Atmosphere Ocean Science Colloquium

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An analytic theory of near-surface relative humidity over land

Speaker: Kaighin McColl, Harvard University

Location: Warren Weaver Hall 1302

Date: Wednesday, April 30, 2025, 3:30 p.m.

Synopsis:

There is no simple explanation for the spatial structure of near-surface relative humidity (RH) over land, despite its importance for human health and ecosystem productivity. The explanation is not obvious: for example, why is the latitudinal profile of RH over land shaped like the letter ‘W’, when both specific humidity and saturation specific humidity essentially decline monotonically from the equator to the poles? I will present a simple theory for RH that shows it is largely controlled by the land surface, particularly soil moisture, and directly answers this question. The theory is then used to address a basic question: do land surfaces get wetter or drier, on average, as the planet warms? It is widely believed that land surfaces rapidly dry, on average. Yet, the same climate models used to project strong aridification show no obvious signs of drastic drying in relevant land surface variables, such as soil water storage or ecosystem productivity. This discrepancy – termed the “aridity paradox” – remains largely unresolved. I will show that the simple theory parsimoniously resolves the aridity paradox, and conclude that there is no reason to expect drier land surfaces, on average, as the planet warms.