On the effects of the ocean on atmospheric CFC-11 lifetimes and emissions

The ocean is a reservoir for CFC-11, a major ozone-depleting chemical. Anthropogenic production of CFC-11 dramatically decreased in the 1990s under the Montreal Protocol, which stipulated a global phase out of production by 2010. However, studies raise questions about current overall emission levels and indicate unexpected increases of CFC-11 emissions of about 10 Gg ⋅ yr⁻¹ after 2013 (based upon measured atmospheric concentrations and an assumed atmospheric lifetime). These findings heighten the need to understand processes that could affect the CFC-11 lifetime, including ocean fluxes. We evaluate how ocean uptake and release through 2300 affects CFC-11 lifetimes, emission estimates, and the long-term return of CFC-11 from the ocean reservoir. We show that ocean uptake yields a shorter total lifetime and larger inferred emission of atmospheric CFC-11 from 1930 to 2075 compared to estimates using only atmospheric processes. Ocean flux changes over time result in small but not completely negligible effects on the calculated unexpected emissions change (decreasing it by 0.4 ± 0.3 Gg ⋅ yr⁻¹). Moreover, it is expected that the ocean will eventually become a source of CFC-11, increasing its total lifetime thereafter. Ocean outgassing should produce detectable increases in global atmospheric CFC-11 abundances by the mid-2100s, with emission of around 0.5 Gg ⋅ yr⁻¹; this should not be confused with illicit production at that time. An illustrative model projection suggests that climate change is expected to make the ocean a weaker reservoir for CFC-11, advancing the detectable change in the global atmospheric mixing ratio by about 5 yr.

Man-made chlorofluorocarbons (CFCs) are the primary cause of the Antarctic ozone hole (1). The atmospheric lifetimes of these chemicals range from about 50 to 500 yr. The Montreal Protocol agreed to a complete phase out of worldwide CFC production and consumption by 2010. Evidence for healing of the Antarctic ozone layer has indeed emerged (2, 3), indicating the overall success of the Montreal Protocol. Atmospheric loss processes of CFC-11, the most abundant ozone-destroying CFC, are due to photolysis and reaction with excited oxygen (O¹D) once the gas reaches the stratosphere. The atmospheric lifetime of CFC-11 is assumed to be inversely related to the atmospheric abundance of the molecule, with due consideration of the lag times between tropospheric and stratospheric burdens (4). Given its lifetime of about 50 to 60 yr and continued emissions from storage banks such as chillers and building insulation foams (5), the CFC-11 inventory in the atmosphere is decreasing slowly. However, the rate of decrease in atmospheric concentrations has been slowing down since about 2012, suggesting higher overall emission and an unexpected additional post-2013 emission increase of CFC-11 of about 7 to 13 Gg ⋅ yr⁻¹ [10 to 20% of the total global emission during that time (6, 7)]. The latter is clearly inconsistent with the global zero new production that has been agreed to by the Montreal Protocol.CFC-11 is soluble in water, and therefore the ocean has absorbed some CFC-11 from the atmosphere. CFC-11 ocean uptake is greatest in high latitudes where cold sea surface temperatures (SSTs) enhance CFC-11 solubility (8), and mixing and transport from the surface into the deep ocean is enhanced. By 1994, the ocean had stored up to 1% of the total anthropogenic emissions of CFC-11 (9), and by 2014, the ocean held roughly 110 Gg of CFC-11 (10), or about 5 to 10% of the CFC-11 inventory in the various anthropogenic storage banks (5). While some CFC-11 is removed in sulfidic anoxic waters (11), this effect is small for the current climate, and CFC-11 has long been employed as a useful passive tracer to study ocean circulation (e.g., refs. 12 and 13). Early studies using a global model incorporating CFC-11 air–sea fluxes suggested that the ocean’s effects on atmospheric CFC-11 lifetimes and concentrations were negligible in the 1980s, when anthropogenic emissions were high (14). However, now that anthropogenic emissions have dramatically decreased and attention is focused on unexpected emissions of 10 Gg ⋅ yr⁻¹ or even less, changes in ocean uptake of CFC-11 could be affecting the atmospheric CFC-11 inventory enough to influence emission estimates and could introduce a time-dependent effect on its total lifetime. Further, as anthropogenic emissions continue to decrease in the future, the ocean must eventually become supersaturated with respect to atmospheric CFC-11 and turn into a source instead of a sink. No study has yet estimated when that should be expected to occur and what its magnitude will be.Here, we address the following questions: 1) How is the ocean affecting the atmospheric CFC-11 inventory, the lifetime of CFC-11 in the atmosphere and its time dependence, and how does this in turn influence emission estimates based on observed concentrations? 2) When will the ocean become a source of CFC-11 to the atmosphere, and how much will ocean outgassing affect the apparent emission and atmospheric mixing ratio in the future? 3) How will climate change affect ocean CFC-11 uptake in the future?For a conceptual understanding, we use a hierarchy of models starting with a simple six-box model that simulates the CFC-11 inventory in the atmosphere, ocean mixed layer, and deep ocean layers (each layer has two boxes representing the two hemispheres, see the schematic in Fig. 1A). CFC-11 in each box is assumed to be well mixed in this illustrative model. The atmospheric CFC-11 lifetime is kept constant at 55 yr and estimated emissions are taken from published work (15). We assume constant interhemispheric exchange timescales for each layer and constant cross-layer timescales for mixed layer to deep ocean exchange (SI Appendix, Table S1). Atmospheric CFC-11’s vertical distribution does affect its lifetime and surface concentration. Here, we subsume stratosphere–troposphere exchange into our adopted atmospheric lifetime estimates assuming a well-mixed atmosphere and focus on the ocean’s effect on atmospheric CFC-11. We then replace the four ocean boxes with a more sophisticated albeit low-resolution representation of the ocean (2.8° × 2.8° horizontal resolution and 15 vertical layers down to 5,000 m), the Massachusetts Institute of Technology general circulation model (MITgcm; 16, 17), which includes a physics-based CFC-11 air–sea flux and transport into the interior ocean and treats CFC-11 as a conservative tracer in the ocean (depicted in Fig. 1B). The MITgcm (for brevity, we refer to the combined coupled box model atmosphere–ocean model simply as the MITgcm) is run in two modes. First, we use the model forced with climatological average wind stress and buoyancy fluxes (Hist run) to assess the influence of parameters (i.e., SST, wind stress, etc.,) on air–sea CFC-11 fluxes. Second, we force the MITgcm using global monthly representative concentration pathway 8.5 condition (RCP8.5) output from the Max Planck Institute Earth System Model low-resolution version (MPI-ESM-LR) fully coupled global climate model (RCP8.5 run; 18, 19). This model has been shown to provide a realistic response of the Southern Ocean (55 to 70 °S), the region that stores the most CFC-11, to the southern annular mode (20). In the RCP8.5 run, interannual variability within the MPI-ESM-LR output provides changes in the forcing of the ocean applied after 1930, but variability in the atmospheric circulation is not explicitly incorporated into the box model atmosphere. We compare these runs to a “no-ocean” run in which the CFC-11 air–sea flux is turned off. Both the box model and MITgcm runs extend from 1930 (essentially the start of emission of this anthropogenic gas) to 2300.Open in a separate windowFig. 1.Schematic diagrams showing the box model (A) and the MITgcm setup (B). The box model has three layers that represent the atmosphere, ocean mixed layer, and deep ocean. Each layer has two boxes that indicate the NH and the SH. The MITgcm setup replaces the four ocean boxes with the MITgcm ocean but keeps the atmospheric boxes unchanged. One-way arrows indicate CFC-11 atmospheric loss; two-way arrows indicate CFC-11 transport into/out of the box.