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 AC&C: Atmospheric Chemistry and Climate A joint initiative of IGBP-IGAC and WCRP-SPARC NOTE:Specifications for the AC&C "Hindcast" and "ACC-MIP/Scenarios" Activities can be found on the HTAP/AC&C Wiki page. AC&C Overview Initiative Coordinators Phil Rasch Martyn Chipperfield 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. In June 2008 AC&C held a workshop joint with TF-HTAP and there defined more specifics of the model runs for three of the AC&C activities. AC&C activities involve:
The activities of AC&C will be pursued under the organizational framework given in the Figure below. AC&C will be implemented in phases. 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) and the global Aerosol model inter-Comparison (AeroCom). 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. A new group, “TropChem”, is acting as the organizing body for tropospheric gas phase runs. In addition AC&C activities are being coordinated with those of the European ACCENT project Model Inter-comParison (ACCENT-MIP) and the Task Force on Hemispheric Transport of Atmospheric Pollutants (TF-HTAP; http://www.htap.org). TF-HTAP is set up under the auspices of the Convention on Long-range Transboundary Air Pollution and focuses on understanding and quantifying northern hemispheric transport of gaseous and particulate air pollutants and their precursors between specific Northern Hemisphere source to receptor regions. 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 TF HTAP. In parallel, emissions for AC&C runs are being coordinated through the Global Emission Inventory Activity (GEIA; http://www.geiacenter.org/).  Three thematic areas help define 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, three activities have been selected and are in progress: To have confidence in near-term (~30 year) climate and regional air quality forecasts we need to know that models can accurately represent the critical chemistry-climate interactions and know the magnitude and source of uncertainties. Four hindcasts will be used to test model skill:
Specifications for the Hindcast model runs can be found on the HTAP/AC&C Wiki page. What controls the distribution of aerosols/gases in the troposphere? Step #1: Investigate what controls the distribution 5km->tropopause Model comparisons conducted under CCMVal (for ozone; e.g. Eyring et al. 2006; Braesicke et al. 2008) and AeroCom (for aerosols; e.g. Textor et al. 2007) have identified particularly large uncertainties in modeled distributions of trace species in the upper troposphere, even when the same emissions are used. Species at these altitudes are radiatively important, and aerosols at these altitudes have a longer lifetime and a large integrated impact. Initial runs will be designed to understand convection and scavenging processes, as these are among the most uncertain and largest “tuning knobs” in the models affecting upper tropospheric distributions. A particular focus will be the distributions of tracers in the tropical tropopause layer (TTL). Future scenarios: Sensitivities & Uncertainties A set of emissions Representative Concentration Pathways (RCPs) are being prepared for the next IPCC assessment (Moss et al., 2008). Many climate modeling centers will perform simulations to define the chemical composition of the atmosphere as needed for these simulations. However, some groups may not have the capability to create their own time-evolving distributions of ozone and aerosols (consistent with each RCP), do not need to carry this information iteratively, or would prefer to use a standard climatology. This activity will provide well-evaluated distributions for use in such climate and Earth System Models. In addition, to date there has been no systematic effort to archive, evaluate and compare the composition changes used in CMIP (Coupled Model Intercomparison Project) simulations, so an atmospheric chemistry & climate MIP is needed. This activity would include diagnostics from the CMIP5 simulations and from additional runs of the composition-climate models, archiving more detailed data on the processes governing the behavior of gas-phase and aerosol species. Additional specific time-slice experiments will enable the participation of chemistry-transport models. Specifications for the Hindcast model runs can be found on the HTAP/AC&C Wiki page. 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. References Braesicke et al. (2008), A model intercomparison analysing the link between column ozone and geopotential height anomalies in January, Atmos. Chem. Phys., 8, 2519-2535. Eyring, V. et al. (2006), Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past, J. Geophys. Res., 111, doi:10.1029/2006JD007327. Moss, R. et al. (2008), Towards new scenarios for analysis of emissions, climate change, impacts, and response strategies, IPCC Expert Meeting Report. () http://www.aimes.ucar.edu/DOCUMENTS/IPCC_Final_Draft_Meeting_Report_3May08.pdf) Textor, C., et al. (2007), The effect of harmonized emissions on aerosol properties in global models - an AeroCom experiment, Atmos. Chem. Phys., 7, 4489-4501. 
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