emissions) Simple box models What is the GMST response to different RF scenarios? Can incorporate multiple time scales to estimate transient and steady-state GMST changes for a variety of scenarios As above; neglects possible dependence of GMST response to the spatial distribution of RF and thus is unlikely to approximate well the response to short-lived air pollutants. Boucher and Reddy, 2008; Held et al., 2010; Pierrehumbert, 2014; Shindell et al., 2012. Adjoint methods in a GCTM Which emissions (species, region, sector) contribute to RF by a particular atmospheric constituent (regional, global)? Receptor-oriented framework allows attribution to multiple factors in a single simulation; complementary to forward modeling source attributions Assumes linear system; by design targets a single quantity (‘receptor’). Henze et al. 2012. Methods applied to study climate impacts on air pollution Observed relationships between chemical and meteorological variables How does air pollution respond to changing weather? Observational constraints on models Does not directly provide information on how climate will change, and may not correctly identify the underlying drivers of the correlation. Bloomer et al., 2009; Dawson et al. 2014; Jacob and Winner, 2009; Rasmussen et al., 2012; Tai et al., 2010, 2012a. Statistical downscaling using observed relationships (above) and climate change projections How will air pollution respond to changes in regional or global climate? Can be applied to a large suite of GCM physical climate change simulations (or their emulators) Assumes stationarity in the air pollutant response to the projected meteorological variable. Holloway et al., 2008; Tai et al., 2012b. CTMs driven by perturbed meteorological variables How does air pollution respond to changes in a meteorological driver? Develops understanding of air pollutant response to changes in a single factor; helps diagnose responses in more complex models Changes in air pollutants do not feed back on the climate system. Section 4 of Jacob and Winner, 2009; Rasmussen et al., 2013; Steiner et al., 2006. Global CTM driven by meteorology from a How do changes in climate alter air Sensitivity simulations separate As above, plus often limited to a few years of meteorology from the Jacob and Winner, 2009; West et al., 2013; Wu et
GCM (or observed for present-day source attributions) quality? role of changes due to meteorology versus emissions or chemistry. parent GCM, complicating attribution to climate change. al. 2008ab; Weaver et al., 2009. Regional CTM driven by meteorology from a GCM or RCM As above, but with a region-specific focus. As above, may also consider feedbacks or processes As above but with additional dependence on chemical and meteorological boundary conditions which may not be consistent. Gao et al., 2013; He et al., 2014; Jacob and Winner, 2009; Kelly et al., 2012; ; Penrod et al., 2014;Trail et al., 2014; Weaver et al., 2009 Global CCMs How do changes in climate alter air quality? As above but with a consistent simulation of meteorology and pollutants Coarse resolution; simplistic or absent representations of some processes, particularly interactions with the biosphere. Clifton et al., 2014; Doherty et al., 2013; Fang et al., 2013; Fiore et al., 2012; Jacobson and Streets, 2009; Lamarque et al., 2011; Rieder et al., 2015; Young et al., 2013. Regional CCMs As above, but with region-specific focus. As above but with higher resolution and/or more complex description of processes Boundary conditions for chemical and meteorological variables not necessarily consistent. Shalaby et al., 2012. Global-to-regional nested CCM (one-way) As above. As above but avoids inconsistent boundary conditions Limited by ability to resolve relevant processes, or if two-way couplings between scales are important. Jacobson, 2008 a. Rapid adjustments are not dependent on the change in temperature, and thus do not act as a feedback to a change in temperature. They include processes such as cloud changes, including those induced by interactions with aerosols, as well as lapse rate changes, geographic temperature variations, and changes in the biosphere that are not direct responses to temperature; some rapid adjustments occur for many atmospheric constituents, including CO2. See Boucher et al. (2013) for a thorough explanation. b. Global Mean Surface Temperature