A 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, as the enhanced stratospheric aerosol scatters solar radiation back to space, cools the Earth’s surface. 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).
Although increased solar scattering from the optically thicker stratospheric aerosol layer is the principal mechanism for volcanic radiative forcing, the large multi-year perturbation can also affect climate in many other ways (see Figure 2):
- The volcanically enhanced stratospheric 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 causing 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.
Figure 2: Schematic illustrating the diverse nature of volcanic impacts on climate
This project will investigate the impacts of tropical eruptions on stratospheric composition and the magnitude of the associated radiative effects Experiments will be carried out to better understand each of the different volcanic aerosol-chemistry and aerosol-radiation interactions described above, to provide an integrated assessment of the effects of volcanic eruptions. Output from the C-IFS experiments will be combined with offline simulations of the SOCRATES radiative transfer model and JULES land surface model to quantify effects on vegetation (Rap et al., 2015).
Expected directions that the PhD studentship could include:
- Simulating the impacts from heterogeneous chemistry and composition-dynamics interactions through the 1991-93 Pinatubo-perturbed period.
- Comparing simulated stratospheric nitrogen species and aerosol properties against aircraft observations to elucidate the drivers of mid-latitude O3 changes.
- Exploring effects on PSCs from the strengthened polar vortex after Pinatubo.
- Investigating the impacts of Pinatubo on the carbon cycle through changes in surface diffuse radiation, temperature and precipitation.
- Assessing how Pinatubo’s effects would have differed if it had erupted into a low chlorine stratosphere, such as at the time of the 1963 Agung eruption
At Leeds we have implemented the GLOMAP aerosol scheme (Mann et al., 2010) into the Copernicus Atmospheric Monitoring System (CAMS) modeling system “Composition-IFS” (C-IFS, Flemming et al., 2015), alongside complementary chemistry schemes from groups around Europe. The GLOMAP module in C-IFS is identical to that in the UK’s new Earth System Model.
In this project, the student will work with leading scientists at Leeds, the Netherlands national meteorological service (KNMI) and the European Centre for Medium-range Weather Forecasts (ECMWF) to investigate the effects of volcanic eruptions on the composition of the stratosphere.
The studentship will also align closely with the Copernicus Atmospheric Monitoring System (CAMS), a major European consortium combining global composition modeling, satellite measurements and in-situ surface observational sites to monitor the Earth’s atmosphere, including preparing for a future volcanic event.
Potential for high impact outcome
Understanding the impacts of volcanoes on climate is an important research area, and accurately charactering their effects is key to attributing anthropogenic influences on historical climate change. Through collaboration with European partners, we are in a unique position at Leeds to answer important unresolved questions about how enhanced stratospheric aerosol influences stratospheric composition and climate. The research topic has immediate policy-relevant findings, and we therefore anticipate the project generating several papers with at least one being suitable for submission to a high impact journal.
The student will be supervised by Dr. Graham Mann, Prof. Martyn Chipperfield and Dr. Alex Rap within the ICAS stratospheric aerosol, 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 Utrecht to work with the CASE partner at KNMI under the supervision of Dr. Vincent Huijnen. 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 has additional funding from the Copernicus Atmospheric Monitoring Service (CAMS) to enhance the NERC student stipend. The project builds on existing collaboration between Leeds, KNMI and ECMWF to develop and couple the aerosol and chemistry modules for Composition-IFS, a key part of the new European Copernicus Atmosphere service. The research for this project will improve the fidelity of simulated volcanic influences on stratospheric composition with C-IFS. The project will therefore impact on the planned ECMWF development of the system to prepare for monitoring the impacts from a potential future major volcanic event.
Aquila et al. (2013) The Response of Ozone and Nitrogen Dioxide to the Eruption of Mt. Pinatubo at Southern and Northern Midlatitudes, J. Atmos. Sci., 70, 894-900, 2013.
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.
Flemming et al. (2015), Tropospheric chemistry in the Integrated Forecasting System of ECMWF, Geosci. Model Dev., 8, 975-1003, 2015
Gu et al., (2003) Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis, Science 299, 2035-2038.
Huijnen et al. (2016) C-IFS-CB05-BASCOE: stratospheric chemistry in the Integrated Forecasting System of ECMWF, Geosci. Model Dev., 9, 3071-3091.
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:
- Physical science