Holistic assessment of the impacts of an enhanced stratospheric aerosol layer

Background

Volcanic eruptions can inject vast quantities of sulphur dioxide and ash into the upper atmosphere, which cools the Earth’s surface by scattering incoming solar radiation back to space. Major tropical eruptions such as Mt Pinatubo in 1991 are particularly effective at forcing climate in this way, with the timescale for decay back to quiescent conditions taking several years (see Figure 1).

While increased solar scattering from the optically thick stratospheric aerosol layer is the principal mechanism for volcanic radiative forcing, the large multi-year perturbation also affects climate in many other ways (see Figure 2):

• The volcanically enhanced aerosol can also absorb terrestrial radiation, offsetting some of the cooling from the additional solar scattering. The balance of these long-wave and short-wave radiative effects is closely associated with how large the aerosol particles grow (i.e. their size distribution).

• Elevated aerosol surface area accelerates heterogeneous chemistry leading to stratospheric ozone loss via changes in NOy partitioning and chlorine activation.

• Major tropical eruptions warm the stratosphere, causing circulation changes. The tropical stratosphere heating increases the equator-pole temperature gradient and cause a stronger and colder polar vortex. This leads to more widespread polar stratospheric clouds (PSCs), increasing polar halogen-induced ozone loss.

• An increase in diffuse radiation reaching the surface can occur after an eruption. This effect was observed after Pinatubo (e.g. Blumenthaler and Ambach, 1994), and likely contributed to the observed pause in CO2 growth rate at that time (Gu et al. 2003), opposing the effects from surface cooling.

Quantifying these volcano-climate interactions, and reducing their uncertainty, also has direct implications for the efficacy and potential environmental risks from hypothesized solar radiation management via stratospheric particle injection.

Project overview

This project will investigate the impacts of tropical eruptions on stratospheric composition and the magnitude of the associated radiative effects. The research will involve global composition-climate model experiments to quantify the different volcanic aerosol-chemistry and aerosol-radiation interactions described above, and thereby provide an integrated assessment of the effects of volcanic eruptions.

The potential research strands within the PhD studentship are:
1. Volcanic aerosol-chemistry interactions assessing impacts on stratospheric NOy species and subsequent effects on ozone in mid- and high-latitudes.
2. Exploring how the stratospheric warming within major tropical volcanic plumes influences ozone changes via composition-dynamics interactions.
3. Investigating the impacts of major eruptions on the terrestrial carbon cycle through changes in surface diffuse radiation, temperature and precipitation.
4. Predicting the effects from a hypothetical future major eruption in a low chlorine stratosphere, and contrasting with the 1963 Agung eruption

The GLOMAP aerosol scheme (Mann et al., 2010) is now a core component of the UK’s community composition-climate model UM-UKCA and the joint NERC-Met Office Earth System Model UKESM. These models have world-leading capability in simulating volcanic impacts on climate, with an “interactive stratospheric aerosol” approach that resolves particle size changes and composition-dynamics interactions.

The UKESM model includes UKCA in the atmosphere model but also marine and terrestrial ecosystem models to resolve key Earth System interactions within coupled atmosphere-ocean experiments.

Combining UKESM simulations with offline analysis based on the SOCRATES radiative transfer and JULES land surface models to better understand the drivers and sensitivity of diffuse radiation effects on vegetation (Rap et al., 2015).