2.3.7 Stratospheric Water Vapour

The TAR noted that several studies had indicated long-term increases in stratospheric water vapour and acknowledged that these trends would contribute a significant radiative impact. However, it only considered the stratospheric water vapour increase expected from CH₄ increases as an RF, and this was estimated to contribute 2 to 5% of the total CH₄ RF (about +0.02 W m^–2).

Section 3.4 discusses the evidence for stratospheric water vapour trends and presents the current understanding of their possible causes. There are now 14 years of global stratospheric water vapour measurements from Halogen Occultation Experiment (HALOE) and continued balloon-based measurements (since 1980) at Boulder, Colorado. There is some evidence of a sustained long-term increase in stratospheric water vapour of around 0.05 ppm yr^–1 from 1980 until roughly 2000, since then water vapour concentrations in the lower stratosphere have been decreasing (see Section 3.4 for details and references). As well as CH₄ increases, several other indirect forcing mechanisms have been proposed, including: a) volcanic eruptions (Considi ne et al., 2001; Joshi and Shine, 2003); b) biomass burning aerosol (Sherwood, 2002); c) tropospheric (SO₂; Notholt et al., 2005) and d) changes in CH₄ oxidation rates from changes in stratospheric chlorine, ozone and OH (Rockmann et al., 2004). These are mechanisms that can be linked to an external forcing agent. Other proposed mechanisms are more associated with climate feedbacks and are related to changes in tropopause temperatures or circulation (Stuber et al., 2001a; Fueglistaler et al., 2004). From these studies, there is little quantification of the stratospheric water vapour change attributable to different causes. It is also likely that different mechanisms are affecting water vapour trends at different altitudes.

Since the TAR, several further calculations of the radiative balance change due to changes in stratospheric water vapour have been performed (Forster and Shine, 1999; Oinas et al., 2001; Shindell, 2001; Smith et al., 2001; Forster and Shine, 2002). Smith et al. (2001) estimated a +0.12 to +0.2 W m^–2 per decade range for the RF from the change in stratospheric water vapour, using HALOE satellite data. Shindell (2001) estimated an RF of about +0.2 W m^–2 in a period of two decades, using a GCM to estimate the increase in water vapour in the stratosphere from oxidation of CH₄ and including climate feedback changes associated with an increase in greenhouse gases. Forster and Shine (2002) used a constant 0.05 ppm yr^–1 trend in water vapour at pressures of 100 to 10 hPa and estimated the RF to be +0.29 W m^–2 for 1980 to 2000. GCM radiation codes can have a factor of two uncertainty in their modelling of this RF (Oinas et al., 2001). For the purposes of this chapter, the above RF estimates are not readily attributable to forcing agent(s) and uncertainty as to the causes of the observed change precludes all but the component due to CH₄ increases being considered a forcing. Two related CTM studies have calculated the RF associated with increases in CH₄ since pre-industrial times (Hansen and Sato, 2001; Hansen et al., 2005), but no dynamical feedbacks were included in those estimates. Hansen et al. (2005) estimated an RF of +0.07 ± 0.01 W m^–2 for the stratospheric water vapour changes over 1750 to 2000, which is at least a factor of three larger than the TAR value. The RF from direct injection of water vapour by aircraft is believed to be an order of magnitude smaller than this, at about +0.002 W m^–2 (IPCC, 1999). There has been little trend in CH₄ concentration since 2000 (see Section 2.3.2); therefore the best estimate of the stratospheric water vapour RF from CH₄ oxidation (+0.07 W m^–2) is based on the Hansen et al. (2005) calculation. The 90% confidence range is estimated as ±0.05 W m^–2, from the range of the RF studies that included other effects. There is a low level of scientific understanding in this estimate, as there is only a partial understanding of the vertical profile of CH₄-induced stratospheric water vapour change (Section 2.9, Table 2.11). Other human causes of stratospheric water vapour change are unquantified and have a very low level of scientific understanding.