Tropospheric ozone in CMIP6 Simulations

Abstract. 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 tropospheric ozone burden increases from 244 ± 30 Tg in 1850 to a mean value of 348 ± 15 Tg for the period 2005–2014, an increase of 40 %. Modelled present day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; ACCMIP: 337 ± 23 Tg and TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden reaches a maximum of 402 ± 36 Tg in 2090, before declining slightly to 396 ± 32 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. There are large differences (30 %) between models in the preindustrial period, with the difference narrowing to 15 % in the present day. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3-LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere-troposphere transport reaches a minimum around the same time, while dry deposition increases steadily across the experiment. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden.