Index links land humidity trends to precipitation and temperature

What the study found: A simple index based on the ratio of precipitation to a modified potential evapotranspiration was able to capture the spatiotemporal variability of land relative humidity and its different historical trends in observations, reanalyses, and Earth system models. The study found that land relative humidity has decreased substantially over 1973-2024.
Why the authors say this matters: The authors say the index provides a physical calibration of biased land relative humidity in reanalyses and a quantitative framework for interpreting land relative humidity changes. They also conclude that the findings indicate a drier land future than current projections.
What the researchers tested: The researchers compared land relative humidity trends in observations, reanalyses, and Earth system models over 1973-2024. They used an index based on precipitation and a modified potential evapotranspiration formulated independently of relative humidity.
What worked and what didn't: The index captured the observed variability and the coherent bias in reanalyses, which overestimated the observed relative humidity decrease. It also captured model spread and the discrepancy between models and observations, while nearly all model runs underrepresented the historical drying.
What to keep in mind: The abstract says model-observation discrepancy is unlikely to be explained by internal variability. Other limitations are not described in the available summary.

Key points

• A simple precipitation-based index captured land relative humidity variability across observations, reanalyses, and models.
• Land relative humidity decreased substantially over 1973-2024.
• Reanalyses overestimated the observed decrease, consistent with exaggerated surface warming and precipitation decline.
• Nearly all model runs underrepresented the historical drying.
• The abstract says the model-observation discrepancy is unlikely to be explained by internal variability.