Abstract
Abstract. Present day tropospheric ozone and its changes between 1850 and 2100 are considered, analysing 15 global models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The ensemble mean compares well against present day observations. The seasonal cycle correlates well, except for some locations in the tropical upper troposphere. Most (75 %) of the models are encompassed with a range of global mean tropospheric ozone column estimates from satellite data, but there is a suggestion of a high bias in the Northern Hemisphere and a low bias in the Southern Hemisphere, which could indicate deficiencies with the ozone precursor emissions. Compared to the present day ensemble mean tropospheric ozone burden of 337 ± 23 Tg, the ensemble mean burden for 1850 time slice is ~30% lower. Future changes were modelled using emissions and climate projections from four Representative Concentration Pathways (RCPs). Compared to 2000, the relative changes in the ensemble mean tropospheric ozone burden in 2030 (2100) for the different RCPs are: −4% (−16%) for RCP2.6, 2% (−7%) for RCP4.5, 1% (−9%) for RCP6.0, and 7% (18%) for RCP8.5. Model agreement on the magnitude of the change is greatest for larger changes. Reductions in most precursor emissions are common across the RCPs and drive ozone decreases in all but RCP8.5, where doubled methane and a 40–150% greater stratospheric influx (estimated from a subset of models) increase ozone. While models with a high ozone burden for the present day also have high ozone burdens for the other time slices, no model consistently predicts large or small ozone changes; i.e. the magnitudes of the burdens and burden changes do not appear to be related simply, and the models are sensitive to emissions and climate changes in different ways. Spatial patterns of ozone changes are well correlated across most models, but are notably different for models without time evolving stratospheric ozone concentrations. A unified approach to ozone budget specifications and a rigorous investigation of the factors that drive tropospheric ozone is recommended to help future studies attribute ozone changes and inter-model differences more clearly.
Highlights
The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) is designed to complement the climate model simulations being conducted for the Coupled Model Intercomparison Project (CMIP), Phase 5 (e.g. Taylor et al, 2012), and both will inform the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5)
This study has analysed tropospheric ozone changes from 1850 to 2100 from the range of chemistry models that contributed to the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), running the latest set of ozone precursor emissions scenarios, and with 14 out of 15 models including representations of the changing climate
The seasonal cycle is well captured, except in some locations in the tropical upper troposphere, and there are suggestions of a high bias in the Northern Hemisphere (NH) and a low bias in the Southern Hemisphere (SH), most model results fall within the range of observed interannual variability
Summary
The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) is designed to complement the climate model simulations being conducted for the Coupled Model Intercomparison Project (CMIP), Phase 5 (e.g. Taylor et al, 2012), and both will inform the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). A primary goal of ACCMIP is to use its ensemble of tropospheric chemistry-climate models to investigate the evolution and distribution of short-lived, chemically-active climate forcing agents for a range of scenarios, a topic that is not investigated in such detail as part of CMIP5. This study is concerned with quantifying the evolution and distribution of tropospheric ozone in the ACCMIP models, detailing the projected ozone changes since the pre-industrial period through to the end of the 21st century, with a focus on where the projected changes from the ensemble are robust. Lelieveld and Dentener, 2000) The magnitudes of these terms are sensitive to the prevailing climate, and the levels and locations of ozone precursor emissions, such as nitrogen oxides (NO and NO2; referred to as NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), including methane A number global model studies have explored how changes in these drivers could affect tropospheric ozone abundances, from the pre-industrial period to future projections (e.g. Johnson et al, 1999; Collins et al, 2003; Prather et al, 2003; Shindell et al, 2003, 2006c; Sudo et al, 2003; Zeng and Pyle, 2003; Mickley et al, 2004; Hauglustaine et al, 2005; Lamarque et al, 2005, 2011; Stevenson et al, 2005; Brasseur et al, 2006; Dentener et al, 2006; West et al, 2007; Wu et al, 2008; Zeng et al, 2008; Jacobson and Streets, 2009; Young et al, 2009; Kawase et al, 2011)
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