Moderate Volcanic Eruptions and Extreme Wildfires Humidify the Stratosphere
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Stratospheric water vapor (SWV) is a key greenhouse gas that influences both global climate and stratospheric chemistry. Volcanic activity has long been expected to humidify the stratosphere via tropopause warming, but direct observational confirmation has been lacking. Using satellite observations and large ensemble simulations, we provide direct observational evidence that frequent moderate eruptions and extreme wildfires since 2005 have increased SWV by warming the tropical cold point tropopause. These events have enhanced SWV by 0.1 parts per million by volume at 83 hPa mainly through aerosol-driven tropopause warming, leading to an accumulated mass of 142-210 million tons during 2005-2021. These activities explain ~40% of observed SWV trend over this period, a contribution comparable to that from global surface temperature trend. Together with surface temperature trend, moderate volcanic eruptions and extreme wildfire events have effectively offset the sudden 10% SWV decrease observed around 2000, revealing a previously unquantified forcing mechanism that warrants consideration in climate assessments.
