PNNL

The Science                                       

Relative humidity (RH), a measure of air wetness, is projected to decrease over land under anthropogenic warming. The decrease in land RH would increase potential water loss from the surface and intensify droughts, heatwaves, and wildfires. The land RH decrease was previously attributed to ocean influences, but this work highlights the importance of interactive soil moisture. Research shows that the land RH decrease is coupled with soil moisture decline. Results from warming simulations that exclude interactive soil moisture and ocean influences illustrate that interactive soil moisture is necessary and sufficient to produce drier land under anthropogenic warming.

The Impact

This study identifies the important role of interactive soil moisture (i.e., where soil moisture is fed by the net precipitation and can in turn affect evaporation through moisture availability) in controlling the land RH response to warming. It shows that the drier land RH under anthropogenic warming is linked with reduced soil moisture. The study further underscores the need for further research to understand how exactly soil moisture adjusts to reach a drier equilibrium. Understanding the robust RH reductions over land lends more confidence in projecting future changes, such as heatwaves and wildfires, affected by the RH changes.

Summary

Under anthropogenic warming, RH is projected to increase over the ocean but decrease over land. The land RH decrease was previously attributed to ocean influences on land changes. New research shows that interactive soil moisture is necessary and sufficient for anthropogenic warming to reduce the land RH. In a comprehensive climate model with realistic oceans, the land RH decrease is absent when soil moisture is prescribed to prevent it from responding to climate change. In a minimalist land model that excludes the influence of oceans, soil moisture naturally decreases under global warming and leads to a decline in RH. The soil moisture decrease can be explained by prior imbalances in evaporation and precipitation increases. Specifically, prior to adjustments in soil moisture and RH, evaporation would increase with warming by ~6% K^-1, outpacing the radiatively constrained ~2% K^-1 precipitation increase. This prior imbalance depletes soil moisture and consequently reduces RH through the coupling between soil moisture and RH. These results suggest that the projected land RH decrease is linked to soil moisture decline and this decline can be understood as a natural response of interactive soil moisture to global warming independent of ocean influences.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Funding

This research was supported by the Department of Energy, Office of Science, as part of research in the Regional and Global Model Analysis program area, Earth and Environmental System Modeling Program.