Natural halogens drive up to half of lower-stratosphere halogen ozone loss

Why it matters

Ozone recovery forecasts mostly treat natural halogens as negligible. This study suggests they matter, especially near the tropopause and over Antarctica. Projections of ozone recovery may need to include them.

What they did

The authors used the CESM2-SLH chemistry-climate model with two sets of runs, each with three members. One set had natural plus human halogen sources (AllHAL). The other had human sources only (NoNatHAL). The difference isolates the natural part. Runs covered 1910 to 2100 under the SSP3-7.0 scenario. The AllHAL runs were checked against observed total ozone and other CMIP6 models.

Key findings

• From 1990 to 2020, natural halogens caused about 15% of halogen ozone loss across the whole stratosphere. In the extrapolar lower stratosphere they caused 49.3%.
• Natural iodine and bromine are far more efficient at destroying ozone than chlorine there. Natural sources are only about 15% of the inorganic halogen burden, yet account for about 40% of the loss.
• By 2080–2099, natural halogens reach 82.8% of halogen-driven ozone loss in the extrapolar lower stratosphere.
• Over the Antarctic periphery (80°–60° S), natural halogens account for 35% of lower-stratosphere halogen ozone loss, mostly through Cl-Br cross-reactions. They cause the ozone hole’s seasonal reversal to start 7–10 years earlier and delay recovery of the natural seasonal cycle by 47–52 years.
• Natural halogens add 0.10 ± 0.04 W m−2 of extra shortwave radiation at the Antarctic polar cap surface, up to 10% of the total halogen-induced increase.

Limitations

• Ocean emissions of short-lived halogen compounds are held constant in time, so climate feedbacks on them are missing. The authors call the projections a conservative lower bound.
• Only one future greenhouse-gas scenario (SSP3-7.0) and three members per set were used.
• The coupled model has Arctic lower-stratosphere temperature biases and cannot reproduce sudden stratospheric warmings, so Arctic estimates need caution.

Glossary

• Short-lived halogens: Bromine, chlorine and iodine compounds with lifetimes under six months, emitted mostly from the ocean.
• Long-lived halogens: Stable compounds such as CFCs that reach the upper stratosphere before releasing reactive chlorine or bromine.
• Cl-Br cross-reactions: Ozone-destroying cycles that involve both chlorine and bromine together.
• Extrapolar lower stratosphere: The air between the tropopause and about 30 hPa, from 60° S to 60° N.