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Publication Date
26 December 2023

Observed Humidity Trends in Dry Regions Contradict Climate Models

Over the past couple of decades, humidity over dry regions has declined while models suggest it should have increased. We do not fully understand this discrepancy, making it an urgent conundrum to resolve in climate modeling.
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Comparing a range of observations and model simulations, we show robust evidence for a discrepancy in vapor pressure (VP) trends. By controlling for possible drivers like sea surface temperatures (SST) or precipitation, we isolate the issue to be most likely related to the land surface component of climate models. 


The discrepancy between observations and models indicates a major gap in our understanding and modeling capabilities which could have severe implications for hydroclimate projections, including fire hazard, moving forward. Resolving this discrepancy is an urgent priority.


Water vapor in the atmosphere is expected to rise with warming because a warmer atmosphere can hold more moisture. However, over the last four decades, near-surface water vapor has not increased over arid and semi-arid regions. This is contrary to all climate model simulations in which it rises at a rate close to theoretical expectations, even over dry regions. This may indicate a major model misrepresentation of hydroclimate-related processes; models increase water vapor to satisfy the increased atmospheric demand, while this has not happened in reality. Given the close links between water vapor and wildfire, ecosystem functioning, and temperature extremes, this issue must be resolved in order to provide more reliable climate projections for arid and semi-arid regions of the world.

Point of Contact
Jerry Meehl
National Center for Atmospheric Research (NCAR)
Funding Program Area(s)
Observed humidity trends in dry regions contradict climate models
Simpson, Isla R., Karen A. McKinnon, Daniel Kennedy, David M. Lawrence, Flavio Lehner, and Richard Seager. 2023. “Observed Humidity Trends In Dry Regions Contradict Climate Models”. Proceedings Of The National Academy Of Sciences 121 (1). Proceedings of the National Academy of Sciences. doi:10.1073/pnas.2302480120.