May 2019 - by the SSiRC Science Steering Group
SSiRC addresses critical elements of the World Climate Research Program (WCRP) mission by focusing on the most variable component of climate forcing: stratospheric aerosol. At most times, aerosol above the tropopause is an optically thin veil with little impact on climate but it can intensify dramatically due to massive, aperiodic volcanic eruptions. Following such events, the stratospheric aerosol influences the Earth's climate by cooling the planet as a whole and creating potentially devastating changes to regional weather patterns that can persist for a number of years. In the modern era, large volcanic events can temporarily slow the pace of human-derived global warming. Since 2012, SSiRC has worked to build and support a community focused on the scientific understanding of stratospheric sulfur and aerosol, and their role in climate. The SSiRC community has been built through two general workshops on stratospheric aerosol (Atlanta, 2013; Potsdam, 2016), one workshop focused on measurement of stratospheric aerosol (Boulder, 2017), and a Chapman Conference on stratospheric aerosol in the post Pinatubo era (Tenerife, 2018). Furthermore, a subgroup that focuses on a rapid response to an ongoing volcanic eruption to determine the potential climate impact (VolRes) has been formed. This groups is supported by the development of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP , Zanchettin et al., 2016); and a new Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP, Timmreck et al., 2018). The SSiRC community also contributed to the development of long-term stratospheric aerosol forcing datasets for climate modeling (GloSSAC , Thomason et al., 2018). A key contribution to the stratospheric aerosol research was the publication of the first extensive review of the status of stratospheric aerosol (Kremser et al., 2016) since the SPARC Assessment of Stratospheric Aerosol Properties (ASAP ) was published in 2006.
Through these contributions, the SSiRC Scientific Steering Group (SSG) believes that SSiRC has proven to be an active and successful SPARC activity. However, it is clear that substantial work remains before we adequately understand stratospheric aerosol, their precursors, and feedback between stratospheric sulfur and climate resulting from the warming induced by increasing atmospheric carbon dioxide. For SSiRC to remain productive, it must be agile with respect to our changing understanding and to scientific priorities regarding our knowledge gaps, and committed to acting as a bridge between the measurement, process modelling, and the wider global climate modelling communities. This document summarizes where SSiRC is and where it is aiming to head as an activity
While much is understood about the impact of stratospheric aerosol on climate, there are a number of open questions relevant to SSiRC, SPARC and the WCRP. For instance, a key knowledge gap is understanding the impact on climate of volcanic eruptions on scales from the modest eruptions observed since 2000 (e.g., Nabro in 2011) to eruptions with large stratospheric impact such as the 1991 eruption of Mt Pinatubo and the 1815 eruption of Tambora. Another poorly understood area is the feedback between the global sulfur cycle and climate. For instance, while we generally understand the sources of biogenic sulfur for the stratosphere, significant gaps remain with virtually no understanding of how these contributions may change in a changing climate. Similarly, the contribution of human-derived SO2 to the stratospheric sulfur budget remains poorly quantified. How changes to the stratospheric sulfur burden, due to the changing climate and its impact on sulfur sources, feed back into circulation changes (particularly the Brewer-Dobson Circulation) is almost completely unknown. Finally, while it has long been known that sulfuric droplets dominate stratospheric aerosol composition, it is becoming clearer that other components such as nitrate aerosol, meteoritic-sulfuric aerosol, organics, soot, and, episodically, volcanic ash are likely relevant in the lower stratosphere and upper troposphere including. How this diverse composition impacts climate and how it can be characterized in climate models is not well understood.
SSiRC addresses these knowledge gaps through directly stimulated activities and associations with related but independently managed projects. The following projects are presently active areas of work or subject of proposals currently underway, and have been at least partly stimulated by SSiRC.
The SSiRC SSG is committed to supporting these activities and others that advance an improved understanding of the role of stratospheric sulfur and aerosol. We will continue to consider the broad overall goal of understanding the sulfur cycle and its impact on stratospheric aerosol and to be self-critical as areas appear where knowledge is particularly deficient. Along these lines we encourage scientists to convey ideas for SSiRC activities to the SSiRC SSG. Promoting these activities through general workshops, topic-specific workshops, and other organizational support are all possible. Extending these ideas leads to the following areas which have been suggested as in need of future investigation.
Strateole2, BATAL, StratoClim, and Asian Monsoon activities, are all focused on the UTLS because of the importance of this source region for the majority of material which maintains the non-volcanic stratospheric aerosol. The only material not controlled by the UTLS is meteoritic and material from explosive volcanic eruptions. SSiRC is committed to encourage these and other activities related to the role of the UTLS as a source region for stratospheric aerosol precursors including processes related to the Asian Monsoon, pyroCbs, and other regional processes which provide avenues for aerosol and aerosol precursor gases such as OCS, DMS, and organics to enter the stratosphere.
SSiRC needs to continue to reach out to the broader scientific community. This includes improving connectivity with other SPARC activities such as the Atmospheric Composition and the Asian Monsoon (ACAM) activity and the emerging Observed Composition Trends and Variability in the Upper Troposphere and Lower Stratosphere (OCTAV-UTLS) activity. In addition, we seek improving connectivity to the ice core community perhaps through the International Partnership in Ice Core Sciences (IPICS). Also, improved communications with organizations like the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) and PAGES/VICS - Past Global Changes/Volcanic Impacts on Climate and Society could be beneficial, particularly as they relate to atmospheric emissions of SO2 and other sulfur species by volcanoes.
We would also like to improve the diversity of the SSiRC Scientific Steering group and the SSiRC membership as reflected in our email distribution list and in attendance at SSiRC and SSiRC-sponsored workshops. In particular, we would like to increase the presence of scientists from Asia, developing countries, women, and early career scientists. At the same time, we recognize that expanding our diversity may place greater demands on financial support in a period when support from SPARC is increasingly limited. Part of outreach will require us to be more creative in how we acquire support in the future. We plan major updates to our web sites at SPARC and at SSiRC to reflect our commitment to diversity as well as our updated science focus and workshop planning.
Meetings are a key part of making SSiRC a unique community. Workshops (both general and topical) allow unique gatherings where the state of the art of SSiRC-related science can be assessed. At the moment, another workshop on stratospheric aerosol in 2020 in Leeds, UK is envisioned. Topical workshops, such as the stratospheric aerosol measurement workshop in Boulder in 2018, are encouraged. Some possible topics include: VolRes, ISA-MIP, Historical data, and developing a new index to define the climate impact of volcanic eruptions. EGU and AGU sessions for topical SSiRC related activities are also encouraged.
Kremser, S., et al. (2016), Stratospheric aerosol-Observations, processes, and impact on climate, Rev. Geophys., 54, 278-335, https://doi.org/10.1002/2015RG000511 .
Timmreck, C. et al (2018), The Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP): motivation and experimental design, Geosci. Model Dev., 11, 2581-2608, https://doi.org/10.5194/gmd-11-2581-2018 .
Thomason, L et al. (2018), A global space-based stratospheric aerosol climatology: 1979-2016, Earth Syst. Sci. Data, 10, 469-492, https://doi.org/10.5194/essd-10-469-2018.
Zanchettin, D. et al. (2016) The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP): experimental design and forcing input data for CMIP6, Geosci. Model Dev., 9, 2701-2719, https://doi.org/10.5194/gmd-9-2701-2016.