Significant Contribution of Paleogeography to Stratospheric Water Vapor Variations in the Past 250 Million Years

Stratospheric water vapor (SWV) variations play an important role in influencing the Earth's energy budget. Here, we investigate the SWV variations in the past 250 million years (Myr) using a fully coupled Earth System Model. It is found that both CO2 concentration and paleogeography have prominent influences on the SWV variations, while solar insolation plays a minor role. The SWV increases with surface warming and stratospheric moistening rate is accelerated during the warm periods in the past 250 Myr except for the Pangea supercontinent stage. The ratio of stratospheric moistening to surface warming is smaller in the warm Pangea supercontinent stage compared to that during the warm Cretaceous Period, which is due to the ascending and consequent cooling of the tropical tropopause layer associated with the severe surface warming over the tropical Pangea supercontinent. Our results suggest that paleogeography is an important factor in regulating SWV variations in deep‐time climate. Plain Language Summary We study the stratospheric water vapor (SWV) variations in the past 250 million years (Myr) using simulations of a fully coupled Earth System Model. The SWV generally increases with surface warming. It is found that surface warming and SWV increase can reach about 12.4 K and 89.2% in the past 250 Myr, respectively, with respect to the preindustrial condition. Both CO2 concentration and paleogeography have prominent influence on the SWV variations, while solar insolation plays a minor role. Although both the Pangea supercontinent stage and Cretaceous period have similar warm climate states, our results show that the SWV is much less in the Pangea supercontinent stage than that during the Cretaceous period. This discrepancy is found to be caused by different continental configurations. The SWV is primarily controlled by the tropical tropopause temperature. The stronger surface warming over the tropical Pangea supercontinent leads to ascending and thus cooling of the tropical tropopause layer. This work helps to understand the contributions of paleogeography to the SWV variations in the paleoclimate. Key Points Both CO2 concentration and paleogeography have prominent influence on the Stratospheric water vapor (SWV) variations in the past 250 million years The SWV increases with surface warming and stratospheric moistening rate is accelerated during warm periods The ratio of stratospheric moistening to surface warming is sensitive to the continental configurat