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

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.

Vol-Clim quantifies the effects of large-magnitude volcanic eruptions on surface temperature changes, stratospheric aerosol particle properties, and ocean heat content since 1250 CE. Vol-Clim aims to resolve a long-standing mismatch between the magnitude of observed and model-simulated surface temperature changes following large-magnitude volcanic eruptions. This will be achieved by deriving physically more realistic estimates of the forcings, feedbacks and responses of the climate system to large volcanic eruptions and by comparing the resulting surface temperature changes to up-to-date and newly compiled proxy records (e.g. tree-ring data).

Our ability to isolate and quantify the contribution of volcanic eruptions to past and future climate change including sea-level rise on decadal- and centennial scales is severely hampered unless we can resolve the persistent inability of climate models to simulate the surface temperature response following volcanic eruptions in agreement with proxy records. Improving the skill to simulate the climatic response to volcanic eruptions in climate models will improve climate model fidelity, which in turn will allow us to better understand other non-volcanic forcings of climate change, climate sensitivity, and decadal-scale climate variability.

Vol-Clim is 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.

Vol-Clim delivers the first volcanic aerosol forcing time-series calculated in a full model of the aerosol system for the period 1250 CE to present, which can be used in models that do not account for the interactions of dynamical, chemical and physical aerosol processes in the stratosphere. International modelling groups running simulations for CMIP6, VolMIP, PMIP4 and RFMIP will be able to compare the new volcanic forcing fields with existing ones and analyze the model simulations further. These groups will also benefit from and the revised 1850 control simulation, which will account for long-term volcanic cooling trends in ocean. The latter will enable more-accurate predictions in sea-level rise.

Vol-Clim exploits and further develops the UK's modelling tools for chemistry-climate studies and the wider research community will be beneficiaries. In particular, the UK Earth System science community using the UK Earth System Model is growing and our work has implications for other applications of the model such as geo-engineering of climate or predictability of future climate variability and sea-level changes.

The general public has a keen interest in volcanic eruptions and also in climate change. Vol-Clim will engage with the public by providing and communicating latest scientific evidence related to the climatic and societal effects of volcanic eruptions via the project's website, news releases and social media.

Vol-Clim will inform government trading fund agencies such as the UK Met Office and policy makers who will benefit from up-to-date information on the climatic effects of volcanic eruptions of different magnitudes and frequencies. This is highly relevant for preparing for and mitigating the societal effects of future eruptions. Schmidt (PI) is already heavily involved in advising UK government on the risks and effects of volcanic eruptions.