Abstract: An expected consequence of a warming atmosphere is that atmospheric humidity would rise as a result of the dependency of the atmospheric water vapor holding capacity on temperature (the Clausius-Clapeyron relationship). But this is only true if there is sufficient availability of water to satisfy the rising atmospheric demand. Given the important implications of atmospheric water vapor content for wildfire and as a general indicator of how the hydroclimate is changing, it is important to ensure that the Earth System Models used for climate projections are accurately representing the historical evolution of atmospheric water vapor. Here, it will be demonstrated that, contrary to expectations, and to the behavior of Earth System Models, atmospheric water vapor over arid and semi-arid regions in observation-based data has not risen. While Earth System Models suggest that atmospheric water vapor should have risen at approximately Clausius-Clapeyron scaling, atmospheric water vapor in these regions in reality has actually remained relatively constant, raising the possibility that Earth System Models are mis-representing processes of relevance to the hydroclimate. Ongoing work to identify the underlying causes of this discrepancy will be discussed including whether models accurately represent evaporative regimes in day-to-day variability and the potential role for inhomogeneities in the observational record.
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