Abstract
Abstract. Water vapour in the atmosphere is the source of a major climate feedback mechanism and potential increases in the availability of water vapour could have important consequences for mean and extreme precipitation. Future precipitation changes further depend on how the hydrological cycle responds to different drivers of climate change, such as greenhouse gases and aerosols. Currently, neither the total anthropogenic influence on the hydrological cycle nor that from individual drivers is constrained sufficiently to make solid projections. We investigate how integrated water vapour (IWV) responds to different drivers of climate change. Results from 11 global climate models have been used, based on simulations where CO2, methane, solar irradiance, black carbon (BC), and sulfate have been perturbed separately. While the global-mean IWV is usually assumed to increase by ∼7 % per kelvin of surface temperature change, we find that the feedback response of IWV differs somewhat between drivers. Fast responses, which include the initial radiative effect and rapid adjustments to an external forcing, amplify these differences. The resulting net changes in IWV range from 6.4±0.9 % K−1 for sulfate to 9.8±2 % K−1 for BC. We further calculate the relationship between global changes in IWV and precipitation, which can be characterized by quantifying changes in atmospheric water vapour lifetime. Global climate models simulate a substantial increase in the lifetime, from 8.2±0.5 to 9.9±0.7 d between 1986–2005 and 2081–2100 under a high-emission scenario, and we discuss to what extent the water vapour lifetime provides additional information compared to analysis of IWV and precipitation separately. We conclude that water vapour lifetime changes are an important indicator of changes in precipitation patterns and that BC is particularly efficient in prolonging the mean time, and therefore likely the distance, between evaporation and precipitation.
Highlights
Water vapour is the largest contributor to the natural greenhouse effect and the source of a major climate feedback mechanism (Boucher et al, 2013)
The core Precipitation Driver Response Model Intercomparison Project (PDRMIP) experiments consist of one base experiment, representing present-day conditions, and five perturbation experiments relative to base: doubling of the CO2 concentration, tripling of the CH4 concentration (CH4x3), total solar irradiance increased by 2 % (Sol+2%), 5 times increase in anthropogenic sulfate concentration or SO2 emissions (SO4x5), and 10 times increase in black carbon (BC) concentration or emissions (BCx10)
The fast response is largest for BCx10, which changes from 7.5 ± 1 % K−1 to 9.8 ± 2 % K−1 between the slow and total response
Summary
Water vapour is the largest contributor to the natural greenhouse effect and the source of a major climate feedback mechanism (Boucher et al, 2013). The global-mean integrated water vapour (IWV) is found to increase by around 7 % K−1 in both models (Held and Soden, 2006; O’Gorman and Muller, 2010) and observations (Wentz et al, 2007; O’Gorman et al, 2012), consistent with the rate of change of saturation vapour pressure with temperatures representative of the lower troposphere and constant relative humidity (Allen and Ingram, 2002; Trenberth et al, 2003; Held and Soden, 2006). Extreme precipitation events are likely to increase with the availability of water vapour (Allen and Ingram, 2002) (at around 7 % K−1), but large uncertainties exist due to non-thermodynamic contributions (O’Gorman and Schneider, 2009; O’Gorman, 2015). Changes in the hydrological cycle will have widespread consequences for humanity, e.g. through changing precipitation patterns and extremes
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