Tropospheric ozone in CMIP6 simulations

The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled atmosphere-ocean chemistry-climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 +/- 36 Tg in 1850 to a mean value of 356 +/- 31 Tg for the period 2005-2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 +/- 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 +/- 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 +/- 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 +/- 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (P-O3) and loss (L-O3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (P-O3-L-O3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere-troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850-2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden.