature trends can be explained partially by stratospheric ozone depletion but it appears inadequate to explain all of the observed cooling at 50 hpa [Shine et al., 2003]. [4] In addition to a significant direct radiative forcing, CH 4 has a significant important indirect effects on O 3,OH (therefore affecting other radiative actively gases), and SWV [Fuglestvedt et al., 1996; Lelieveld and Crutzen, 1992; Lelieveld et al., 1998; Shindell et al., 2005]. CH 4 is oxidized in the atmosphere by OH, which in the stratosphere leads to SWV and is the most important source for SWV. Since the mid-18th century CH 4 has increased from 700 ppmv to 1750 ppmv. A large trend in the SWV cannot be entirely explained by the increase in CH 4. Hansen et al. [2005] calculated the SWV from CH 4 oxidation in a two-dimensional model. Their radiative forcing due to SWV for a change in CH4 between the years 1750 to 2000 is 0.07 Wm 2. [5] Several explanations of the increase in SWV have been proposed [Joshi and Shine, 2003; Rockmann et al., 2004; Rosenlof, 2003; Sherwood, 2002]. It is likely that increased transport of water vapour from the troposphere to the stratosphere has contributed to the SWV trend [Hansen et al., 2005; Rosenlof, 2003]. Rockmann et al. [2004] suggested that increased level of chlorine, reduced ozone, and more OH as a feedback of increased CH 4 in the stratosphere were three mechanisms that had increased the oxidation of CH 4 in the stratosphere. [6] In this study we use vertical CH 4 profiles from satellite data to calculate the vertical profile of the increase in SWV since pre-industrial times from the oxidation of CH 4, and the resulting radiative forcing and temperature changes in the stratosphere. In addition to changes since pre-industrial time, we focus on changes since 1950 and L of5

2 Figure 1. HALOE zonal mean vertical profile of CH 4 over the period October 1991 throughout These time scales are chosen since SWV trends are observed, at least crudely, since 1950 and satellite data of stratospheric temperature trends are available after Methods 2.1. SWV Change Based on Satellite Data [7] We use data from the Halogen Occultation Experiment (HALOE) onboard the Upper Atmosphere Research Satellite (UARS), described by Russell et al. [1993]. The HALOE instrument provides high-quality vertical profiles of methane and water vapor, derived from solar occultation experiments. The vertical resolution for the version used here is approximately 1.3 km. The HALOE measurements extend almost down to the local tropopause level, below which cloud effects contaminate a significant fraction of the retrievals. The data are obtained from the SPARC Reference Climatology Project ( ref_clim.html). HALOE CH 4 measurements from October 1991 throughout 1999 are averaged following the method of Randel et al. [1998], making use of the Microwave Limb Sounder (MLS) data, also onboard UARS, to fill in where HALOE data are lacking. The HALOE CH 4 has been validated by Park et al. [1996]. [8] The HALOE vertical profiles of CH 4 are used to calculate the SWV from the oxidation of CH 4 (see Figure 1). The CH 4 concentration is nearly constant up to the tropopause. In the stratosphere the sum of water vapour and twice the CH 4 concentration is nearly constant [Nassar et al., 2005; Rosenlof, 2002]. This is important for our method to extract SWV changes from CH 4 oxidation. Above the tropopause oxidation of one CH 4 molecule generates two H 2 O molecules. Based on the reduction CH 4 with altitude from the HALOE measurements we calculate the increase in SWV. We consider only the fractional part of the CH 4 oxidation to SWV that is anthropogenic. This is obtained by using the present and pre-industrial (in some cases since 1950 and 1979 concentrations) CH 4 concentration. The time-lag of the methane has not been taken into account and with the rather modest increase in the tropospheric methane during the 1990s, this neglect is not expected to cause a significant error. [9] An assumption in our method is that shape of the vertical profile of CH 4 in the stratosphere has not changed over the industrialized era. This assumption is likely to be applicable to a large extent, but some caveats should be mentioned. In reality, a small part of the decrease in water vapour with height will result from the several years delay for the increases in tropospheric methane to propagate to the upper stratosphere. In addition, both Evans et al. [1998] and Nedoluha et al. [1998] find decreases in stratospheric methane from HALOE data during the mid- 1990s which could be driven by an increased rate of oxidation of methane to water vapour. For example Nedoluha et al. [1998] estimate that the increase in tropospheric methane gives a SWV trend at km of 20 ppbv/year between 1992 and 1997 from the trend in tropospheric methane, but this is actually exceeded by an increase in the conversion of methane to SWV (38 ppbv/ year). However, Nedoluha et al. [2003] discuss the lack of a trend in SWV between , using observations from a variety of sources, and so an increased methane conversion does not appear to be occurring during this later period Radiative Transfer Calculations [10] In the calculations of radiative forcing and temperature changes in the stratosphere we use separate longwave and shortwave radiative schemes. For the longwave calculations a broad band model is adopted [Myhre and Stordal, 1997], whereas for shortwave a model using the discrete ordinate method [Stamnes et al., 1988] is used. The two schemes that are used for global calculations are compared to line-by-line codes [Myhre and Stordal, 2001] for a mid-latitude-summer profile. A change of SWV from 6.0 ppmv to 6.7 ppmv gave a shortwave radiative forcing of 0.11 Wm 2 for the global model and the LBL (with a 6% difference). For similar change in SWV the longwave radiative forcing was 0.32 Wm 2 in the broad band model and the LBL model (2 % difference). Stratospheric temperature adjustment is calculated using the fixed dynamical heating approximation as in Myhre and Stordal [1997]. The meteorological data (including water vapour) used for the reference calculation is from ECMWF on a T21 ( degree) resolution and with 60 vertical layers. 3. Results 3.1. SWV Change [11] Figure 2a shows the SWV change from CH 4 oxidation based on the HALOE data over the industrialized era, with small changes in the lower stratosphere but almost 2 ppmv at altitudes at km. The latitudinal gradient in the SWV is particularly strong in the lower stratosphere. In the upper stratosphere the contribution from CH 4 is significant and can amount to nearly one third of the present abundance of SWV [Rosenlof, 2002]. More than half of the CH 4 change since pre-industrial time has occurred after The SWV from CH 4 oxidation since 1950 is shown in Figure 2b; it has a similar pattern to Figure 2a, but with maximum changes of somewhat more than 1 ppmv (or 20 ppbv/year) at high latitudes in the upper stratosphere. A significant increase of 0.7 ppmv in SWV is obtained in the middle stratosphere (30 km) at high latitudes, but only half 2of5

3 shown in Figure 2c, with also maximum changes up to 20 ppbv/year Radiative Forcing [12] Table 1 shows results of the radiative forcing calculations due to the increase in SWV from CH 4 oxidation. Results are given for changes since 1750 (the beginning of the industrial era), 1950, and 1979 until present. The net radiative forcing over the industrial era of Wm 2 consists of a longwave forcing of 0.10 Wm 2 and a shortwave forcing of 0.02 Wm 2. This net forcing is slightly larger than that presented by Hansen et al. [2005] based on a two-dimensional model with a radiative forcing of 0.07 Wm 2 over the period 1750 to 2000, but much larger than the value of 0.02 Wm 2 over the period estimated by Lelieveld et al. [1998]. The radiative forcing of the indirect effect of CH 4 oxidation to SWV is 15 20% of the direct radiative forcing due to methane. Note that this result is of significance for the inclusion of the indirect effect of stratospheric water vapour changes on the Global Warming Potential for methane. In recent IPCC reports [e.g., Intergovernmental Panel on Climate Change (IPCC), 2001] it is assumed that water vapour contributes up to 5% of the forcing due to methane; our results indicate that this figure should be much higher, and indeed closer to the values given in earlier IPCC reports [see IPCC, 1995, section 4.2.2]. Further, the radiative forcing from 1950 to present is 60% of the methane-induced SWV forcing over the industrial era. Our estimate of SWV from CH 4 oxidation accounts for less than 15% of the upper estimate of radiative forcing of total SWV change since 1950 by Forster and Shine [2002]. Additional simulations we have performed show that SWV change from CH 4 oxidation above 30 km is of relatively minor importance in terms of the radiative forcing of SWV changes. Figure 2. Stratospheric water vapour (ppmv) increase from CH 4 oxidation (a) from 1750 until 2000, (b) from 1950 until 2000, and (c) from 1979 until of this at the same height over the equator. At lower levels in the stratosphere the SWV from CH 4 oxidation gives only a small percentage change. Therefore, CH 4 oxidation can only partly explain the proposed trend in SWV since The change in SWV from CH 4 oxidation since 1979 is 3.3. Temperature Change [13] To illustrate the importance of the vertical profile of the SWV change Figure 3a shows the calculated temperature change in the stratosphere based on a constant SWV change from 6.0 to 6.7 ppmv throughout the stratosphere. The cooling is around 0.5K in the lower part of the stratosphere at high latitudes and weaker at higher altitudes as well as in the whole tropical stratosphere. The pattern is very similar to the simulations of Forster and Shine [1999] for the same SWV change. The more realistic change in the SWV profile of Shine et al. [2003]; Smith et al. [2001] show weaker cooling for SWV than for a constant profile. Figure 3b shows the temperature change for SWV increase due to CH 4 oxidation based on the HALOE data over the industrial era. Importantly the temperature change in this case differs significantly from the change with constant SWV change in Figure 3a. The cooling is shifted upward and we find a warming in the lower stratosphere. This warming is a result of amplified absorption of downwelling longwave radiation due to increased SWV at higher altitudes. The cooling in the lower stratosphere in Figure 3a is mainly a result of local cooling from the SWV increase. This local cooling is almost absent in the results in Figure 3b since the lower SWV change due to CH 4 oxidation is so small. Cooling of more than 1 K at high latitudes upward from the middle stratosphere and globally at 50 km is simulated. Observations 3of5

4 Table 1. Radiative Forcing Due to SWV Change From CH 4 Oxidation and the Direct Radiative Forcing Due to CH 4 Period Radiative Forcing, Wm 2 SWV Change From CH 4 Direct Due to CH show that the upper stratosphere is cooling [Shine et al., 2003] with models indicating that ozone depletion and the increase in CO 2 are the dominant contributors. Figures 3c and 3d show the temperature change from SWV from CH 4 oxidation over the period since 1950 and 1979, respectively. For 1950 to present the simulated temperature change is less than 0.2 K warming in the lower stratosphere and a maximum cooling around 50 km of 0.7 K. The corresponding values since 1979 are approximately a factor of 3 lower. 4. Summary [14] In this study we use satellite data from HALOE of CH 4 to estimate the SWV change from CH 4 oxidation and the corresponding radiative forcing. The simulations presented here indicate a radiative forcing slightly weaker than 0.1 Wm 2 and that a significant fraction of the forcing has occurred since The forcing from water vapour is approximately 15 20% of the direct methane forcing, which is much higher than the value of 5% used in recent IPCC reports for the water vapour contribution to the global warming potential for methane. [15] An important result from this study is that the temperature change due to increases in SWV from CH 4 oxidation is quite different from a homogeneous SWV change. Shine et al. [2003]; Smith et al. [2001] showed Figure 3. Temperature change (K) from stratospheric water vapour: (a) change in SWV from 6.0 to 6.7 ppmv, (b) change in SWV from CH 4 oxidation in the period from 1750 until 2000, (c) change in SWV from CH 4 oxidation in the period from 1950 until 2000, and (d) change in SWV from CH 4 oxidation in the period from 1979 until of5

5 that HALOE deduced SWV changes result in different temperature change compared to homogeneous SWV change. Actually, the results presented here show a small increase in temperature just above the tropopause for the SWV increase from CH 4 oxidation. Therefore, emphasis should be made to investigate further the change in the vertical profile of SWV. The contribution to SWV from CH 4 increases since pre-industrial time seems to account for only a fraction of the reported increase, particularly in the lower stratosphere, although these observations are uncertain. Furthermore, the impact on the stratospheric temperature trend from SWV increase is strongly dependent on the height profile of the increase. Therefore, both the magnitude of the SWV increase and impact on stratospheric temperatures are uncertain. Further modeling of the SWV from CH 4 oxidation should be performed to investigate the importance of increased chlorine level in the atmosphere and its impact on CH 4 destruction in the stratosphere. If this reaction is of large importance our SWV change may be slightly overestimated. However, global transport models that should be used for such purposes must have a top boundary above the stratosphere, otherwise the boundary conditions decide to a large extent the SWV change, since a large portion of the CH 4 destruction occurs in the upper stratosphere. [16] The largest uncertainty in our estimate of SWV change and the resulting radiative forcing is probably related to the observed vertical profile of CH 4. It is shown that the vertical profile in the CH 4 from HALOE has some interannual variability [Ruth et al., 1997]. This interannual variability is also found in high precision balloon-borne measurements [Patra et al., 2003] and importantly the gradient in the vertical profile of CH 4 from these measurements compare well with HALOE data. [17] Acknowledgments. GM acknowledges The Research Council of Norway project /S30 and KPS acknowledges NERC grant NERC/L/ S/2001/ References Evans, S. J., et al. (1998), Trends in stratospheric humidity and the sensitivity of ozone to these trends, J. Geophys. Res., 103, Forster, P. M. D., and K. P. Shine (1999), Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling, Geophys. Res. Lett., 26, Forster, P. M. de F., and K. P. Shine (2002), Assessing the climate impact of trends in stratospheric water vapor, Geophys. Res. Lett., 29(6), 1086, doi: /2001gl Fuglestvedt, J. S., I. S. A. Isaksen, and W. C. Wang (1996), Estimates of indirect global warming potentials for CH 4, CO and NO x, Clim. Change, 34, Hansen, J., et al. (2005), Efficacy of climate forcings, J. Geophys. Res., 110, D18104, doi: /2005jd Intergovernmental Panel on Climate Change (IPCC) (1995), Climate Change 1994: Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios, edited by J. T. Houghton et al., Cambridge Univ. Press, New York. Intergovernmental Panel on Climate Change (IPCC) (2001), Climate Change 2001: The Scientific Basis, edited by J. T. Houghton et al., Cambridge Univ. Press, New York. Joshi, M. M., and K. P. Shine (2003), A GCM study of volcanic eruptions as a cause of increased stratospheric water vapor, J. Clim., 16, Lelieveld, J., and P. J. Crutzen (1992), Indirect chemical effects of methane on climate warming, Nature, 355, Lelieveld, J., P. J. Crutzen, and F. J. Dentener (1998), Changing concentration, lifetime and climate forcing of atmospheric methane, Tellus, Ser. B, 50, Myhre, G., and F. Stordal (1997), Role of spatial and temporal variations in the computation of radiative forcing and GWP, J. Geophys. Res., 102, 11,181 11,200. Myhre, G., and F. Stordal (2001), On the tradeoff of the solar and thermal infrared radiative impact of contrails, Geophys. Res. Lett., 28, Nassar, R., P. F. Bernath, C. D. Boone, G. L. Manney, S. D. McLeod, C. P. Rinsland, R. Skelton, and K. A. Walker (2005), Stratospheric abundances of water and methane based on ACE-FTS measurements, Geophys. Res. Lett., 32, L15S04, doi: /2005gl Nedoluha, G. E., R. M. Bevilacqua, R. M. Gomez, D. E. Siskind, B. C. Hicks, J. M. Russell III, and B. J. Connor (1998), Increases in middle atmospheric water vapor as observed by the Halogen Occultation Experiment and the ground-based Water Vapor Millimeter-wave Spectrometer from 1991 to 1997, J. Geophys. Res., 103(D3), Nedoluha, G. E., R. M. Bevilacqua, R. M. Gomez, B. C. Hicks, J. M. Russell III, and B. J. Connor (2003), An evaluation of trends in middle atmospheric water vapor as measured by HALOE, WVMS, and POAM, J. Geophys. Res., 108(D13), 4391, doi: /2002jd Oinas, V., A. A. Lacis, D. Rind, D. T. Shindell, and J. E. Hansen (2001), Radiative cooling by stratospheric water vapor: Big differences in GCM results, Geophys. Res. Lett., 28, Oltmans, S. J., H. Vömel, D. J. Hofmann, K. H. Rosenlof, and D. Kley (2000), The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado, Geophys. Res. Lett., 27, Park, J. H., et al. (1996), Validation of Halogen Occultation Experiment CH 4 measurements from the UARS, J. Geophys. Res., 101, 10,183 10,204. Patra, P. K., S. Lal, S. Venkataramani, and D. Chand (2003), Halogen Occultation Experiment (HALOE) and balloon-borne in situ measurements of methane in stratosphere and their relation to the quasi-biennial oscillation (QBO), Atmos. Chem. Phys., 3, Randel, W. J., et al. (1998), Seasonal cycles and QBO variations in stratospheric CH 4 and H 2 O observed in UARS HALOE data, J. Atmos. Sci., 55, Randel, W. J., et al. (2004), Interannual changes of stratospheric water vapor and correlations with tropical tropopause temperatures, J. Atmos. Sci., 61, Randel, W. J., F. Wu, H. Vömel, G. E. Nedoluha, and P. Forster (2006), Decreases in stratospheric water vapor after 2001: Links to changes in the tropical tropopause and the Brewer-Dobson circulation, J. Geophys. Res., 111, D12312, doi: /2005jd Rockmann, T., J. U. Grooss, and R. Muller (2004), The impact of anthropogenic chlorine emissions, stratospheric ozone change and chemical feedbacks on stratospheric water, Atmos. Chem. Phys., 4, Rosenlof, K. H. (2002), Transport changes inferred from HALOE water and methane measurements, J. Meteorol. Soc. Jpn., 80, Rosenlof, K. H. (2003), Atmospheric science How water enters the stratosphere, Science, 302, Rosenlof, K. H., et al. (2001), Stratospheric water vapor increases over the past half-century, Geophys. Res. Lett., 28, Russell, J. M., III, L. L. Gordley, J. H. Park, S. R. Drayson, W. D. Hesketh, R. J. Cicerone, A. F. Tuck, J. E. Frederick, J. E. Harries, and P. J. Crutzen (1993), The Halogen Occultation Experiment, J. Geophys. Res., 98, 10,777 10,797. Ruth, S., R. Kennaugh, L. J. Gray, and J. M. Russell (1997), Seasonal, semiannual, and interannual variability seen in measurements of methane made by the UARS halogen occultation experiment, J. Geophys. Res., 102, 16,189 16,199. Sherwood, S. (2002), A microphysical connection among biomass burning, cumulus clouds, and stratospheric moisture, Science, 295, Shindell, D. T., G. Faluvegi, N. Bell, and G. A. Schmidt (2005), An emissions-based view of climate forcing by methane and tropospheric ozone, Geophys. Res. Lett., 32, L04803, doi: /2004gl Shine, K. P., et al. (2003), A comparison of model-simulated trends in stratospheric temperatures, Q. J. R. Meteorol. Soc., 129, Smith, C. A., J. D. Haigh, and R. Toumi (2001), Radiative forcing due to trends in stratospheric water vapour, Geophys. Res. Lett., 28, Stamnes, K., S. C. Tsay, W. Wiscombe, and K. Jayaweera (1988), Numerically stable algorithm for discrete-ordinate-method radiative-transfer in multiple-scattering and emitting layered media, Appl. Opt., 27, L. Gulstad, I. S. A. Isaksen, G. Myhre, and B. Rognerud, Department of Geosciences, University of Oslo, PO Box 1022, Blindern, N-0315 Oslo, Norway. (gunnar.myhre@geo.uio.no) J. S. Nilsen, Ullevål University Hospital, N-0407 Oslo, Norway. K. P. Shine, Department of Meteorology, University of Reading, Earley Gate, P.O. Box 243, Reading RG6 6BB, UK. 5of5