Exeter-NCAR collaborative Development (EXTEND)

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Maintaining global mean temperatures within 1.5C or 2C of pre-industrial levels is a cornerstone of humanity's battle against climate change. Climate model simulations suggest that even limiting change to 1.5C above pre-industrial levels will lead to increased detrimental climate extremes such as heat-waves and extreme precipitation events at a regional, and global scale and these extremes are further exacerbated under 2C temperature targets. The realization by the scientific community of the difficulty of limiting global mean temperatures to within these 1.5 or 2.0C targets has led to increased calls for climate intervention via so called "solar radiation management (SRM)" techniques which aim to increase planetary albedo and induce a cooling that acts to partially offset global warming. SRM is not a new idea, but such an approach is extremely controversial, with many scientific, ecological, moral and philosophical arguments against such proposals. In March 2021, The US National Academy of Sciences recommended in their integrated agenda for SRM research robust, unbiased, multi-model assessments of SAI with a funding stream of $200m, highlighting that "The program should, from the outset, prioritize development of international coordination and co-development of research with other countries". Haywood is an ideal partner for this work. Haywood has authored many cautionary studies including detrimental teleconnection impacts on Amazonian rainfall, termination effect, impacts on Sahelian drought and hurricane frequency of hemispherically asymmetric SRM, and stressed that any practical SRM deployment should only be used to temporally ameliorate the worst impacts of climate change whilst transitioning to a net carbon zero economy.Recent assessments of SRM support a cautionary and objective analysis using models that can represent the detailed mechanisms of SRM. For SAI, they must represent the dynamics and chemistry of the stratosphere such as the Brewer-Dobson circulation, the Quasi-biennial oscillation (QBO), homogeneous and heterogeneous chemistry relevant to the ozone layer, and the detailed sulphate microphysical evolution such as gas phase oxidation, nucleation, condensation, coagulation, evaporation and gravitational settling. These dynamical, chemical and microphysical processes determine the spatial distribution of stratospheric sulphate aerosol and the associated radiative forcing and are far more complex than those associated with well-mixed carbon dioxide where global concentrations and spectroscopic properties are well known. Only a handful of GCMs within the most advanced GeoMIP G6sulph experiment adequately; the two best models are arguably the UK Met Office Hadley Centre UKESM1 and the USA's National Center for Atmospheric Research (NCAR) CESM2-WACCM This proposal uses the UK's climate model (UKESM1) to EXTEND the large-ensemble approach pioneered by NCAR with CESM2-WACCM. Direct collaboration between the University of Exeter and NCAR promises a dual-model assessment of the various pros, cons, perils and pitfalls of SAI climate intervention utilising two of the most advanced GCMs currently available. The collaboration will not only position the UK scientific community at the forefront of SAI climate intervention, but provide a balanced assessment of the potential of SAI as a climate intervention strategy to policy-makers worldwide.The proposal promises sustainability as matched funding has been promised should EXTEND be funded and there are opportunities to tap into the USA's proposed funding stream of $200m that has been recommended on 25 March 2021, particularly as the USA recognises "The program should, from the outset, prioritize development of international coordination and co-development of research with other countries". Thus sustainability is assured.