Holistic assessment of the impacts of an enhanced stratospheric aerosol layerG.W.Mann@leeds.ac.uk
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).
Figure 1: 15-year satellite record of tropical stratospheric aerosol combining vertical profiles measured by SAGE-II and CALIOP instruments (from Vernier et al., 2011).
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 nitrogen (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.
Figure 2: Schematic illustrating the different ways volcanoes influence climate.
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:
- Assessing volcanic aerosol-chemistry interactions and impacts on stratospheric NOy species and subsequent effects on ozone in mid- and high-latitudes.
- Exploring how the stratospheric warming within major tropical volcanic plumes influences ozone changes via composition-dynamics interactions.
- Investigating the impacts of major eruptions on the terrestrial carbon cycle through changes in surface diffuse radiation, temperature and precipitation.
- 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).
Potential for high impact outcome
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. Through ongoing collaboration with scientists at the UK Met Office and internationally through 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 a NASA-initiated activity to co-ordinate international modelling and observational capabilities to prepare for monitoring the effects from a future major volcanic eruption. The research in the project will likely lead to several papers, with the potential for submission to high impact journals.
The student will be supervised by Dr Graham Mann, Prof. Martyn Chipperfield and Dr Alex Rap within the ICAS stratospheric aerosol, atmospheric chemistry and physical climate research groups. This project provides a high level of specialist scientific training in: (i) State-of-the-science application and analysis of global atmospheric composition-climate models; (ii) techniques to analyse and compare with satellite measurements of atmospheric composition; (iii) numerical modeling and use of cutting-edge supercomputers. The student will have extended visits to Exeter to work with the CASE partner at the Met Office under the supervision of Prof. Jim Haywood and Dr. Andy Jones. The student will have access to a broad spectrum of training workshops in Leeds including extensive training workshops in numerical modelling and generic skills (http://www.emeskillstraining.leeds.ac.uk/).
The project will be part of the extensive collaborations in atmospheric science between Leeds and the UK Met Office. The research for this project will improve the fidelity of simulated volcanic influences on stratospheric composition and climate within UKESM. The experiments and research findings are also part of a wider activity to prepare for simulating the potential impacts from a future major volcanic event.
- Blumenthaler and Ambach (1994) Changes in solar radiation fluxes after the Pinatubo eruption, Tellus, 46B, 76-78.
- Dhomse et al. (2014) Aerosol microphysics simulations of the Mt. Pinatubo eruption with the UM-UKCA composition-climate model, Atmos. Chem. Phys., 14, 11221–11246.
- Fahey et al. (1993) In-situ measurements constraining the role of sulphate aerosols in mid-latitude ozone depletion, Nature, 363, 509-514.
- Gu et al., (2003) Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis, Science 299, 2035-2038.
- Mann et al. (2010) Description and evaluation of GLOMAP-mode: a modal global
- aerosol microphysics model for the UKCA composition-climate model, Geosci. Mod. Dev., 3, 519–551, 2010.
- Rap et al. (2015) Fires increase Amazon forest productivity through increases in diffuse radiation, Geophys. Res. Lett., 42, 4654–4662.
- Timmreck (2012) Modeling the climatic effects of large explosive volcanic eruptions, WIREs Clim Change 2012, 3:545–564.
- Vernier et al. (2011) Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade, Geophys. Res. Lett., 38, L12807.
Related undergraduate subjects:
- Applied mathematics
- Atmospheric science
- Chemical engineering
- Earth science
- Earth system science
- Environmental science
- Geological science
- Natural sciences
- Physical science
- Remote sensing