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 AC&C: Atmospheric Chemistry and Climate A joint initiative of IGBP-IGAC and WCRP-SPARC NEW: Summary of outcomes from 2nd AC&C Planning meeting, held joint with HTAP. Download .pdf here. AC&C Overview Initiative Coordinators Phil Rasch A. R. Ravishankara Sarah Doherty Background A significant part of the current human-induced climate forcing occurs through chemically active species. Changes in climate can lead to changes in the chemical composition of the atmosphere both by altering emissions and through changes in the chemical processes that occur in the atmosphere. The study of climate-chemistry interactions represents one of the most important and, at the same time, most difficult foci of global change research. Further, chemically active species are more amenable to short term manipulations through changes in emissions and are therefore of major policy relevance. Changes in emissions themselves can be brought on by climate trends or a change in climate variability. These factors also strongly couple the emerging issue of the coupling of climate and air quality, from both scientific and policy perspectives. Provision of high-quality, policy-relevant information on the current state of climate and its possible future states, as well as options for mitigation / control / change / adaptation are strongly dependent on the progress in studies in this area. In addition, at least two major assessments -- The World Meteorological Organization (WMO) Ozone Assessment and the Intergovernmental Panel on Climate Change (IPCC) climate change Assessment -- would benefit by improved understanding of chemistry-climate interactions; such improvements would help society through better information and policy. Significant progress to this end has been made through SPARC's Chemistry Climate Model Validation (CCMVal) effort, which has focused specifically on stratospheric chemistry-climate models and has fed directly into the latest WMO/UNEP International Ozone Assessment. Additional progress can be made by coupling this effort with studies using tropospheric chemistry-climate models and through coordinated studies with tropospheric chemistry-climate and aerosol models. The next IPCC assessment needs better information on emissions and abundances of chemical active constituents to address not only global climate change attribution but also the needed regional emphases for attribution and predictions. Improvements to the representation of these species in chemistry-climate models will also allow for better representation of the climate system in global models. Because of the importance of chemistry-climate interactions, much work is already going on in this area. Modeling centers are rapidly expanding the scope of their modeling efforts (for example, to include biogeochemical processes at the surface, chemical processes in the troposphere and middle atmosphere, and the impact of each of these on climate). Within IGAC, efforts to date have focused primarily on constraining atmospheric chemistry components and processes through measurements. Within WCRP's SPARC (Stratospheric Processes and their Role in Climate) project, the focus has been on modeling activities in the middle atmosphere, with less emphasis on field experiments of chemistry and chemical processes and the troposphere. The steering groups of SPARC and IGAC and their parent organizations, WCRP and IGBP, believe that a synergy would result from a coordinated effort by the SPARC and IGAC communities that focuses specifically on the representation of chemistry-climate interactions in earth system models. This effort would both be informed by inputs from the observational community (in-situ and remote sensing) and would help inform decisions about how to optimize future measurement campaigns. AC&C GoalsThe Atmospheric Chemistry and Climate Initiative (AC&C) was endorsed in March 2006 as a joint effort of WCRP and IGBP, with the SPARC and IGAC projects tasked to take the lead in its implementation. An initial scoping meeting for the Atmospheric Chemistry and Climate Initiative (Boulder, Colorado; August, 2006) laid the groundwork for the basic structure and goals of the Initiative. Using this as a starting point, a first set of AC&C activities, more specific goals, and a time-line were set at the 1st AC&C Workshop, which was held in January 2007 in Geneva, Switzerland. AC&C will be implemented in phases, with the first phase planned to end in 2009. In Phase I, the primary focus will be on improving process representation in chemistry-climate models but the effort will also be useful for Earth system and regional/global air quality models. The role of the AC&C project is coordination, in that it is not an independently funded effort. The mission of AC&C is to help the scientific community to define a common set of scientific themes and facilitate their execution once defined. Some of this coordination will involve defining new activities. Other advances on aspects of this problem will be made by connecting to and influencing the direction of several existing activities linked to AC&C -- e.g. the Chemistry-Climate Model Validation activity of SPARC (CCMVal), the global Aerosol model inter-Comparison (AeroCom), the European ACCENT project Model Inter-comParison (ACCENT-MIP), and the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF HTAP; The Task Force on Hemispheric Transport of Air Pollution is set up under the auspices of the Convention on Long-range Transboundary Air Pollution. More information is available at http://www.htap.org). CCMVal is a model inter-comparison and validation effort for stratospheric chemistry-climate models. Under AeroCom, global tropospheric aerosol models were compared and tested against satellite, lidar, and sun photometer measurements. The ACCENT-MIP effort previously focused on coordinating and comparing IPCC scenarios, contrasting the climate between 2030 vs. 2000 across a suite of tropospheric chemistry-climate models, with an eye toward capturing how climate change might affect air quality (gas species only). This effort has now been extended to encompass the activities of the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF HTAP). The TF HTAP activities focus on understanding and quantifying northern hemispheric transport of gaseous and particulate air pollutants and their precursors from source to receptor region. For all of these activities and for the AC&C objectives in general, emissions characterization (time-history, uncertainty, etc.) are of critical importance. Therefore, the IGBP-AIMES Global Emission Inventory Activity (GEIA) and other emissions activities will also be associated with AC&C. AC&C activities involve:
At the first workshop in Geneva, leaders from CCMVal, AeroCom, ACCENT-MIP, TF HTAP and GEIA were asked to give overviews of their project, with an emphasis on how the activity relates to the goals of AC&C and on how the AC&C initiative might benefit that activity. There were several resounding messages on this latter point: 1) Physical system interdependenciesThe science within each project would benefit through cross-fertilization with related projects, given the inter-dependencies of the different components of the system -- for example, the physical connections between the troposphere and stratosphere and between the aerosol and gas phases of atmospheric chemistry. To date, there have been largely separate efforts addressing stratospheric chemistry, tropospheric gas phase chemistry, and tropospheric aerosols. The science and models have recently become sufficiently advanced to address these "compartments" as a single system. 2) Emissions inventories An overarching activity such as AC&C could be used to promote an expansion and a thorough evaluation of emissions inventories. Current emissions inventories are effectively a database, with little or no accompanying meta-data and with little effort toward assessing the characteristics of the inventories for any given application. GEIA has a strong activity in assembling emission inventories relevant to atmospheric chemistry and climate. There are also efforts (e.g. within the TF HTAP community) that focus on emissions for air quality research, and efforts from the socio-economic scenarios community to produce future emissions scenarios. A systematic assessment of uncertainties, harmonization of the emissions data bases, and the addition of meta-data about how the inventory was derived, what applications is it useful for, etc., would be highly beneficial to this community and for the other communities, such as for model evaluation and for process-related field studies. Furthermore by helping to define the criteria by which the modeling community judges these inventories we might influence the methodologies used to produce next generation inventories. 3) Common database and tools for model output Within each of the existing projects (CCMVal, AeroCom, ACCENT-MIP, TF-HTAP) a common data format has been established; however, the chosen data format differs between some of these projects. It would be beneficial to have a common data center (even if it were only a "virtual" link), a single formatting standard, a set of visualization tools that could be utilized across the whole community, and a single meta-database with information on the models themselves. 4) Observational/Laboratory data sets First steps have been made within the existing projects to go beyond model inter-comparisons, e.g. to model evaluation via comparison with observations/measurements. However, in many cases the comparisons help reveal which models might be in error but do not provide information on which model processes are causing the error. In addition, the observations themselves might be biased, making results sometimes inconclusive. Therefore a comprehensive comparison to multiple types of observations/measurements is needed, and these comparisons should be crafted wherever possible to reveal information about the performance of processes in models. Barriers to achieving this include a lack of understanding within each community of what the other needs or can provide and where to go for information; the lack of a centralized, standardized observational database that includes information on data quality/uncertainties and other meta-data; difficulties around mis-matches in the scales (spatial and temporal) of model output vs. measurements; and differences in the model input and output parameters versus those physical parameters that are measured. Under AC&C, some of these barriers could be overcome through coordination of the relevant communities. 5) Need for advanced planning and "legwork" to meet assessment demands It was clear to the participants that advanced planning, preparation, and initiation of activities help a better assessment and are essential for the science community. There is very little time between assessments and there is a serial nature to the sequence of events that lead to the assessments, so that any planning and preparation is greatly helpful. In some cases, chemistry model runs have to be done even before climate runs are carried out. AC&C provides a pathway for the international scientific community to carry out this early work. Phase I Activities & Structure The activities of AC&C will be pursued under the organizational framework given in the Figure below. This framework has been presented to the governing bodies of both IGBP and WCRP; they have both accepted and endorsed this approach. Two existing activities -- CCMVal and AeroCom -- cover two of the areas of interest under AC&C: stratospheric chemistry and tropospheric aerosols. A third area of interest, tropospheric gas phase chemistry, is covered only in part by other activities (e.g. ACCENT-MIP/TF HTAP). Thus a new group, dubbed "TropChem" , will act as a liaison to the gas-phase tropospheric chemistry/climate modeling community and will build on existing activities such as under TF-HTAP and ACCENT-MIP. These three groups will act as the Research Implementation Bodies for AC&C activities. It should be emphasized that the activities of AC&C will comprise components of the science pursued by CCMVal and AeroCom and will require buy-in from these projects' communities. As discussed below, AC&C activities were selected with this in mind.  In Boulder, three thematic areas were decided on for AC&C: the impacts of climate on atmospheric chemistry; the impact of atmospheric chemistry on climate; the impact of climate on air quality. As AC&C is an unfunded activity, improvements in each of these areas will only be made through the efforts of independently funded research groups. Thus its success is contingent on buy-in from the scientific community and on being able to take advantage of already-planned or existing activities/model runs. Given limited time and financial resources, not all aspects of these thematic areas can be addressed simultaneously. Conversely, activities under AC&C -- which is by definition a coordination activity -- should require the participation of three or more modeling groups and two or more of the Research Implementation Bodies. Thus, discussions in Boulder and at the 1st AC&C Workshop focused on selecting a set of activities based on scientific questions that:
Using the above criteria, four projects were selected in Geneva for pursuit under AC&C in Phase I of
the project (nominally to end in 2009): In addition, a Data Center Committee was formed to explore issues/options for having a centralized data center/tools under AC&C and an Emissions Harmonization Committee was formed to work with GEIA and HTAP to try and improve the utility of emissions databases for use by models. It was decided that there would not be a centralized effort to consolidate observational data sets as a general task of AC&C, as this is beyond the scope, capability, and resources of the initiative. Instead, observations and laboratory data will be utilized within each of the four AC&C activities as appropriate for validating and understanding model processes and output and as a way of increasing our ability to represent processes in models. The observational and laboratory community will be engaged in each of the AC&C activities for this purpose. In Geneva, broad outlines were drawn up for each of the AC&C Activities, and draft steering committees were established. These committees are currently working on the details of how each activity will proceed. Below, brief descriptions of each, as defined at the 1st AC&C Workshop, are given. ACTIVITY 1: 20 Year Hindcast Simulation Five or more models would do a 20-25 year "hindcast" to address the questions:- Can we replicate the observed changes in chemical composition over the past 20 years? - Can we understand what processes have acted to change the tropospheric chemistry of the atmosphere, particularly ozone and aerosols? Model results would be compared with each other and with available observations to assess them for where uncertainties lie. The 20 year runs might be designed to incorporate special "focus" or "snapshot" periods; e.g. El Nino vs. La Nina years, periods when field campaign data are available as ground-truth, etc. To the best degree possible, everyone would run with the same anthropogenic emissions. Diagnostics would be designed to reveal information on model processes. Such an experiment can be based on experiences gained from the European RETRO project, in which 3 global models have simulated the period 1960-2000 and investigated changes in tropospheric ozone and its precursors. ACTIVITY 2: What controls the distribution of aerosols/gases in the troposphere? Step #1: Investigate what controls the distribution 5km->tropopause. > This activity would contribute to the next WMO Ozone Assessmen The upper troposphere was chosen as an area of interest because a) the processes that control trace constituents in this region differ from model to model, and this results in dramatic difference in the distributions themselves from model to model; b) species in this altitude range (e.g. ozone, dust, black carbon) can have a significant radiative impact, and may affect other components of the climate system (like cirrus clouds); c) the processes that control the distribution of species in this altitude range (e.g. vertical lofting; wet deposition; cloud processing) also control the long-range transport of these species; and d) the distribution of species in this region depends on and influences processes in the upper troposphere/lower stratosphere. ACTIVITY 3: Cloud, Aerosol, Chemical Interactions This activity will address the question: How well can we characterize warm cloud / aerosol interactions in global models, with a specific focus on the interactions with gas chemistry photochemistry? It will explore the impact of aerosols on atmospheric chemistry through their modification on clouds. This would be done employing a paradigm of controlling iterative sets of parameters (cloud droplet number, etc.) during model runs, slowly adding in links to aerosols and investigating the impacts on chemistry in clouds. ACTIVITY 4: Future scenarios: Sensitivities & Uncertainties > This activity would contribute to the next IPCC Assessment The goal here would be specifically to have a better representation of aerosol and chemistry in the next IPCC Assessment Report 5 (AR5; should there be one). This group could define the pre-industrial to present to future scenarios, based on emissions that are consistent with other AR5 runs (to the best degree possible). By running multiple models with constrained emissions, it would be possible to define a "best guess" and uncertainties. The model runs would further be designed to explore sensitivities to model processes. More details on these activities will be given in a future journal article. If you are interested in participating in one of the AC&C activities described above or have other input, please contact Sarah Doherty of the IGAC Seattle Core Project Office. Boulder AC&C Initial Planning Meeting Attendees:Mary Barth (NCAR-ACD), Guy Brasseur (NCAR-ACD), William Collins (U.K. Met Office), Sarah Doherty, (IGAC Core Project Office), Anne Douglass (NASA Goddard), Veronika Eyring (DLR), Andrew Gettelman (NCAR-ACD), Claire Granier (Service d Aeronomie CNRS), Didier Hauglustaine (LSCE, CEA-CNRS), Peter Hess (NCAR-ACD), Kathy Hibbard (AIMES Core Project Office), Larry Horowitz (NOAA-GFDL), Ivar Isaksen (Univ. Oslo), Jean Francois (J.F.) Lamarque, (NCAR-ACD), Phil Rasch (NCAR-ACD & IGAC co-chair), A. R. Ravishankara, (NOAA-ESRL & SPARC co-chair), Michael Schulz (LSCE, CEA-CNRS-IPSL), Ted Shepherd (Univ. Toronto), Drew Shindell (NASA-Goddard) 1st AC&C Workshop Attendees: Yves Balkanski, Gufran Beig, Isabelle Bey, Bill Brune, Philip Cameron-Smith, Mian Chin, Martyn Chipperfield, Bill Collins, Frank Dentener , Sarah Doherty, Veronika Eyring, Arlene Fiore, Savitri Garavait, Claire Granier, Volker Grewe, Ann Henderson-Sellers, Peter Hess, Hans-Werner Jacobi, Terry Keating, Jean Francois Lamarque, Kathy Law, Mark Lawrence, Hong Liao, Jennifer Logan, Tatsuya Nagashima, Thanos Nenes, David Parrish, Vincent-Henri Peuch, Joyce Penner, David Plummer, Michael Prather, Phil Rasch, Sebastian Rast, A. R. Ravishankara, Andreas Richter, Jose Rodriguez, Vladimir Ryabinin, Martin Schultz, Drew Shindell, David Stevenson, Kengo Sudo, Christiane Textor, Michiel van Weele, Oliver Wild, André Zuber. 
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