SSiRC Implementation Plan

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

Knowledge Gaps - Stratospheric Sulfur and Aerosol

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.

SSiRC Projects

• Estimating the climate impact of historical eruptions remains an on-going challenge. Work here is currently focused on investigating the radiative effects from historical major eruptions, particularly those in the post-1950s era, to address the sources of uncertainties in the capacity of current models to simulate the impacts of volcanic eruptions of very different scales. Outgrowths of this work include the Interactive Stratospheric Aerosol Model Intercomparison Project (ISA-MIP ) led by Valentina Aquila and Graham Mann. A closely related activity is focused on the recovery of historical data led by Juan Carlos Antuña. These data would contribute to new or updated volcanic forcing datasets and can provide observational constraints for interactive stratospheric aerosol models. The work is primarily focused on early lidar and searchlight data, but is also highlighting some of the early in situ data which are archived at NDACC. We encourage efforts to archive any recovered data at NDAAC or a similar facility, preferably with a DOI.
• VolRes , led by Jean-Paul Vernier and Claudia Timmreck, is focused on creating a document to describe activities recommended to follow a significant volcanic eruption. The idea is to have an easily accessible, and annually updated, document available for reference by observationalists and modelers which specifies a measurement and modeling strategy to promote the capture of the most important information in the short term following a major volcanic eruption. While this document could be generally described in a refereed publication, the ultimate goal is a living document since volcanic eruptions are not predictable and instrumentation, observational capabilities, and models are always in flux.
• Measured Sulfur Burden In the Stratosphere : Summarizing and reviewing the measured stratospheric sulfur burden is the goal of this effort which is led by Terry Deshler. The goals and commitments are to provide this review of both gas and particle phase sulfur.
• Volcanic Impact on Climate Index: SSiRC recognizes that VEI cannot adequately relate the impact of volcanic eruptions on climate. As a result, a new activity which is led by Landon Rieger and Matthew Toohey, was initiated to develop a simple index to convey the likely significance to climate due to the injection of sulfur into the stratosphere by a volcanic eruption.
• Database of data bases: At this point we are abandoning the idea of a wiki page hosting a list of sulfur and aerosol measurements. Partly this is due to the lukewarm response to the idea by the community and due to new EU regulations on data protection that would make such a wiki difficult to maintain. In lieu of the wiki, we will add a page to the SSiRC web site to promote key data sets available at data centers around the world.

Associated projects

• The Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP ) is a climate model intercomparison project that seeks to understand the wide discrepancy in various climate model responses to large eruptions that were observed in the CMIP5 comparisons. VolMIP is an endorsed MIP as part of CMIP6. VolMIP?s key goal is to compare the response of the coupled ocean-atmosphere system in coupled climate models when the same strong volcanic forcing is applied.
• The Geoengineering Model Intercomparison Project (GeoMIP ) addresses the effectiveness and perhaps unintended consequences of attempting to modify climate through a variety of radiation management schemes. Since a prominent geoengineering proposal is to modify sulfuric aerosol loading in the stratosphere, overlap between GeoMIP and SSiRC science interests is clear. GeoMIP is an endorsed MIP as part of CMIP6.
• The Balloon Measurement Campaigns of the Asian Tropopause Aerosol Layer (BATAL ) is a NASA-sponsored field campaign that aims to characterize the optical, physical, and chemical properties of the ATAL; assess its impacts on water vapor and ozone; and understand the role of convection in its formation. Insight into ATAL may lead to an improved understanding of the processes of aerosol and aerosol precursor transport into the stratosphere.
• Stratéole 2 is an initiative to study the Tropical Tropopause Layer (TTL) and lower stratosphere using long-duration balloon platforms drifting near and within the TTL. The three ballooning campaigns planned between 2018 and 2024 present unique opportunities to make measurements of the physics and chemistry of the TTL from nearly Lagrangian platforms. In situ measurements of temperature, water vapor and cirrus profiles across the TTL and aerosol size distributions, ozone, and three dimensional winds in the lower stratosphere, just above the TTL, will advance scientific inquiries into the TTL concerning the role of gravity and equatorial wave dynamics, clouds and aerosols, stratospheric hydration, and the validation of satellite retrievals and model simulations.
• Stratospheric and upper tropospheric processes for better climate predictions (StratoClim ) seeks to improve the understanding of the microphysical, chemical and dynamical processes that determine the composition of the UTLS, such as the formation, loss and redistribution of aerosol, ozone and water vapor, and how these processes will be affected by climate change. An atmospheric observatory has been established on Palau and two successful aircraft campaigns have been carried out within StratoClim; one from Kalamata, Greece and one from Katmandu, Nepal.
• The construction of long-term stratospheric aerosol forcing datasets for climate modeling has long been associated with SPARC stratospheric aerosol projects beginning with ASAP in 2006 and continuing with SSiRC. It consists of two components, a climatology of stratospheric aerosol optical properties primarily based on space missions (GloSSAC ) and a climate forcing data set produced by ETH Zurich available at input for MIPS .
• The AEROCOM -project is focused on improving the understanding of global aerosol and its impact on climate. AEROCOM is primarily focused on tropospheric aerosol but a component is related to stratospheric and upper tropospheric aerosol observations.

New Activities

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.

Extending Outreach

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.

References

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.