Possible Impacts on Precipitation
AR5 continues to predict a warmer future climate due to increasing greenhouse gases. A warming climate will cause rapid responses and feedbacks of clouds and aerosols. These cloud and aerosol responses can be direct changes in their radiative forcing, or be changes in cloud fields (in forms of cloud fraction, thickness, spatial distributions, etc.). It can also be alteration of aerosol composition or mixing state, or other aerosol properties. All these changes might affect the large-scale circulation, directly or indirectly altering the hydrological cycles, precipitation patterns and extremes. These possible impacts on precipitation are assessed by chapter 7.
1. on Hydrological Cycle
A warming climate has a warmer troposphere, which will have a reduced net radiative cooling effect, due to higher downwelling radiation and almost unchanged outgoing longwave radiation (see the ‘fixed anvil-temperature’ theory in “Cloud feedbacks and rapid responses”). The reduced net cooling effect on troposphere will cause a weakening of the atmospheric overturning circulations (e.g. Hadley, Walker circulations) and a reduction in precipitation rate [Bony et al., 2013]. Modeled large-scale atmospheric overturning responses to warming are shown in Figure 7.20 (left).
2. on Large-scale Precipitation Trend
The 'wet-get-wetter' and 'dry-get-drier' response is likely caused by a change in the water vapor content carried by circulations. A positive feedback mechanism driven by warmer temperatures increases the transport of moisture from dry regions to wet regions.
3. on Precipitation Extremes
Intensity of precipitation extreme is determined by local water vapor availability and not affected much by the large-scale circulation. In AR5, results from cloud process modeling and GCM studies, as well as observations enables us to predict an increase in the intensity of extreme precipitation with warming climate is expected with high confidence. However, there are more uncertainties in determining the magnitude of this increase in extreme intensity. AR5 estimates an increasing rate of ~5 to 10% °C^-1 warming, varying with time scale, location and season with medium confidence.
Figure 7.21 | Estimate (5 to 95% range) of the increase in precipitation amount per degree Celsius of global mean surface temperature change. At left (blue) are climate model predictions of changes in time-averaged global precipitation; at centre and right (orange) are predictions or estimates of the typical or average increase in local 99.9th percentile extremes, over 24 hours (centre) and over one hour or less (right). Data are adapted from (A) GCM studies (Allen and Ingram, 2002; and Lambert and Webb, 2008, for time average; O’Gorman and Schneider, 2009 for extremes), (B) long-term trends at many sites globally (Westra et al., 2013), (C) GCMs constrained by present- day observations of extremes (O’Gorman, 2012), (D, E) cloud-resolving model (CRM) and large-eddy simulation (LES) studies of radiative convective equilibrium (Muller et al., 2011; Romps, 2011).