Projected Global Changes in CMIP6 Simulations of Extreme Low Relative Humidity Days

Extreme low relative humidity (RH) occurrences desiccate the landscape, fueling wildfire risk, in addition to other impacts on drought, plant and animal life, and human health. Rising global temperatures due to anthropogenic climate change are generally expected to cause increases in specific humidity where surface moisture is plentiful, though the effects on humidity extremes are more uncertain. RH, by contrast, is modulated by both water availability and air temperature. In this study, projected global changes in the frequency of low RH extremes through the end of the 21st century were analyzed from fifteen General Circulation Model (GCM) simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6). The threshold for extreme low RH days was determined based on the 1, 5, and 10% daily minimum RH values from the historical period (1950-2014) for each model. Projections from simulations based on several Shared Socioeconomic Pathways (SSPs) were compared, including SSP2-4.5, SSP3-7.0, and SSP5-8.5. Global trends in the frequency of these extreme low RH days are presented here. Relationships between the trends in low RH extremes and trends in specific humidity and extreme temperatures were also explored. Case studies within the United States are presented that highlight potential risks for wildfire and social vulnerability through the upcoming century.