Impact of unmitigated HFC emissions on stratospheric ozone at the end of the 21st century as
simulated by chemistry-climate models*Hideharu Akiyoshi1, Eric Dupuy1, and Yousuke Yamashita1,21National Institute for Environmental Studies, 2Japan Agency for Marine-Earth Science and Technology

5. Results from CCMs (3-D)

1. Introduction
Hydrofluorocarbon (HFC) concentrations in the atmosphere have increased rapidly since
the early 1990s because of their use as substitutes for chlorofluorocarbons (CFCs) andhydrochlorofluorocarbons (HCFCs).
HFCs do not contain chlorine and bromine atoms, thus do not contribute to direct
chemical ozone destruction. However, because of their long lifetimes and strong infraredabsorption in the atmospheric window, they affect stratospheric temperature and circulation
patterns. This, in turn, influences the concentration and variations of stratospheric ozone.
In this study, we performed numerical experiments using two 3-dimensional (3-D)
Chemistry-Climate Models (CCMs) to investigate the effect of unregulated HFC concentration
increases on stratospheric ozone. Results of the two CCMs are cross-compared andcompared with the previous 2-D model results of Hurwitz et al. (2015).

5-1. Temperature response (zonal mean, annual mean, 100-member ensemble mean)

Grey-shaded areas are regions with statistical significancebelow the threshold (t-test, 95%)

2. Future radiative forcing from HFC

5-2. Ozone response (zonal mean, annual mean, 100-member ensemble mean)

Radiative forcing from HFCs could becomecomparable with that of CO2 by the end of thiscentury if HFCs are not regulated and the future CO2
concentration follows the RCP2.6 scenario. In this case,
how seriously would the ozone layer be affected byHFCs?

(Velders et al., 2014)

3. Preceding study: HFC effect estimation from a 2-D model (Hurwitz et al., 2015)
HFC scenario

Grey-shaded areas are regions with statistical significancebelow the threshold (t-test, 95% for MIROC3.2-CCM and
66.7% for MIROC5-CCM)

Effect on circulation (2050)

Streamlines: annual mean circulation
change induced by the HFC increase

5-3. Colmun ozone response (zonal mean, annual mean, 100-member ensemble mean)

Color： Annual mean vertical motion
changes (mm s-1)
Red：upwards Blue：downwards

Effect on temperature (2050)

Effect on ozone (2050)

Due to the vertically alternating positive and negativeozone concentration responses, magnitude of thetotal ozone response is very small.
Proportionality of the response between the low-HFC
and high-HFC cases is not clear.

Hurwitz et al. (2015)

Are the responses similar or different between the 2-D model and our 3-D models?

5-4. Residual mean circulation response (zonal mean, annual mean, 100-member ensemble mean)

4. Method⚫Two CCMs (MIROC3.2-CCM and MIROC5-CCM) are used for the numerical experiments.
⚫The basic model atmospheric state is set to the 2095 atmosphere under the RCP2.6
scenario, when radiative forcing caused by HFCs is nearly equal to that of CO2.
⚫100-member ensemble simulations are performed to evaluate statistical significance ofthe response.

Arrows： Annual mean
circulation changes
induced by the HFC
increase

Color： Annual mean verticalmotion anomalies (mm.s-1)
Red：upwards Blue：
downwards

Grey-shaded areas are regions with statisticalsignificance below the threshold (t-test, 95%)

5-5. EP-flux and EP-flux divergence response (High-HFC case)(zonal mean, annual mean, 100-member ensemble mean)
Control run0 ppbv

0 ppbv

0 ppbv

0 ppbv

Low-HFC
High-HFC➢HFC profiles are calculated from fixed surface concentrations and chemical reactions with OH,O(1D), and Cl.
➢The 2095 output from a sensitivity run of CCMI・REF-C2 with the RCP2.6 scenario is used as
initial data. Practically, a 110-year continuous run is performed, then data from the last 100years are analyzed as 100 distinct ensemble members.
➢The lower and upper limits of the projected HFC radiative forcing in 2095 are twice and threetimes the 2050 forcing, respectively (Velders et al., 2014). Assuming that HFC radiative forcing
is proportional to the amount, HFC concentrations in the unregulated increase experiments are
set to twice or three times the 2050 concentrations of Hurwitz et al. (2015).

Seasonal evolution of EP flux and EP flux divergence aresimilar between the models, but responses to the HFC
increase are different.

Red: divergence
Blue: convergence

Grey-shaded areas are regions with statisticalsignificance below the threshold (t-test, 66.7%)6. Concluding Remarks(1)The estimated impact of HFCs on total ozone was small, at most 1% (4.2 DU) for MIROC3.2-CCM and 0.3% (1.2 DU) for MIROC5-CCM. In both cases, the net global effect was positive.
(2)The small magnitude of the total ozone response is caused by the altitude-dependent, alternately negative and positive ozone responses.
(3)The observed alternating anomaly pattern can reasonably be explained by competing effects of residual vertical motion anomalies in the lower and middle stratosphere, and temperature
anomalies in the upper stratosphere.
(4)Large differences were observed at high latitudes, notably in the NH polar region, not only between the 2-D and 3-D simulations but between the 3-D models themselves. These discrepancies
were attributed to differences in wave activity during winter.
This research was published in JGR. https://doi.org/10.1029/2021JD035307

The simulations were completed with the supercomputer (NEC SX-Aurora TSUBASA) at NIES.