Quantifying the Environmental Effects of Stratospheric Geo-engineering

Graham Mann (gmann(at)env.leeds.ac.uk), Ken Carslaw (k.s.carslaw(at)leeds.ac.uk) CASE partner supervisor: Nicolas Bellouin (Met Office).

Successive IPCC climate assessment reports (e.g. Forster et al, 2007) have become more certain that human-induced increases in greenhouse gases are causing substantial warming of the earth.  It is increasingly evident that the world’s emissions of greenhouse gases are not being reduced quickly enough to avoid dangerous climate change.  Much attention, including from the Nobel Laureate Paul Crutzen (Crutzen, 2006) has focussed on whether the cooling effects of aerosol particles, in scattering and absorbing solar radiation, could be exploited to offset the warming from increased greenhouse gases. However, large-scale injection of sulphur would cause numerous side effects to the earth’s environment.  Robock (2008) lists 20 reasons why geoengineering the stratosphere in this way would be a bad idea.

Nevertheless, Rasch et al (2008) reviewed findings from climate model studies at that time, and concluded that, were a delivery mechanism found that could produce the required enhancement of the stratospheric sulphate aerosol layer, temperature increases from increased greenhouse gases could, on the global average, be counteracted. A key caveat to the findings from these initial model experiments, was that, “Representations for the formation, evolution and removal of aerosol and distribution of particle size are still very crude”. 

Three key areas that research on geoengineering needs to provide information on are:

• How would the enhanced aerosol change the scattering/absorption of solar & terrestrial radiation?
• How much additional depletion of the stratospheric ozone layer would occur?
• What regional climate effects would occur, e.g. impacts on Asian/African monsoon systems?

The UK Chemistry and Aerosol project (http://www.ukca.ac.uk) is a collaboration since 2005 between the University of Leeds, University of Cambridge and the Met Office Hadley Centre  has developed a new aerosol-chemistry-climate model, based around a whole-atmosphere version of the Hadley Centre climate model HadGEM3.

A key capability of the new HadGEM3-UKCA model is that it includes an “aerosol microphysics” scheme which simulates the evolution of the particle size distribution, and is coupled to stratospheric chemistry scheme to find the influence on stratospheric ozone layer.

More recent studies (e.g. Heckendorn et al, 2009; Niemeyer et sl, 2010) using size-resolved aerosol microphysics models shows that the growth in particle size following sustained injection of sulphur dioxide, and the subsequent sedimentation of the larger particles, causes the lifetime of the aerosol to decrease as the emission rate increases, making the technique substantially less effective at cooling the earth than suggested by the previous studies.

In this project, the PhD studentship will work with leading scientists at Leeds and the UK Met Office to apply HadGEM3-UKCA to investigate the effects of stratospheric geoengineering. The project will also contribute to the Integrated Assessment of Geoengineering Proposals (http://www.iagp.ac.uk), alongside other complimentary research activities at School of Earth & Environment.

The studentship will include:

• HadGEM3-UKCA experiments simulating the effects on stratospheric aerosol, ozone and surface temperature, from the 1991 Pinatubo eruption will be extended to simulate larger one-off eruptions and then to investigate effects from sustained injections of sulphur of differing magnitudes.  The evolution of the stratospheric aerosol properties will be analysed, and perturbations to radiative fluxes, and on stratospheric ozone quantified.
• Fully coupled ocean-atmosphere HadGEM3-UKCA experiments, similar to those in the Geoengineering Model Intercomparison (GeoMIP) initiative (e.g. Kravitz et al, 2011) and/or further simulations to investigate potential impacts on regionally sensitive phenomena such the Asian and African monsoons.

The project is expected to lead to a high profile publication of how stratospheric geoengineering would perturb stratospheric ozone and aerosol properties and give improved estimates of the resulting regional climate effects.