We develop a proxy to infer humidity from the geochemistry of paleosols in order to enhance reconstructions of ancient paleoclimate beyond trends in mean annual precipitation. Geochemical transfer functions, developed herein, are used to estimate net primary production and evapotranspiration along three latitudinal transects of modern soils in the coterminous United States. Mean annual precipitation and the degree of chemical weathering are estimated from the major element concentrations in soils. The ratio of evapotranspiration, estimated from our proxy, and mean annual precipitation provides a method of determining the humidity province of ancient climates that is more robust and meaningful than previous methods, and our approach differs from existing methods since both the influx and efflux of moisture are explicitly determined. The required input parameters for application of this proxy are (1) the soil morphology, (2) accurate and complete major element concentration data for the active soil and parent material, and (3) the latitude or mean annual temperature (±5°C) of the soil or paleosol. The correlation coefficient between the measured climate and the modeled climate regime using only the latitude, morphologic, and major element data of soils is r=0.95⁠. We conclude that this proxy provides a refined determination of humidity and floral regimes that can be applied to paleosols.

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