Holistic assessment of the impacts of a volcanically-enhanced stratospheric aerosol layer

Background

The historical climate record includes periods of strong surface cooling after major eruptions (Fig 1a), a tropical reservoir of volcanic aerosol slowly dispersing to mid-latitudes over several years (Fig 1b). The forcings after 1991 Pinatubo, 1982 El Chichon and 1963 Agung are known to dominate decadal forcing trends, yet their uncertainty within IPCC historical climate integrations is not currently represented.

While increased solar scattering from the volcanic aerosol cloud is the main way major eruptions force climate, the multi-year perturbation to the stratosphere has a range of other important impacts (see Figure 2):

  • The volcanic aerosol also absorbs outgoing long-wave 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 particles in the volcanic aerosol cloud 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 (e.g. Hofmann and Solomon, 1989; Fahey et al., 1993).
  • LW aerosol absorption after major eruptions warms the tropical stratosphere, changing the stratospheric circulation. The heating increases tropical upwelling, increasing stratospheric water vapour but reduces tropical stratosphere ozone, the effect also causing greater export of tropical ozone to mid-latitudes.
  • An increase in diffuse radiation reaching the surface also occurs after major eruptions, (e.g. as observed after Pinatubo: Blumenthaler and Ambach, 1994). The greater diffuse radiation and reduced surface temperature combine for a complex volcanic influence on the terrestrial carbon cycle (e.g. Gu et al. 2003).

Project Overview

This project will investigate the impacts of tropical major 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.

Potential research strands the PhD student could choose to explore in the 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, or considering how Pinatubo’s influence differed from 1963 Agung.

The GLOMAP aerosol scheme (Mann et al., 2010) is a core component of the joint NERC-Met Office Earth System Model UKESM, and the Leeds team have further extended UKESM capability for   volcanic impacts, the evolution of dispersion of major eruption clouds now simulated interactively (see Dhomse et al., 2020).

The project will analyse results from UKESM simulations with offline analysis with the SOCRATES radiative transfer and JULES land surface models, to consider how one or more of these individual volcanic aerosol indirect effects add to the main “direct volcanic aerosol radiative effect” from scattering of incoming solar radiation.

References

Fit to NERC science

Fully understanding the impacts of volcanoes on climate remains an important research area, and accurately charactering their effects is key to attributing anthropogenic influences on historical climate change. 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.

Through existing collaborations with other scientists in the UK and internationally, including via the SPARC initiative on stratospheric sulphur, this project can answer important unresolved questions about how volcanoes influence stratospheric composition and climate.

The Leeds team are involved with an international activity “VolRes”, aligned to a NASA-led activity (e.g. Carn et al., 2021) to co-ordinate modelling and observational capabilities to prepare a rapid response plan for monitoring the progression of the aerosol cloud from a future major volcanic eruption. The research in the studentship has the potential for several papers, with the intention for the student to submit PhD by publication.