Moderate volcanic eruptions and extreme wildfires humidify the stratosphere

Stratospheric water vapour (SWV) is a key greenhouse gas that influences both global climate and stratospheric ozone chemistry^{1, 2, 3–4}. Its abundance is strongly modulated by natural climate variability^{1,5, 6, 7–8}. Volcanic eruptions have long been expected to humidify the stratosphere via tropopause warming^9,10, but observational confirmation has been lacking. Here we provide observational evidence that moderate volcanic eruptions and extreme wildfires since 2005 have systematically increased SWV. Both contribute through aerosol-induced tropopause warming; however, extreme wildfires reveal an additional self-lofting pathway that transports water vapour into the stratosphere. Complementary analysis of satellite observations and climate model simulations reveals an SWV enhancement of about 0.1 ppmv at 83 hPa, accumulating 76–203 million tons of water vapour during 2005–2021. This contribution explains 36 ± 7% of the observed SWV trend over this period, comparable to that from the global surface temperature increase. SWV changes induced by the surface temperature trend, moderate volcanic eruptions and extreme wildfire events have together effectively offset the sudden 10% SWV decrease observed around 2000. Episodic aerosol perturbations from moderate volcanic eruptions and extreme wildfires therefore emerge as a previously overlooked driver of SWV variability. Future projections of stratospheric composition, radiative forcing and ozone recovery should account for these aerosol-mediated processes, especially as extreme fires intensify in a warming world.