Using reduced-complexity volcanic aerosol and climate models to produce large ensemble simulations of Holocene temperature

Abstract. Volcanic eruptions are one of the most important drivers of climate variability, but climate model simulations typically show stronger surface cooling than proxy-based reconstructions. Uncertainties associated with eruption source parameters, aerosol-climate modelling and internal climate variability might explain those discrepancies but their quantification using complex global climate models is computationally expensive. In this study, we combine a reduced-complexity volcanic aerosol model (EVA_H) and a climate model (FaIR) to simulate global mean surface temperature from 6755 BCE to 1900 CE (8705 to 50 BP) accounting for volcanic forcing, solar irradiance, orbital, ice sheet, greenhouse gases and land-use forcing. The models’ negligible computational cost enables us to use a Monte Carlo approach to propagate uncertainties associated with eruption source parameters, aerosol and climate model parameterisations, and internal climate variability. Over the last 9000 years, we obtain a global-mean volcanic forcing of -0.15 W.m^-2 and an associated surface cooling of 0.12 K. For the 14 largest eruptions (injecting more than 20 Tg of SO₂) of 1250 CE – 1900 CE, a superposed epoch analysis reveals an excellent agreement on the mean temperature response between our simulations, scaled to Northern Hemisphere summer temperature, and tree ring-based reconstructions. For individual eruptions, discrepancies between the simulated and reconstructed surface temperature response are almost always within uncertainties. At multi-millennial timescales, our simulations reproduce the Holocene global warming trend, but exhibit some discrepancies on centennial to millennial timescales. In particular, the Medieval Climate Anomaly to Little Ice Age transition is weaker in our simulations, and we also do not capture a relatively cool period in climate reanalyses between 3000 BCE and 1000 BCE (5000 and 3000 BP). We discuss how uncertainties in land-use forcing and model limitations might explain these differences. Our study demonstrates the value of reduced-complexity volcanic aerosol-climate models to simulate climate at annual to multi-millennial timescales.