Climate Change 2001:
Working Group I: The Scientific Basis
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6.6.4 Stratospheric Water Vapour

The oxidation of CH4 produces water vapour that can contribute significantly to the levels of H2O in the stratosphere. Water vapour is also directly emitted by aircraft in the lower stratosphere. Lelieveld and Crutzen (1992), Lelieveld et al. (1993, 1998), Hauglustaine et al. (1994) and Fuglestvedt et al. (1996) have estimated a contribution of stratospheric water vapour of 2 to 5% of the total CH4 forcing. Oltmans and Hofmann (1995) have reported an increase in lower stratospheric H2O (18 to 20 km) measured at Boulder of 0.8% per year for the period 1981 to 1994. Measurements of stratospheric H2O from the HALOE instrument on board the UARS satellite also show an increase in stratospheric H2O between 30 and 60 km of 40 to 100 ppbv/year for the period 1992 to 1997 (Evans et al., 1998; Nedoluha et al., 1998). A similar trend has also been reported by Randel et al. (1999) based on HALOE data for the period 1993 to 1997. In this study, a time variation and a flattening of the trend has been determined after 1996. This stratospheric H2O increase is well above that expected from the rising CH4 levels in the atmosphere. Based on these observed stratospheric H2O trends, Forster and Shine (1999) have estimated a radiative forcing of 0.2 Wm-2 since 1980. Note that, just as for stratospheric O3, there exists considerable uncertainty concerning the trend near the tropopause region globally. It should also be noted that if the changes in water vapour were a result of CH4 oxidation, the changes in H2O would be a forcing. However, if they result from changes in tropical tropopause temperature change or in dynamics, then they should be viewed as a feedback (as defined in Section 6.2). Additional measurements and analyses are clearly needed to explain the observed trends.

6.7 The Direct Radiative Forcing of Tropospheric Aerosols

Anthropogenic aerosols scatter and absorb short-wave and long-wave radiation, thereby perturbing the energy budget of the Earth/atmosphere system and exerting a direct radiative forcing. This section concentrates on estimates of the global mean direct effect of anthropogenic tropospheric aerosols and is necessarily dependent upon global models. Field campaigns which provide essential input parameters for the models, and satellite observational studies of the direct effect of tropospheric aerosols, which provide useful validation data for the models, are considered in detail in Chapter 5.

6.7.1 Summary of the IPCC WGI Second Assessment Report and Areas of Development

The SAR considered three anthropogenic aerosol species; sulphate, biomass burning aerosols, and fossil fuel black carbon (or soot). The SAR suggested a radiative forcing of -0.4 Wm-2 with a factor of two uncertainty for sulphate aerosols, -0.2 Wm-2 with a factor of three uncertainty for biomass burning, and +0.1 Wm-2 with a factor of three uncertainty for fossil fuel black carbon aerosols. The level of scientific understanding (referred to as a confidence level in the SAR, see Section 6.13) was classified as "low" for sulphate aerosol and "very low" for both fossil fuel black carbon and biomass burning aerosols. Since the SAR, there have been advances in both modelling and observational studies of the direct effect of tropospheric aerosols (see reviews by Shine and Forster (1999) and Haywood and Boucher (2000)). Global chemical transport modelling studies encompass a greater number of aerosol species and continue to improve the representation of the physical and chemical processes (see Chapters 4 and 5). Global models are more numerous and include more accurate radiative transfer codes, more sophisticated treatments of the effects of relative humidity for hygroscopic aerosols, better treatment of clouds, and better spatial and temporal resolution than some earlier studies. The present day direct radiative forcing due to aircraft emissions of sulphate and black carbon aerosol have been calculated to be insignificant (IPCC, 1999) and are not considered further. Spatial patterns of the calculated radiative forcings are not discussed in detail here but are presented in Section 6.14.

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