Content area
Full Text
1. Introduction
Future changes in surface humidity under global warming could affect human comfort (Willett and Sherwood 2012), labor capacity (Dunne et al. 2013), and even, for extreme warming scenarios, the habitability of some land regions (Sherwood and Huber 2010; Pal and Eltahir 2016). Humidity changes are also closely linked to changes in surface evaporation, transpiration, soil moisture, mean and extreme precipitation, and clouds (e.g., Emori and Brown 2005; Fasullo 2012; Sherwood and Fu 2014; Byrne and O’Gorman 2015; Chadwick 2016; Kamae et al. 2016).
Over the oceans, energy balance considerations (described in Held and Soden 2000; Schneider et al. 2010) suggest that surface specific humidity (q) should increase approximately in line with surface air temperature under fixed relative humidity (RH), which implies increases of around 7% degree−1 of local warming under the Clausius-Clapeyron equation (Held and Soden 2006). Climate model projections agree well with this prediction (Held and Soden 2006; Sherwood et al. 2010; O’Gorman and Muller 2010), with only small RH increases projected over the oceans. Observed and modeled trends of historical surface q change over the oceans are also consistent with this Clausius-Clapeyron rate of increase (Dai 2006).
Over land it is far less obvious how surface humidity will respond to warming, as the moisture supply from the land surface is often limited, and the energy balance arguments that apply to oceanic [Inline formula omitted: See PDF] are much less relevant. Net moisture transport from ocean to land increases under warming in future model projections (Zahn and Allan 2013), but the exact implications of this for humidity change are unclear. Observed trends from the period of 1973-99 are consistent with a local Clausius-Clapeyron response under fixed RH in many land regions, but the relationship between local temperature and q change breaks down in drier regions (Dai 2006; Willett et al. 2010). Over the more recent period of 1999-2015 there is evidence of significant decreases in RH over land (Simmons et al. 2010; Willett et al. 2014, 2015), causing this local Clausius-Clapeyron model to be brought into question. Agreement between the in situ record and reanalyses is good (Willett et al. 2015), but natural variability over the observed period and the sparseness of the observational network over many land...