Recent CH 2 Cl 2 trends and future growth scenarios

Figure 1 shows the measured surface abundance of CH 2 Cl 2 from the National Oceanic and Atmospheric Administration (NOAA) long-term surface monitoring network. Globally, CH 2 Cl 2 concentrations approximately doubled between 2004 and 2014 (Fig. 1a), although growth rates varied considerably during this period (Fig. 1b). The abundance of CH 2 Cl 2 at mid-latitudes in the NH is around a factor of 3 greater than in the Southern Hemisphere (SH), reflecting NH industrial sources. At present, it is unknown if a single industrial application of CH 2 Cl 2 , or several, is contributing to the observed upward trend. As a common solvent, CH 2 Cl 2 has numerous applications, which include use in metal cleaning/degreasing, in paint remover, and use by the pharmaceutical industry for preparing drugs. It is also used as blowing agent in production of foam plastics. A specific use of CH 2 Cl 2 , which seems likely to have increased in recent years, is in the manufacture of hydrofluorocarbons—the non-ozone-depleting chemicals used as replacements for CFCs and hydrochlorofluorocarbons (HCFCs). Given these sources, it is probable that demand for CH 2 Cl 2 from developing countries now, and in coming years, will be relatively high. This is supported by elevated levels of CH 2 Cl 2 detected over Asia, where Indian emissions are estimated to have increased by two- to fourfold between 1998 and 2008 (ref. 23).

Figure 1: Observed trends and growth rate of surface CH 2 Cl 2 and simulated stratospheric loading of chlorine. (a) CH 2 Cl 2 surface mixing ratio in p.p.t. from 2004 to 2015 derived from NOAA measurements as the annual mean observed at 4 sites in the SH, and 5 sites in the NH between 30° and 60° N (ref. 36). The time series is an update of ref. 21, years 2014 and 2015 are new data. Error bars denote ±1 s.d. and the solid lines denote a linear fit to these data with the shaded regions representing ±1 s.d. uncertainty on the fit. (b) Corresponding CH 2 Cl 2 growth rates (% per year). (c) Observed surface CH 2 Cl 2 mixing ratio in the NH (green circles, as in a) and trend (black line), along with projections of surface CH 2 Cl 2 between 30 and 60° N latitude under future scenarios (dashed lines); CH 2 Cl 2 increases at the mean rate observed over the 2004–2014 period (Scenario 1, blue), CH 2 Cl 2 increases at the mean rate observed over the 2012–2014 period (Scenario 2, red) and CH 2 Cl 2 remains at 2016 levels (Scenario 3, no future growth, orange). (d) Modelled chlorine (p.p.t.) from CH 2 Cl 2 entering the stratosphere in the recent past and projections. This is derived by multiplying the simulated CH 2 Cl 2 mixing ratio at the tropical tropopause by 2, to account for the 2 Cl atoms in the molecule. Data between 2005 and 2013 are an update of ref. 22, while subsequent years and future projections are from this study. Annual means in decadal intervals (2020–2050) are shown (filled circles) with ±1 s.d. (error bars) for Scenarios 1 (blue) and 2 (red). Solid lines denote a linear fit to these data, dashed portions extrapolate this fit prior to 2020. The orange line (dashed throughout) represents Scenario 3 (no future growth). Inset; Enlarged model curve for 2004–2014 with observed estimates from NASA aircraft measurements (stars). Full size image

Based on the NOAA surface measurements presented in Fig. 1a, we estimate a global emission source of around 1 Tg CH 2 Cl 2 per year to sustain the observed CH 2 Cl 2 concentrations in recent years (Fig. 2). We note that this is a far larger source than that of individual CFCs and other long-lived ozone-depleting gases (for example, carbon tetrachloride) in the 1980s, when emissions of those gases peaked. For CH 2 Cl 2 , and other VSLS more generally, relatively large emissions do not have the same impact on atmospheric concentrations, compared to say CFCs, as CH 2 Cl 2 is more rapidly oxidized in the troposphere and has a much shorter atmospheric lifetime.

Figure 2: Time trend in global halocarbon emissions. Emissions derived from a simple 1-box model for CCl 4 (dotted line), CFC-11 (dashed line) and CH 2 Cl 2 (crosses) in units of Gigagrams (Gg) of source gas per year. Calculation for CH 2 Cl 2 based on a parameterized global mean lifetime of 0.43 years. Also shown are recent independent estimates of CH 2 Cl 2 emissions (orange points) from the AGAGE 12-box model15. Error bars denote uncertainty range. Full size image

Two future scenarios encompassing potential surface CH 2 Cl 2 increases from 2015 to 2100 have been derived and are considered in our forward model simulations. Both are based on observed long-term surface trends (Fig. 1c), and are designed to test the sensitivity of ozone to potential future changes in chlorine derived from CH 2 Cl 2 growth. Scenario 1 assumes that surface CH 2 Cl 2 continues to increase at the mean rate observed during the 2004–2014 period: 2.85 parts per trillion (p.p.t.) per year at mid-latitudes in the NH. Scenario 2, a more extreme growth scenario to test the sensitivity of ozone to larger CH 2 Cl 2 increases, assumes CH 2 Cl 2 continues to increase at the mean rate observed in the 2012–2014 period only: 6.1 p.p.t. per year. This period saw comparatively large CH 2 Cl 2 growth compared to other recent years (Fig. 1b). In addition to the two growth scenarios, we also consider a third scenario in which no further CH 2 Cl 2 growth occurs post 2016 (Methods section). In this scenario (Scenario 3), surface CH 2 Cl 2 concentrations are fixed at 2016 levels throughout the forward simulation.

Constrained by the growth scenarios, our model simulations show a monotonic increase in chlorine from CH 2 Cl 2 entering the stratosphere (Fig. 1d) in coming decades, from ∼70 p.p.t. Cl in 2014, to ∼180 p.p.t. Cl or ∼360 p.p.t. Cl by 2050, under Scenarios 1 or 2, respectively. Critically, the model reproduces well observed levels of CH 2 Cl 2 around the tropopause in the recent past (Fig. 1d, inset) and, therefore, the stratospheric chlorine perturbation in response to increasing surface CH 2 Cl 2 concentrations is realistic in our simulations.

Impact of CH 2 Cl 2 growth on stratospheric inorganic chlorine

The dissociation of ozone-depleting compounds in the stratosphere liberates chlorine radicals which catalyse ozone loss. Owing to its relatively short stratospheric partial lifetime (of the order of 1–2 years in our model outside of the poles), CH 2 Cl 2 dissociates rapidly and thereby makes its largest relative contribution to the pool of inorganic chlorine (Cl y ) in the lowermost stratosphere, at low latitudes (Fig. 3a–c). At present, CH 2 Cl 2 accounts for <10% of stratospheric Cl y , although this contribution would increase significantly in coming decades if CH 2 Cl 2 growth continues and as chlorine from long-lived gases decreases. By 2050 under Scenario 1, CH 2 Cl 2 is projected to account for 20–30% of total Cl y in the lower stratosphere. An examination of the stratospheric Cl y trend in recent decades in our model reveals a peak in Cl y around the turn of the century at mid-latitudes (Fig. 3d,e). In the absence of CH 2 Cl 2 , here lower stratospheric Cl y is projected to return to pre-1980 levels by 2049, in line with ongoing decreases in levels of CFCs and other controlled long-lived Cl y precursors7. When CH 2 Cl 2 growth is considered, this Cl y return date is delayed by around 15–17 years under Scenario 1. Under Scenario 2—an extreme scenario—the Cl y return date occurs after 2080.

Figure 3: Contribution of CH 2 Cl 2 to stratospheric inorganic chlorine and changes in total inorganic chlorine relative to 1980 baseline. (a–c) Percentage (%) of total stratospheric inorganic chlorine (Cl y ) derived from CH 2 Cl 2 in 2015, 2030 and 2050 under model run Scenario 1 (assuming CH 2 Cl 2 continues to increase at the mean rate observed over the 2004–2014 period). (d,e) Modelled annual mean mid-latitude Cl y change in northern (35–60° N) and southern (35–60° S) hemisphere. The Cl y change is expressed in parts per billion (p.p.b.) relative to 1980 baseline at 50 hPa (lower stratosphere). Cl y changes are shown for model simulations without CH 2 Cl 2 (black) and with CH 2 Cl 2 under growth Scenarios 1 (blue, see above) and 2 (red, assuming CH 2 Cl 2 increases at the mean rate observed over the 2012–2014 period), and for the no additional growth Scenario 3 (orange). The projected dates when Cl y returns to 1980 levels in the NH are 2050 (no CH 2 Cl 2 ), 2067 (Scenario 1) and 2054 (Scenario 3). In the SH: 2049 (no CH 2 Cl 2 ), 2064 (Scenario 1) and 2053 (Scenario 3). The horizontal purple lines show best estimated range of 1980 return dates from previous CCMs6 which did not include CH 2 Cl 2 . Full size image

The delay in the Cl y return date discussed above under Scenario 1 is significant and is additional to, and of a similar magnitude, to other factors which have previously been considered when assessing the uncertainty in return dates, for example, coupled chemistry-transport differences between climate models or different future greenhouse gas scenarios. The effect on the Cl y return date is also much larger than the influence of potentially eliminating remaining small levels of production or emission of CFCs and HCFCs24. In the upper stratosphere, Cl y return dates occur later than in the lower stratosphere and are less sensitive to future CH 2 Cl 2 growth (Supplementary Fig. 1). Here, the contribution of CH 2 Cl 2 to total Cl y remains at <10% in 2050 (Fig. 3c).

Impact of past and potential future CH 2 Cl 2 growth on ozone

We consider next the impact of CH 2 Cl 2 on ozone. Ozone is most sensitive to CH 2 Cl 2 in polar regions; by the time air reaches high latitudes all chlorine that entered the stratosphere as CH 2 Cl 2 has been converted to Cl y . The largest ozone decreases attributable to CH 2 Cl 2 are simulated in the SH, where the Antarctic Ozone Hole—the most drastic manifestation of the effect of halogen-driven ozone loss—forms each spring. Although modest, the impact of CH 2 Cl 2 is non-negligible in the present day with springtime column ozone up to ∼3%, or 6 dobson units (DU), lower in simulations in which CH 2 Cl 2 is considered relative to an atmosphere without CH 2 Cl 2 in 2016 (Supplementary Fig. 2). The equivalent relative ozone decrease in the spring of 2010 was ∼1.5% (3 DU), which allows quantification of CH 2 Cl 2 increases on polar ozone depletion during spring, over this period, and highlights that this impact has already doubled in the past 6 years alone.

Figure 4 shows simulated ozone changes due to CH 2 Cl 2 in the recent past and future period. Ozone loss due to further CH 2 Cl 2 growth is projected to increase significantly in coming decades. By 2050, expressed as an annual mean, ozone is ∼6% lower in the Antarctic lower stratosphere under growth Scenario 1 (Fig. 4a), and annual mean column ozone is decreased by up to 8 DU, relative to the no CH 2 Cl 2 simulation (Fig. 4c), against a background of recovering ozone. At mid-latitudes, column ozone decreases are smaller (up to several DU) and despite CH 2 Cl 2 making a relatively large contribution to Cl y in the tropics, ozone decreases here remain small (<1%). Recall, our simulations in the future period reflect the ozone response to changes in projected stratospheric composition only, comparing scenarios with CH 2 Cl 2 growth to one with no CH 2 Cl 2 . Future ozone is also expected to be influenced by other factors, including climate-driven changes to stratospheric temperature and circulation7,12,25 and possibly due to changes in natural halocarbon emissions26. By isolating the ozone response due to CH 2 Cl 2 , we highlight its increasing influence on ozone evolution in coming decades. Such findings could also be relevant from a climate perspective21 as ozone absorbs both ultraviolet and infrared radiation, and as ozone perturbations in the lower stratosphere cause a relatively large radiative effect27.

Figure 4: Stratospheric ozone decreases due to CH 2 Cl 2 . (a) Difference in zonal mean annual mean ozone (%) between run Scenario 1, assuming surface CH 2 Cl 2 continues to increase at the mean rate observed over the 2004–2014 period, and run no_CH 2 Cl 2 in 2050. (b) As a for Scenario 2, assuming CH 2 Cl 2 continues to increase at the mean rate observed over the 2012–2014 period. (c) Difference in zonal mean annual mean column ozone (DU) between run Scenario 1 and run no_CH 2 Cl 2 as a function of year. (d) As c for Scenario 2. Full size image

Although the severity of ozone loss over Antarctica is most strongly affected by the abundance of reactive halogens, inter-annual variability in temperature and dynamical influences that determine strength of the polar vortex provide additional influence. For example, relatively large levels of springtime (September–November) ozone have been observed (that is, less loss) following stratospheric warming events, such as in 2002 (ref. 28), during which the occurrence of polar stratospheric clouds was relatively low29. The sensitivity of the SH column ozone decrease due to CH 2 Cl 2 to a range of assumed stratospheric meteorology in the future period (see Methods) is shown in Fig. 5. In all cases, the impact of CH 2 Cl 2 is substantial, with the greatest ozone decreases towards the end of the century predicted under the assumed 2012 (base meteorology) and 2006 (relatively cold Antarctic winter) conditions. However, note, these CTM simulations did not explicitly consider climate-driven cooling of the upper stratosphere or changes in stratospheric dynamics (see below).