8.6.3.1 Water Vapour and Lapse Rate

Absorption of LW radiation increases approximately with the logarithm of water vapour concentration, while the Clausius-Clapeyron equation dictates a near-exponential increase in moisture-holding capacity with temperature. Since tropospheric and surface temperatures are closely coupled (see Section 3.4.1), these constraints predict a strongly positive water vapour feedback if relative humidity (RH) is close to unchanged. Furthermore, the combined water vapour-lapse rate feedback is relatively insensitive to changes in lapse rate for unchanged RH (Cess, 1975) due to the compensating effects of water vapour and temperature on the OLR (see Box 8.1). Understanding processes determining the distribution and variability in RH is therefore central to understanding of the water vapour-lapse rate feedback. To a first approximation, GCM simulations indeed maintain a roughly unchanged distribution of RH under greenhouse gas forcing. More precisely, a small but widespread RH decrease in GCM simulations typically reduces feedback strength slightly compared with a constant RH response (Colman, 2004; Soden and Held, 2006; Figure 8.14).

In the planetary boundary layer, humidity is controlled by strong coupling with the surface, and a broad-scale quasi-unchanged RH response is uncontroversial (Wentz and Schabel, 2000; Trenberth et al., 2005; Dai, 2006). Confidence in GCMs’ water vapour feedback is also relatively high in the extratropics, because large-scale eddies, responsible for much of the moistening throughout the troposphere, are explicitly resolved, and keep much of the atmosphere at a substantial fraction of saturation throughout the year (Stocker et al., 2001). Humidity changes in the tropical middle and upper troposphere, however, are less well understood and have more TOA radiative impact than do other regions of the atmosphere (e.g., Held and Soden, 2000; Colman, 2001). Therefore, much of the research since the TAR has focused on the RH response in the tropics with emphasis on the upper troposphere (see Bony et al., 2006 for a review), and confidence in the humidity response of this region is central to confidence in modelled water vapour feedback.

The humidity distribution within the tropical free troposphere is determined by many factors, including the detrainment of vapour and condensed water from convective systems and the large-scale atmospheric circulation. The relatively dry regions of large-scale descent play a major role in tropical LW cooling, and changes in their area or humidity could potentially have a significant impact on water vapour feedback strength (Pierrehumbert, 1999; Lindzen et al., 2001; Peters and Bretherton, 2005). Given the complexity of processes controlling tropical humidity, however, simple convincing physical arguments about changes under global-scale warming are difficult to sustain, and a combination of modelling and observational studies are needed to assess the reliability of model water vapour feedback.

In contrast to cloud feedback, a strong positive water vapour feedback is a robust feature of GCMs (Stocker et al., 2001), being found across models with many different schemes for advection, convection and condensation of water vapour. High-resolution mesoscale (Larson and Hartmann, 2003) and cloud-resolving models (Tompkins and Craig, 1999) run on limited tropical domains also display humidity responses consistent with strong positive feedback, although with differences in the details of upper-tropospheric RH (UTRH) trends with temperature. Experiments with GCMs have found water vapour feedback strength to be insensitive to large changes in vertical resolution, as well as convective parametrization and advection schemes (Ingram, 2002). These modelling studies provide evidence that the free-tropospheric RH response of global coupled models under climate warming is not simply an artefact of GCMs or of coarse GCM resolution, since broadly similar changes are found in a range of models of different complexity and scope. Indirect supporting evidence for model water vapour feedback strength also comes from experiments which show that suppressing humidity variation from the radiation code in an AOGCM produces unrealistically low interannual variability (Hall and Manabe, 1999).

Confidence in modelled water vapour feedback is dependent upon understanding of the physical processes important for controlling UTRH, and confidence in their representation in GCMs. The TAR noted a sensitivity of UTRH to the representation of cloud microphysical processes in several simple modelling studies. However, other evidence suggests that the role of microphysics is limited. The observed RH field in much of the tropics can be well simulated without microphysics, but simply by observed winds while imposing an upper limit of 100% RH on parcels (Pierrehumbert and Roca, 1998; Gettelman et al., 2000; Dessler and Sherwood, 2000), or by determining a detrainment profile from clear-sky radiative cooling (Folkins et al., 2002). Evaporation of detrained cirrus condensate also does not play a major part in moistening the tropical upper troposphere (Soden, 2004; Luo and Rossow, 2004), although cirrus might be important as a water vapour sink (Luo and Rossow, 2004). Overall, these studies increase confidence in GCM water vapour feedback, since they emphasise the importance of large-scale advective processes, or radiation, in which confidence in representation by GCMs is high, compared with microphysical processes, in which confidence is much lower. However, a significant role for microphysics in determining the distribution of changes in water vapour under climate warming cannot yet be ruled out.

Observations provide ample evidence of regional-scale increases and decreases in tropical UTRH in response to changes in convection (Zhu et al., 2000; Bates and Jackson, 2001; Blankenship and Wilheit, 2001; Wang et al., 2001; Chen et al., 2002; Chung et al., 2004; Sohn and Schmetz, 2004). Such changes, however, provide little insight into large-scale thermodynamic relationships (most important for the water vapour feedback) unless considered over entire circulation systems. Recent observational studies of the tropical mean UTRH response to temperature have found results consistent with that of near-unchanged RH at a variety of time scales (see Section 3.4.2.2). These include responses from interannual variability (Bauer et al., 2002; Allan et al., 2003; McCarthy and Toumi, 2004), volcanic forcing (Soden et al., 2002; Forster and Collins, 2004) and decadal trends (Soden et al., 2005), although modest RH decreases are noted at high levels on interannual time scales (Minschwaner and Dessler, 2004; Section 3.4.2.3). Seasonal variations in observed global LW radiation trapping are also consistent with a strong positive water vapour feedback (Inamdar and Ramanathan, 1998; Tsushima et al., 2005). Note, however, that humidity responses to variability or shorter time-scale forcing must be interpreted cautiously, as they are not direct analogues to that from greenhouse gas increases, because of differences in patterns of warming and circulation changes.

8.6.3.1.1 Evaluation of water vapour/lapse rate feedback processes in models

Evaluation of the humidity distribution and its variability in GCMs, while not directly testing their climate change feedbacks, can assess their ability to represent key physical processes controlling water vapour and therefore affect confidence in their water vapour feedback. Limitations in coverage or accuracy of radiosonde measurements or reanalyses have long posed a problem for UTRH evaluation in models (Trenberth et al., 2001; Allan et al., 2004), and recent emphasis has been on assessments using satellite measurements, along with increasing efforts to directly simulate satellite radiances in models (so as to reduce errors in converting to model-level RH) (e.g., Soden et al., 2002; Allan et al., 2003; Iacono et al., 2003; Brogniez et al., 2005; Huang et al., 2005).

Major features of the mean humidity distribution are reasonably simulated by GCMs, along with the consequent distribution of OLR (see Section 8.3.1). In the important subtropical subsidence regions, models show a range of skill in representing the mean UTRH. Some large regional biases have been found (Iacono et al., 2003; Chung et al., 2004), although good agreement of distribution and variability with satellite data has also been noted in some models (Allan et al., 2003; Brogniez et al., 2005). Uncertainties in satellite-derived data sets further complicate such comparisons, however. Skill in the reproduction of ‘bimodality’ in the humidity distribution at different time scales has also been found to differ between models (Zhang et al., 2003; Pierrehumbert et al., 2007), possibly associated with mixing processes and resolution. Note, however, that given the near-logarithmic dependence of LW radiation on humidity, errors in the control climate humidity have little direct effect on climate sensitivity: it is the fractional change of humidity as climate changes that matters (Held and Soden, 2000).

A number of new tests of large-scale variability of UTRH have been applied to GCMs since the TAR, and have generally found skill in model simulations. Allan et al. (2003) found that an AGCM forced by observed SSTs simulated interannual changes in tropical mean 6.7 µm radiance (sensitive to UTRH and temperature) in broad agreement with High Resolution Infrared Radiation Sounder (HIRS) observations over the last two decades. Minschwaner et al. (2006) analysed the interannual response of tropical mean 250 hPa RH to the mean SST of the most convectively active region in 16 AOGCMs from the MMD at PCMDI. The mean model response (a small decrease in RH) was statistically consistent with the 215 hPa response inferred from satellite observations, when uncertainties from observations and model spread were taken into account. AGCMs have been able to reproduce global or tropical mean variations in clear sky OLR (sensitive to water vapour and temperature distributions) over seasonal (Tsushima et al., 2005) as well as interannual and decadal (Soden, 2000; Allan and Slingo, 2002) time scales (although aerosol or greenhouse gas uncertainties and sampling differences can affect these latter comparisons; Allan et al., 2003). In the lower troposphere, GCMs can simulate global-scale interannual moisture variability well (e.g., Allan et al., 2003). At a smaller scale, a number of GCMs have also shown skill in reproducing regional changes in UTRH in response to circulation changes such as from seasonal or interannual variability (e.g., Soden, 1997; Allan et al., 2003; Brogniez et al., 2005).

A further test of the response of free tropospheric temperature and humidity to surface temperature in models is how well they can reproduce interannual correlations between surface temperature and vertical humidity profiles. Although GCMs are only partially successful in reproducing regional (Ross et al., 2002) and mean tropical (Bauer et al., 2002) correlations, the marked disagreement found in previous studies (Sun and Held, 1996; Sun et al., 2001) has been shown to be in large part an artefact of sampling techniques (Bauer et al., 2002).

There have also been efforts since the TAR to test GCMs’ water vapour response against that from global-scale temperature changes of recent decades. One recent study used a long period of satellite data (1982–2004) to infer trends in UTRH, and found that an AGCM, forced by observed SSTs, was able to capture the observed global and zonal humidity trends well (Soden et al., 2005). A second approach uses the cooling following the eruption of Mt Pinatubo. Using estimated aerosol forcing, Soden et al. (2002) found a model-simulated response of HIRS 6.7 µm radiance consistent with satellite observations. They also found a model global temperature response similar to that observed, but not if the water vapour feedback was switched off (although the study neglected changes in cloud cover and potential heat uptake by the deep ocean). Using radiation calculations based on humidity observations, Forster and Collins (2004) found consistency in inferred water vapour feedback strength with an ensemble of coupled model integrations, although the latitude-height pattern of the observed humidity response did not closely match any single realisation. They deduced a water vapour feedback of 0.9 to 2.5 W m^–2 °C^–1, a range which covers that of models under greenhouse gas forcing (see Figure 8.14). An important caveat to these studies is that the climate perturbation from Mt Pinatubo was small, not sitting clearly above natural variability (Forster and Collins, 2004). Caution is also required when comparing with feedbacks from increased greenhouse gases, because radiative forcing from volcanic aerosol is differently distributed and occurs over shorter time scales, which can induce different changes in circulation and bias the relative land/ocean response (although a recent AOGCM study found similar global LW radiation clear sky feedbacks between the two forcings; Yokohata et al., 2005). Nevertheless, comparing observed and modelled water vapour response to the eruption of Mt. Pinatubo constitutes one way to test model ability to simulate humidity changes induced by an external global-scale forcing.

At low latitudes, GCMs show negative lapse rate feedback because of their tendency towards a moist adiabatic lapse rate, producing amplified warming aloft. At middle to high latitudes, enhanced low-level warming, particuarly in winter, contributes a positive feedback (e.g., Colman, 2003b), and global feedback strength is dependent upon the meridional warming gradient (Soden and Held, 2006). There has been extensive testing of GCM tropospheric temperature response against observational trends for climate change detection purposes (see Section 9.4.4). Although some recent studies have suggested consistency between modelled and observed changes (e.g., Fu et al., 2004; Santer et al., 2005), debate continues as to the level of agreement, particularly in the tropics (Section 9.4.4). Regardless, if RH remains close to unchanged, the combined lapse rate and water vapour feedback is relatively insensitive to differences in lapse rate response (Cess, 1975; Allan et al., 2002; Colman, 2003a).

In the stratosphere, GCM water vapour response is sensitive to the location of initial radiative forcing (Joshi et al., 2003; Stuber et al., 2005). Forcing concentrated in the lower stratosphere, such as from ozone changes, invoked a positive feedback involving increased stratospheric water vapour and tropical cold point temperatures in one study (Stuber et al., 2005). However, for more homogenous forcing, such as from CO₂, the stratospheric water vapour contribution to model sensitivity appears weak (Colman, 2001; Stuber et al., 2001, 2005). There is observational evidence of possible long-term increases in stratospheric water vapour (Section 3.4.2.3), although it is not yet clear whether this is a feedback process. If there is a significant global mean trend associated with feedback mechanisms, however, this could imply a significant stratospheric water vapour feedback (Forster and Shine, 2002).

8.6.3.1.2 Summary of water vapour and lapse rate feedbacks

Significant progress has been made since the TAR in understanding and evaluating water vapour and lapse rate feedbacks. New tests have been applied to GCMs, and have generally found skill in the representation of large-scale free tropospheric humidity responses to seasonal and interannual variability, volcano-induced cooling and climate trends. New evidence from both observations and models has reinforced the conventional view of a roughly unchanged RH response to warming. It has also increased confidence in the ability of GCMs to simulate important features of humidity and temperature response under a range of different climate perturbations. Taken together, the evidence strongly favours a combined water vapour-lapse rate feedback of around the strength found in global climate models.