The Scientific Assessment Panel for the Montreal Protocol has released highlights of the Scientific Assessment of Ozone Depletion: 2022, and it shows that actions taken under the Montreal Protocol have continued to decrease atmospheric abundances of controlled ozone-depleting substances, and advance the recovery of the stratospheric ozone layer.
HIGHLIGHTS of the Scientific Assessment of ozone Depletion: 2022
Science has been one of the foundations of the Montreal Protocol’s success. This document highlights advances and updates in the scientific understanding of ozone depletion since the 2018 Scientific Assessment of Ozone Depletion and provides policy-relevant scientific information on current challenges and future policy choices.
Major achievements of the Montreal Protocol
Actions taken under the Montreal Protocol continued to decrease atmospheric abundances of controlled ozone-depleting substanc- es (ODSs) and advance the recovery of the stratospheric ozone layer. The atmospheric abundances of both total tropospheric chlorine and total tropospheric bromine from long-lived ODSs have continued to decline since the 2018 Assessment. New studies support previous Assessments in that the decline in ODS emissions due to compliance with the Montreal Protocol avoids global warming of approximately 0.5–1 °C by mid-century compared to an extreme scenario with an uncontrolled increase in ODSs of 3–3.5% per year.
Actions taken under the Montreal Protocol continue to contribute to ozone recovery. Recovery of ozone in the upper stratosphere is progressing. Total column ozone (TCO) in the Antarctic continues to recover, notwithstanding substantial interannual variability in the size, strength, and longevity of the ozone hole. Outside of the Antarctic region (from 90°N to 60°S), the limited evidence of TCO recovery since 1996 has low confidence. TCO is expected to return to 1980 values around 2066 in the Antarctic, around 2045 in the Arctic, and around 2040 for the near-global average (60°N–60°S). The assessment of the depletion of TCO in regions around the globe from 1980–1996 remains essentially unchanged since the 2018 Assessment.
Compliance with the 2016 Kigali Amendment to the Montreal Protocol, which requires phase down of production and consumption of some hydrofluorocarbons (HFCs), is estimated to avoid 0.3–0.5°°C of warming by 2100. This estimate does not include contribu- tions from HFC-23 emissions.
Current Scientific and Policy Challenges
The recent identification of unexpected CFC-11 emissions led to scientific investigations and policy responses. Observations and analyses revealed the source region for at least half of these emissions and substantial emissions reductions followed. Regional data suggest some CFC-12 emissions may have been associated with the unreported CFC-11 production. Uncertainties in emissions from banks and gaps in the observing network are too large to determine whether all unexpected emissions have ceased.
Unexplained emissions have been identified for other ODSs (CFCs-13, 112a, 113a, 114a, 115, and CCl4), as well as HFC-23. Some of these unexplained emissions are likely occurring as leaks of feedstocks or by-products, and the remainder is not understood.
Outside of the polar regions, observations and models are in agreement that ozone in the upper stratosphere continues to recover. In contrast, ozone in the lower stratosphere has not shown signs of recovery. Models simulate a small recovery in mid-latitude low- er-stratospheric ozone in both hemispheres that is not seen in observations. Reconciling this discrepancy is key to ensuring a full understanding of ozone recovery.
The existing network of atmospheric monitoring stations provides measurements of global surface concentrations of long-lived ODSs and HFCs resulting from anthropogenic emissions. However, gaps in regional atmospheric monitoring limit the scientific community’s ability to identify and quantify emissions of controlled substances from many source regions.
Several space-borne instruments providing vertically resolved, global measurements of ozone-related atmospheric constituents (e.g., reactive chlorine, water vapor, and long-lived transport tracers) are due to be retired within a few years. Without replacements of these instruments, the ability to monitor and explain changes in the stratospheric ozone layer in the future will be impeded.
The impact on the ozone layer of stratospheric aerosol injection (SAI), which has been proposed as a possible option to offset global warming, has been assessed following the terms of reference for the 2022 SAP Assessment Report. Important potential consequenc- es, such as deepening of the Antarctic ozone hole and delay in ozone recovery, were identified. Many knowledge gaps and uncertain- ties prevent a more robust evaluation at this time.
Heightened concerns about influences on 21st century ozone include impacts of: further increases in nitrous oxide (N2O), methane (CH4), and CO2 concentrations; rapidly expanding ODS and HFC feedstock use and emissions; climate change on TCO in the tropics; extraordinary wildfires and volcanic eruptions; increased frequency of civilian rocket launches and the emissions of a proposed new fleet of supersonic commercial aircraft.
Future Policy Considerations
If ODS feedstock emissions as currently estimated were to be eliminated in future years, the return of mid-latitude Equivalent Effective Stratospheric Chlorine (EESC) to 1980 abundances could be advanced by almost 4 years, largely due to reductions in CCl4, and there- by reduce total climate forcing from ODSs.
Eliminating future emissions of methyl bromide (CH3Br) from quarantine and pre-shipment applications currently allowed by the Montreal Protocol would accelerate the return of mid-latitude EESC to 1980 abundances by two years (as noted in previous Assessments).
Emissions of anthropogenic very short-lived chlorine substances, dominated by dichloromethane (CH2Cl2), continue to grow and contribute to ozone depletion. If CH2Cl2 emissions continue at their current level, they will continue to deplete approximately 1 DU of annually averaged global TCO. Elimination of these emissions would rapidly reverse this depletion.
A 3% reduction in anthropogenic N2O emissions, averaged over 2023–2070, would lead to an increase in annually averaged global TCO of about 0.5 DU over the same period, and a decrease of about 0.04 Wm–2 in radiative forcing, averaged over 2023–2100.
Global emissions of long-lived HFC-23, which are largely a by-product of HCFC-22 production, are as much as eight times larger than expected and are likely to grow unless abatement increases during HCFC-22 production or feedstock use of HCFC-22 decreases.
The current combined GWP-weighted emissions of CFCs plus HCFCs are comparable to those of HFCs. Reductions in the future emissions of CFCs and HCFCs requires addressing releases from banks and continuing production and use in allowed manufacturing of feedstocks, in by-products, or in unknown uses, depending upon the compound.