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We describe and evaluate atmospheric chemistry in the newly developed Geophysical Fluid Dynamics Laboratory chemistry‐climate model (GFDL AM3) and apply it to investigate the net impact of preindustrial (PI) to present (PD) changes in short‐lived pollutant emissions (ozone precursors, sulfur dioxide, and carbonaceous aerosols) and methane concentration on atmospheric composition and climate forcing. The inclusion of online troposphere‐stratosphere interactions, gas‐aerosol chemistry, and aerosol‐cloud interactions (including direct and indirect aerosol radiative effects) in AM3 enables a more complete representation of interactions among short‐lived species, and thus their net climate impact, than was considered in previous climate assessments. The base AM3 simulation, driven with observed sea surface temperature (SST) and sea ice cover (SIC) over the period 1981–2007, generally reproduces the observed mean magnitude, spatial distribution, and seasonal cycle of tropospheric ozone and carbon monoxide. The global mean aerosol optical depth in our base simulation is within 5% of satellite measurements over the 1982–2006 time period. We conduct a pair of simulations in which only the short‐lived pollutant emissions and methane concentrations are changed from PI (1860) to PD (2000) levels (i.e., SST, SIC, greenhouse gases, and ozone‐depleting substances are held at PD levels). From the PI to PD, we find that changes in short‐lived pollutant emissions and methane have caused the tropospheric ozone burden to increase by 39% and the global burdens of sulfate, black carbon, and organic carbon to increase by factors of 3, 2.4, and 1.4, respectively. Tropospheric hydroxyl concentration decreases by 7%, showing that increases in OH sinks (methane, carbon monoxide, nonmethane volatile organic compounds, and sulfur dioxide) dominate over sources (ozone and nitrogen oxides) in the model. Combined changes in tropospheric ozone and aerosols cause a net negative top‐of‐the‐atmosphere radiative forcing perturbation (−1.05 W m−2) indicating that the negative forcing (direct plus indirect) from aerosol changes dominates over the positive forcing due to ozone increases, thus masking nearly half of the PI to PD positive forcing from long‐lived greenhouse gases globally, consistent with other current generation chemistry‐climate models. Document and evaluate atmospheric chemistry in GFDL‐AM3Net forcing from PI to PD short‐lived pollutant emission changes is ‐1.05 Wm‐2

Climate models incorporate a number of adjustable parameters in their cloud formulations. They arise from uncertainties in cloud processes. These parameters are tuned to achieve a desired radiation balance and to best reproduce the observed climate. A given radiation balance can be achieved by multiple combinations of parameters. We investigate the impact of cloud tuning in the CMIP5 GFDL CM3 coupled climate model by constructing two alternate configurations. They achieve the desired radiation balance using different, but plausible, combinations of parameters. The present‐day climate is nearly indistinguishable among all configurations. However, the magnitude of the aerosol indirect effects differs by as much as 1.2 Wm − 2, resulting in significantly different temperature evolution over the 20th century. We construct alternate model configurations by modifying cloud parametersThe present‐day climate is nearly indistinguishable between all configurationsThe warming over the 20th century is substantially impacted

The NOAA Geophysical Fluid Dynamics Laboratory three‐dimensional Global Chemical Transport Model (GFDL GCTM) is used to examine the winter‐spring evolution of pollution (fossil fuel combustion and biomass burning) from South and Southeast Asia with special focus on the Indian Ocean region. We find that during the monsoonal winter‐spring outflow, pollution over the Indian Ocean north of the ITCZ is concentrated in the maritime boundary layer and originates from both regions. South Asian emissions dominate over the Arabian Sea and the Western Indian Ocean, while the Southeast Asian emissions have the greatest impact over the Bay of Bengal and Eastern Indian Ocean. Over these oceanic regions, CO pollution in both source regions, most of which is from biomass burning, accounts for 30–50% of the boundary layer CO. It is transported equatorward from South and Southeast Asian source regions and episodically lofted into the upper troposphere by tropical convection events. This transport path has a noticable impact (10–20%) on total CO at 300 mb and produces a maximum in a tropical belt over and north of the ITCZ. Another free troposphere transport path, primarily open to Southeast Asian emissions, carries CO from that region out over the North Pacific and around the Northern Hemisphere. O3production is driven by NOx, which, unlike CO, comes almost equally from biomass burning and fossil fuel combustion in this region and has a chemical lifetime of a few days or less. The resulting NOxdistributions, while qualitatively similar to CO, have much steeper gradients, are transported much less widely, have a much lower background, and over the Indian and Pacific Oceans, are strongly dominated by pollution. O3resulting from these anthropogenic sources generally exhibits patterns similar to those found for CO and NOx. Pollution accounts for 20–50% of the near‐surface O3and 5–10% of the O3in the upper troposphere. South and Southeast Asian emissions only produce 25% of the boundary layer O3in the continental source regions. The maximum impact of the emissions occurs over the Indian Ocean (25–40%) with comparable contributions from O3produced in the continental emission regions and O3produced over the ocean by transported precursors. Convective lifting of the transported pollution O3supplies ∼10% of the O3in the tropical upper troposphere. While both emission regions have modest impacts on O3(5–10%) outside of the Indian Ocean region, Southeast Asian pollution impacts free troposphere O3in a midlatitude belt across the North Pacific, similar to NOx.