Leverage Points in the Earth System: Soil Moisture

The 2004 Science paper – Regions of Strong Coupling Between Soil Moisture and Precipitation – by Koster et al. used a dozen independent climate models to estimate ‘hot spots’ on Earth’s surface where precipitation is affected by soil moisture anomalies during Northern Hemisphere summer. They propose that these hot spots are, in a sense, land-surface analogs to the ocean’s “El Niño hot spot” in the eastern tropical Pacific.

Soil moisture is a slowly vary aspect of the Earth system (relative to weather). Soil moisture can persist for months. Soil moisture, influences evaporation and other surface energy fluxes can influence weather.

Soil moisture atmospheric coupling

Figure: Hot spots of soil moisture – local precipitation coupling appear in the central Great Plains of North America, the Sahel, equatorial Africa, and India. Less intense hot spots appear in South America, central Asia, and China.

The hot spots are located in regions that in areas that are at intermediate moisture levels. The authors argue that this is because in wet climates, soil water is plentiful and evaporation is controlled not by soil moisture but by net radiative energy. In dry climates evaporation rates are sensitive to soil moisture but they are small. Consquently the biggest impact of soil moisture on evaporation is in the transition areas between dry and wet climates.

What this analysis suggests is that these hotspots are areas in which changes in land use – especially those that alter soil moisture – such as irrigation or land clearing, will have a larger impact of regional climate.

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