Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850 to 2100

Abstract

Abstract. Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼ 300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer–Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions.

Climate change favours large seasonal loss of Arctic ozone

“…A few of the GCMs did not archive output for all four of the SSP scenarios used in our paper; in this case, we simply averaged output from all available GCMs. These ten CMIP6 GCMs tend, on average, to underestimate observed H 2 O in the tropical lower stratosphere 110 by nearly 1 ppmv from 1984 to the present, as shown in the upper panel of Figure 12 of Keeble et al 43 . The abundance of H 2 O in the tropical lower stratospheric is governed by thermodynamics, whereas the abundance of H 2 O in the polar stratosphere is driven by this process as well as the oxidation of CH 4 .…”

“…Temperature fields within these GCMs often exhibit biases with respect to observed temperature that can approach 5 K, with most models being biased warm 42 . Stratospheric H 2 O tends to be biased low in many models 43 , which together with a high-temperature bias will lead to an underestimation of the accumulated exposure to PSCs in the Arctic. To compensate for the temperature biases, the temperature threshold for the existence of PSCs has been offset by a constant value specific to each model such that the overall magnitude of PFP LM in the GCM matches the observed magnitude of PFP LM over the modern satellite era.…”

“…combined with historical and future projections of CH 4 from the SSP-database, and the thermodynamic-based estimate results from an analysis of CMIP6 GCM output 43 (see "Methods").…”

“…The projections for the effect of warming of the tropical tropopause shown in Fig. 2c are based on the analysis of output from CMIP6 GCMs shown in Figure 15 of Keeble et al 43 . They document results from ten CMIP6 GCMs, for the four SSP scenarios shown in Fig.…”

The impact of increasing stratospheric radiative damping on the quasi-biennial oscillation period

“…First of all, the QBO exerts a marked influence on the distribution and transport of various chemical constituents such as ozone (O 3 ) (e.g., Hasebe, 1994), water vapor (H 2 O) (e.g., Kawatani et al, 2014), methane (CH 4 ), nitrous oxide (N 2 O), hydrogen fluoride (HF), hydrochloric acid (HC1), odd nitrogen species (NO y ) (e.g., Zawodny and McCormick, 1991), and volcanic aerosol (Trepte and Hitchman, 1992). Secondly, it is well appreciated that the QBO influences the extratropical circulation in the winter stratosphere, which is commonly known as the Holton-Tan effect (Holton and Tan, 1980;Labitzke, 1982). It has been noted that the effect of the QBO on the extratropical winter stratosphere impacts the severity of stratospheric ozone depletion (e.g., Lait et al, 1989).…”