Understanding Decreases in Land Relative Humidity with Global Warming: Conceptual Model and GCM Simulations

Abstract

Climate models simulate a strong land-ocean contrast in the response of near-surface relative humidity to global warming: relative humidity tends to increase slightly over oceans but decrease substantially over land. Surface energy balance arguments have been used to understand the response over ocean but are difficult to apply over more complex land surfaces. Here, a conceptual box model is introduced, involving moisture transport between the land and ocean boundary layers and evapotranspiration, to investigate the decreases in land relative humidity as the climate warms. The box model is applied to idealized and full-complexity (CMIP5) general circulation model simulations, and it is found to capture many of the features of the simulated changes in land relative humidity. The box model suggests there is a strong link between fractional changes in specific humidity over land and ocean, and the greater warming over land than ocean then implies a decrease in land relative humidity. Evapotranspiration is of secondary importance for the increase in specific humidity over land, but it matters more for the decrease in relative humidity. Further analysis shows there is a strong feedback between changes in surface-air temperature and relative humidity, and this can amplify the influence on relative humidity of factors such as stomatal conductance and soil moisture.

Understanding the Dynamic Contribution to Future Changes in Tropical Precipitation From Low‐Level Convergence Lines

“…The difference pattern therefore predominantly highlights the wet-getwetter, dry-get-drier regions. It has been suggested that, as the world warms, there will be small changes in the sensitivity of precipitation to convergence (i.e., the slope (a 1 ) of the regression model as shown in Figure S4a in the supporting information; e.g., Byrne & O'Gorman, 2016;Singh & O'Gorman, 2013). It has been suggested that, as the world warms, there will be small changes in the sensitivity of precipitation to convergence (i.e., the slope (a 1 ) of the regression model as shown in Figure S4a in the supporting information; e.g., Byrne & O'Gorman, 2016;Singh & O'Gorman, 2013).…”

Thermodynamic and dynamic responses of the hydrological cycle to solar dimming

“…Previous studies have proposed that simulated and observed land-sea contrasts in relative humidity responses to global warming can be explained by the stronger temperature-driven increase in saturation specific humidity over land, which is not sufficiently compensated for by moisture transport from the ocean (Byrne and O'Gorman, 2016). Byrne and O'Gorman (2016) develop a conceptual box model that quantitatively supports the fact that this ocean control mechanism, as well as changes in evapotranspiration, explains simulated relative humidity anomalies over land.…”

Weak hydrological sensitivity to temperature change over land, independent of climate forcing

“…This difference can, to a large degree, be understood in terms of differences in the surface heating, atmospheric response to regional forcing and differences in surface relative humidity. [25][26][27][28] For all forcers, the land surface temperature changes more rapidly than over the ocean (see Supplementary Fig. 1).…”

“…While there is a general increase in convective precipitation, this response is virtually absent over land. Recently, Byrne and O'Gorman 28,31 have discussed the land/ ocean contrast under global warming in terms of differences in changes to relative humidity. While they find consistent fractional changes in specific humidity over land and ocean, the larger thermal response over land causes a decrease in relative humidity here, which will inhibit precipitation.…”