A Critical Assessment of the Recent Rapid Decline in Land Surface Relative Humidity

About This Grant

Surface relative humidity (RH) has major impacts on human comfort and health, as well as droughts and wildfires. Historical data show rapid declines in surface RH over many land areas since the late 1990s. Such a RH decrease enhances atmospheric demand for moisture under rising temperatures, leading to increased risk of drought and wildfires in the western U.S. and other regions. However, the exact cause of this RH decrease is unknown. Given the broad impacts of surface RH, improved understanding of the RH decline is critically important. This project will help explain the rapid decline in recent surface RH, thereby benefiting research in human health, drought and wildfires in the U.S. and other regions. It will also help the PI train two graduate students, teach climate-related courses, and educate the public about our changing climate. A comprehensive analysis and critical assessment of the recent RH trend will be carried out to reveal its causes and assess its reliability. Utilizing the Principal Investigator's expertise in homogenizing climate records, monthly RH data series from global weather stations since 1973 will be homogenized using advanced statistical methods combined with available metadata to detect and remove discontinuities associated with instrumental changes. This will help quantify the impact of such discontinuities on the recent RH trend over land. Further, RH variations and changes will be compared with those in physically related but independently observed meteorological variables, including cloud cover and precipitation. The relationships among RH, cloud cover, precipitation, surface latent and sensible heat fluxes, and soil moisture in a reanalysis dataset (ERA5) will be examined, with a focus on whether land-air interactions have amplified the RH decrease. Finally, after removing artificial changes in the RH records, the researchers investigate the key drivers of the remaining RH trend, including a) externally-forced changes estimated from large-ensembles of climate model simulations, and b) the contribution from decreasing topsoil moisture through a series of climate model simulations with and without observed drying in topsoil forced with observed sea surface temperatures over the last four decades. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.