Scientific Summary Chapter 5: Scenarios, Information, and Options for Policymakers
A new baseline scenario for ozone-depleting substances (ODSs) is presented in Chapter 5 that reflects our current understanding of atmospheric mixing ratios, production levels, and bank sizes. Elimination of future emissions, from either production or existing banks of various ODSs, is applied to this scenario to evaluate the maximum impacts of various hypothetical policy options including phase- outs and destruction (see Table S5-1). Some specific findings corresponding to this table include:
Emissions from the current banks (taking 2015 as being current) over the next 35 years are projected to lead to greater future ozone depletion and climate forcing than those caused by future ODS production. Capture and destruction of these banks could avoid 1.8 million Ozone Depletion Potential-tonnes (ODP-tonnes) of future emission through 2050; this compares with an estimated 1.6 million ODP-tonnes of emissions that have occurred over the last decade (from 2005–2014).
Of all of the ODS banks, the banks of halons in 2015 are projected to contribute most to ozone depletion over the next 35 years (as Equivalent Effective Stratospheric Chlorine, EESC), while chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC) banks are expected to contribute most in terms of Global Warming Potential-weighted (GWP- weighted) emissions.
If future production and all ODS 2015 banks are considered, HCFCs represent the halocarbon group that contributes most to future GWP-weighted emissions. HCFCs can be reduced in the future by both bank recapture and destruction and by production elimination.
Table S5-1. Summary of mitigation options for accelerating the recovery of the ozone layer and reducing CO2-equivalent emissions.
The table gives the reductions in integrated EESC (Equivalent Effective Stratospheric Chlorine) and integrated CO2-equivalent emissions relative to the baseline scenario that can be achieved for mitigation options beginning in 2015 or 2020. The integrated EESC is defined as the total EESC amount integrated from 2015 until the time EESC returns to the 1980 level (before 2050 for all scenarios). Bank recapture and destruction is assumed to be 100% effective and either applies to the bank existing in 2015 or the bank existing in 2020. Any potential contribution from very short-lived substances is neglected. These calculations use the lifetimes derived from SPARC, 2013 (Stratosphere-troposphere Processes And their Role in Climate).
Substance or Group of Substances
Reductions (%) in Integrated Midlatitude EESC Integrated from
2015 until EESC Returns to 1980 Levels
Reduction in Cumulative GWP- Weighted Emissions from 2015 to 2050 (Gigatonnes of CO2- equivalent)
Bank recapture and destruction in 2015 or 2020:
Production elimination from 2015 onward:
CH3Br (only quarantine and pre-shipment)
Total emissions elimination from 2015 onward:
Up to 165*
* Reduction relative to hypothetical future upper range scenario and would depend on actual growth rate of HFC use.
The impact on ozone-layer recovery of further policy actions on already controlled ozone- depleting substances is becoming smaller. Nonetheless, if all ODS emissions – including those emanating from many widely dispersed banks – were to be stopped in 2015, then the return to 1980 midlatitude EESC values would be brought forward to 2036 compared with 2047 in the baseline scenario.
Updated Ozone Depletion Potentials (ODPs) are almost all numerically smaller, ranging from no change (for carbon tetrachloride, CCl4) to more than a factor of two smaller (for CFC-115), with most of these smaller by 10–30% than the values reported in WMO (2011). These changes largely reflect the revised estimate for the atmospheric lifetime of CFC-11 (from 45 to 52 years) reported in SPARC (2013); CFC-11 is the reference gas in determining ODPs so this change affects all ODPs. Uncertainties in the atmospheric lifetimes, the fractional release values, and atmospheric chemistry generally result in overall uncertainties in ODPs on the order of 30% for the CFCs and CCl4, but are higher for HCFCs and halons (about 60% for the HCFCs and halon-1301, to over 80% for halon-1202 and halon-1211).
New atmospheric model studies continue to emphasize that ODPs for very short-lived substances (VSLS) that contain bromine or chlorine are strongly dependent on the geographic location and season of emission. Impacts from VSLS are much larger (with ODPs approaching values of 1) if emissions occur in regions close to convective regions in the tropics. There is still insufficient research available to confidently compare the mitigation options of anthropogenic VSLS emissions with those of the longer-lived halogenated hydrocarbons; overall the VSLS have smaller ODPs than longer-lived ODS. However, if long-lived controlled halocarbons (and their banks) follow their projected decline, then chlorine- and bromine-containing anthropogenic VSLS emissions will play a relatively larger role in future ozone depletion, but the absolute effects are smaller than that of ODSs today while remaining uncertain.
The projection of CCl4 remains more uncertain than projections for other ODSs due to our incomplete understanding of the current CCl4 budget (likely a missing source; see Chapter 1). In the scenarios examined (see table above), CCl4 human-related emissions from 2015 through 2050 are comparable to those of the HCFCs in terms of ODP-weighted emissions and are about 10% in terms of GWP-weighted emissions. It is expected that future emissions of CCl4 will remain an important factor in the evolution of EESC.
The total anthropogenic emissions of methyl bromide (CH3Br) have declined in response to controls of the Montreal Protocol. Overall, reported consumption has gone down from ~70,000 tonnes/yr in the late 1990s to ~13,000 tonnes/yr in 2012.
Quarantine and pre-shipment (QPS) uses of CH3Br are exempted uses (not controlled) by the Montreal Protocol and in 2012 constitute an annual consumption of CH3Br (~9,000 tonnes) that is larger than the annual consumption for 2012 from uses controlled by the Protocol (~4,000 tonnes). The elimination of future emissions from QPS uses could bring forward the date of EESC return to 1980 levels by 1.1 years, smaller than the 1.6 years estimated in the previous Assessment. Critical-use exemptions continue to be granted, but at levels significantly reduced compared with four years ago. A continuation of critical-use exemptions at the current level would delay the return of EESC to 1980 levels by 0.2 years.
Carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) are each important to climate forcing and to the levels of stratospheric ozone (see Chapter 2). In terms of the globally averaged ozone column, additional N2O leads to lower ozone levels, whereas additional CO2 and CH4 lead to higher ozone levels. Ozone depletion to date would have been greater if not for the historical increases in CO2 and CH4. The net impact on ozone recovery and future levels of stratospheric ozone thus depends on the future abundances of these gases. For many of the scenarios used in the most recent Intergovernmental Panel on Climate Change (IPCC) Assessment (IPCC, 2013), global ozone will increase to above pre-1980 levels due to future trends in the gases. Latitudinal and altitudinal responses are expected to vary. Note that scenarios used in IPCC consider a future with all three major greenhouse gases increasing and thus it is important to assess the net balance of these perturbations on stratospheric ozone.
Global Warming Potentials (GWPs) for a range of halocarbons have been updated based on IPCC (2013) and SPARC (2013). The CO2 Absolute Global Warming Potential (AGWP; the denominator for the GWP of other greenhouse gases) has increased by 6% compared to the previous Assessment (WMO, 2011). As a result, GWP values for many non-CO2 greenhouse gases decreased slightly. GWPs also changed because of revised values for the lifetime and the radiative efficiency of the individual greenhouse gases. The revised SPARC-based lifetimes for a range of ODSs have been updated due to new analyses of observations and models and are included here; the largest differences in GWPs are found for CFC-11, CFC-115, halon-1301, halon-2402, and halon-1202. For hydrofluorocarbons (HFCs), some examples of the IPCC 100-year GWPs and the SPARC lifetime adjusted values are given below. The numbers in parentheses represent the effects of uncertainties in the SPARC lifetimes, radiative efficiency, and the AGWP for CO2 based on 90% confidence. In addition, the IPCC (2013) stated uncertainties in the 100-year GWP for HFC-134a is ±35% (90% confidence) as representative for similar gases. The IPCC and updated GWPs that use the SPARC lifetimes are consistent within their uncertainties.
IPCC AR5 100-yr GWP
Updated 100-yr GWP (90% uncertainty range)
Global Temperature Potentials (GTPs) are discussed and values reported for the first time in a WMO-UNEP Ozone Assessment. The GTP metric gives the relative temperature increase at a specified time horizon due to emissions of a greenhouse gas, relative to that caused by the same weight of CO2 emissions. This metric may be useful as an alternative to GWPs. These metrics are different in construction and have both advantages and disadvantages. The revised SPARC-based lifetimes affect GTPs (relative to IPCC, 2013) in the same way as GWPs. The table below shows updated GTPs for the same HFCs listed above.
Updated 20-yr GTP
Updated 50-year GTP
Updated 100-year GTP
The current direct radiative forcing (RF) from ODS halocarbons (CFCs, halons, and HCFCs) is about 0.33 W m-2 and is near its expected peak. The RF is projected to decrease to about 0.20 W m-2 by about 2050, depending on the particular emission scenario adopted. By 2100, the radiative forcing from these halocarbons is projected to be near 0.10 W m-2 independent of the specific emission scenarios considered here.
While HFCs currently constitute less than 1% of the radiative forcing on climate (0.02 W m-2), if the current mix of HFCs is unchanged, increasing demand could imply a radiative forcing for HFCs as high as 0.4 W m-2 by 2050. For all scenarios (Special Report on Emissions Scenarios (SRES) and Representative Concentration Pathway (RCP)) used in the recent IPCC Assessments, the HFC radiative forcing increases by 0.1 W m-2 or less by 2050; however, these scenarios did not consider recent market trends. Scenarios based on projections of HFC markets yield radiative forcings that range from 0.16 W m-2 to 0.4 W m-2 by 2050.
Replacements of the current mix of high-GWP HFCs with low- or zero-ODP, low-GWP compounds, could lead to a decrease in the radiative forcing on climate over the coming decades, possibly by as much as 0.07 W m-2 by 2030 relative to baseline scenarios assuming continued growth in high-GWP HFC production. Such reductions are comparable to possible reductions in radiative forcing for some other non-CO2 emissions (e.g., for black carbon emissions). Even by 2050, the RF from the low-GWP replacement compounds would be negligibly small. For the uses projected, such replacements are likely to have a negligible effect on stratospheric ozone, despite some replacements containing chlorine or bromine and having non-zero ODPs.
The impact of HFC mitigation on future climate change that only considers radiative forcing of HFCs through a particular year is underestimated if the future commitment to climate forcing in the HFC banks is neglected. This bank size represents a substantially larger fraction of the cumulative HFC production and emission than was the case for CFCs in the 1980s; this is because current and projected applications for HFCs emit those HFCs much more slowly than applications historically did for CFCs.
Unsaturated HFCs (also known as hydrofluoro-olefins, HFOs) are replacement compounds for long-lived HCFCs and HFCs. Unsaturated HFCs have short atmospheric lifetimes (days) and small GWPs (<10). Atmospheric degradation of one of these substances (HFC-1234yf) produces the persistent degradation product trifluoroacetic acid (TFA). While the environmental effects of TFA are considered negligible over the next decades, potential longer-term impacts could require future evaluations due to the environmental persistence of TFA and uncertainty in future uses of HFC- 1234yf.
CFC-316c ((E)- and (Z)- isomers of cyclic 1,2-C4F6Cl2) are possible ODS replacement compounds, and have long lifetimes (75 and 114 years), with correspondingly high ODPs (0.46 and 0.54) and GWPs (4160 and 5400).
Emissions of biogenically produced bromocarbons will likely increase as a result of changes in the management of their human-related production (e.g., marine aquaculture). However, uncertainties in all natural emissions and in transport to the stratosphere are large, making it difficult to quantify their effects on ozone.
Current emissions from aviation and rockets have only a small effect on total ozone (<1%). However, new technologies and potential market growth in aviation and rockets will require further assessment as they could potentially lead to effects on ozone.
Geoengineering the climate system via anthropogenic increases of stratospheric sulfate aerosols within the next few decades would be expected to deplete stratospheric ozone, with the largest effects in the polar regions. The current level of understanding of how other possible geoengineering approaches would affect the stratosphere is limited.
The proposed cosmic-ray-driven breakdown of CFCs in ice particles is of negligible importance in polar ozone loss.