The Net Effective Radiative Forcing from Ozone-Depleting Substances and its Uncertainty

Abstract 

The direct effective radiative forcing (ERF) of long-lived ozone-depleting substances (ODS) is around 15%–20% of the pre-industrial to present-day ERF of , but their net ERF, including indirect chemical adjustments, remains poorly constrained. We use an adapted simple climate model, trained on bulk indirect ODS forcing from complex climate models, to quantify uncertainty in net ERF over time and the net ERF across individual ODS. We find that the direct ODS ERF in 2019 of 0.35 (0.31, 0.39) W  is reduced to a net ERF of 0.11 (−0.02, 0.26) W  when chemical adjustments are included. Most net ERF arises from CFC-12 and HCFC-22 (0.18 W ), with chemical adjustments substantially reducing the net ERF of many other substances. Since the implementation of the Montreal Protocol, the total ODS net ERF has increased much less rapidly than the direct ERF. 

Plain Language Summary

Ozone-depleting substances, such as chlorofluorocarbons (CFCs), damage the ozone layer and also contribute to climate change by trapping heat. Their direct warming effect is quite well understood. However, their net total impact on climate, including the climate impact due to changes in the chemistry of the atmosphere, is less certain. In this study, we used a simple climate model to account for these chemical effects. Our model helps identify how much each substance contributes to the net total impact on climate, which is difficult in larger climate models. We find that most of the warming comes from one, CFC-12, with much of the additional warming coming from HCFCs, which are one class of replacement gases for CFCs and cause less damage to the ozone layer. Our results highlight the need for further research to better understand these climate effects.