VOlcanic climate forcing over the onset of the Little ice age through TEphra geochemistry and Stratospheric Sulfate isotope analyses in polar ice cOres

The most recent climatic shift in the Northern Hemisphere before anthropogenic forcing became dominant was the transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), a cooling that lasted until the late 19th century. A leading hypothesis for the LIA onset is that a series of large volcanic eruptions during the MWP-LIA transition, amplified by sea ice/ocean-related feedbacks, drove sustained cooling. Seven presumed tropical and six northern high-latitude eruptions have been identified from ice cores; however, their precise sources and the amounts of stratospheric sulfate aerosol injection remain poorly constrained, hampering robust assessments of their climatic forcing. This project addresses these gaps by applying state-of-the-art tephra geochemistry and sulfur isotope analyses to polar ice cores to determine eruption sources and quantify stratospheric sulfate aerosol loadings for major eruptions between 1150 and 1350 CE. The new volcanic forcing record will be combined with existing paleoclimate data products to determine the climate impacts of closely timed eruptions at the start of the LIA. The project will also use aerosol-chemistry-climate models to simulate the impacts of Northern Hemisphere and tropical eruption sequences, assessing which scenarios – if any – could plausibly trigger the LIA through sea ice/ocean-related feedbacks. This comprehensive approach – combining novel ice-core measurements, comparative paleoclimate records, and model evaluation– will substantially advance understanding of whether, and by what processes and feedbacks, a sequence of major tropical and northern high-latitude volcanic eruptions contributed to Northern Hemisphere cooling or acted as a trigger for the onset of the LIA. Ultimately, the findings will provide critical observational constraints to refine climate model sensitivities to sulfur aerosol forcing, a key uncertainty in future climate predictions.