CMIP forcing - volcanic

Summary

Stratospheric aerosols, formed primarily from sulphur dioxide (SO₂) released during volcanic eruptions, influence the amount of energy entering and leaving the atmosphere
- and therefore Earth's climate. Climate models rely on accurate representations of such climate forcings to reliably simulate past climate variations and predict future climate behaviour.

This project aims to provide stratospheric aerosol forcing for the next generation of climate model simulations (via CMIP) and to quantify uncertainties in this forcing. The project focuses on two challenges. Firstly it will provide a dataset for volcanic sulphur emissions suited to state-of-the-art models need to interactively simulate stratospheric aerosols from precursor emissions. Secondly, the project will attempt to fill the gaps in small but frequent eruptions that were mostly unaccounted for in predecessor datasets covering prior to 1979 - the pre-satellite era.

Volcanoes - a natural driver of climate change

One of the most important natural driver of climate change is stratospheric aerosol, primarily formed from sulphur gases injected into the stratosphere by powerful volcanic eruptions. These aerosol particles can reside a couple years in the stratosphere and spread globally. Their main effect is to reflect a small fraction of incoming Sun radiation, thus resulting in a cooling of Earth surface.

To provide volcanic forcing inputs to climate modelling centers for 1850-present day, a key challenge is that there is no direct observation of stratospheric aerosol prior the onset of the satellite era, around 1979. Before that time period, we typically rely on volcanic aerosol forcing models which take as input the amount of sulphur gases erupted by volcanoes reconstructed from ice core measurement. Ice core are our main archive to reconstruct past atmospheric composition, and some of the stratospheric volcanic aerosols are deposited at the poles, in the ice, recording past volcanic events.

Scientific challenge of our project are thus:

1. Determining the exact characteristic of a volcanic injection (e.g. latitude and altitude) based mostly on information on sulphate deposited in ice-core.
2. Filling gaps in small-magnitude but frequent volcanic eruptions. These eruptions are largely undetected in mainstream ice-core dataset, but they contribute up to 50% of long-term volcanic injections into the stratosphere.
3. Improving volcanic aerosol models used to translate volcanic injections into stratospheric aerosol forcing
4. Combining our model-derived, 1850-1978 dataset with direct satellite observations from 1979 onwards.

To address this challenge, the project team is highly interdisciplinary, including ice-core experts, satellite experts, aerosol modellers and climate modellers, and volcanologists.

Project aims and objectives

The overarching project objective is to provide stratospheric aerosol optical properties and volcanic stratospheric sulphur emission datasets for phase 7 of the Coupled Model Intercomparison Project (CMIP7). Specifically, we will:

• Provide best estimate for both optical properties and emissions for the Fast Track Phase of CMIP7
• Develop uncertainty products for both datasets
• Document both datasets
• Quantify how uncertainties in stratospheric aerosol forcing translates in terms of uncertainties in historical (1850-present day) climate model simulations

Project plans

The first task of the project is to deliver the 1850-present day volcanic forcing dataset. To achieve this, key steps are to:

1. Collate state-of-the-art ice-core and satellite records of volcanic sulfur injections and stratospheric aerosols.
2. Combine ice-core sulfur injection records with volcanological records to best constrain volcanic sulfur injection parameters
3. Fix biases in small-magnitude eruptions under-recorded in ice-core records
4. Develop a volcanic aerosol model capturing well how volcanic sulfur injection translate in terms of stratospheric aerosol perturbations.
5. Produce a consistent dataset combining the pre-satellite and satellite era information.

Once this first objective is achieved, two critical objectives are to extensively document the dataset to facilitate its use by scientists, and to facilitate operationalisation of volcanic forcing production, i.e. regular updates and improvements to this dataset in the future.

Last, reconstructions of past volcanic forcing are characterized by large uncertainties. Accordingly, our last two objectives are to:

1. Create uncertainty products associated with our main dataset
2. Quantify how volcanic uncertainty propagate in terms of simulated historical climate, and implications for evaluating climate models as well as attributing past climate change.