Reconciling Volcanic Forcing and Climate Records throughout the Last Millennium (Vol-Clim)

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Volcanic eruptions are an important driver of climate variability and climate change, yet climate model simulations do not agree with data on the magnitude of temperature changes caused by large-magnitude volcanic eruptions. The Vol-Clim project will resolve this discrepancy by deriving new and improved estimates of volcanic forcing using a state-of-the-art Earth System Model developed in the UK (UKESM1), which will allow us to quantify and better understand how large explosive volcanic eruptions affected the climate system since 1250 CE. The model explicitly accounts for the interaction of chemical, dynamical and aerosol microphysical processes during volcanic eruptions, all of which affect the magnitude of the climate response. However, these processes have not been taken into account in previous assessments of climate change and natural climate variability caused by volcanic eruptions since 1250 CE. In detail, at least 60 volcanic eruptions have been detected based on volcanic deposits in polar ice-cores since 1250 CE. Large-magnitude eruptions emit sulphur dioxide high into the stratosphere where it is oxidized to form sulphuric acid vapour, which nucleates and condenses to form sulphate aerosol particles. These aerosol particles scatter and absorb energy from the Sun thereby cooling the Earth's surface. In terms of the magnitude of this surface cooling, tree-rings (and other data) appear to show a smaller hemispheric temperature response (of up to 1 degree Celsius) to volcanic eruptions than simulated by current climate models. This mismatch means that at present we do not fully understand how the climate system including clouds responds after volcanic eruptions. We also do not fully understand how tree growth and subsequently tree-rings respond as a consequence of the cooling induced by a volcanic eruption. Overall, these uncertainties affect our ability to use climate models to simulate past, present and future changes of climate. Current climate models have simple implementations of volcanic effects, ignoring many key chemical and physical processes relevant after volcanic eruptions. Using UKESM1 we will be able to simulate the evolution of volcanic aerosol particles with unprecedented sophistication, which has the potential to greatly improve the fidelity of predicted climatic effects and reconcile model-simulated and observational records of climate change after volcanic eruptions. Our simulations in UKESM1 will cover the period 1250 CE to present, which will enable us to characterize and evaluate annual to centennial-scale effects on global and hemispheric surface temperatures, climate variability and impacts on surface ocean temperatures for eruptions of different frequencies and intensities. Vol-Clim is an ambitious project that aligns closely with international initiatives and NERC's main goals. Quantifying the contribution of volcanic eruptions to climate variability over the past millennium is key to understanding present day and future decadal-scale climate variability; this is in line with NERC's main goal 'to understand and predict how the planet works'. Vol-Clim will also help prepare society for the effects of future eruptions. Vol-Clim is also strongly aligned to international activities such as the new Past Global Changes (PAGES) working group "Volcanic Impacts on Climate and Society (VICS)" and the PMIP (Paleoclimate Modelling Intercomparison Project) and CMIP (Coupled Model Inter-comparison Project) communities. We will generate a volcanic aerosol forcing time-series (1250 CE to present) for use in those models that do not account for the chemical and physical aerosol processes in the stratosphere. These deliverables are relevant for CMIP6-endorsed activities such as VolMIP (Model Inter-comparison Project on the Climatic Response to Volcanic Forcing) and RFMIP (Radiative Forcing Model Inter-comparison Project), and also the IPCC.