Bottom-up global HFC-23 emissions

Inventory-based (bottom-up) estimates of global HFC-23 emissions can be derived from emissions reported to the UNFCCC and from reports of HCFC-22 production submitted to the United Nations Environment Programme (UNEP), combined with emissions factors (i.e. emissions to the atmosphere of HFC-23 per tonne of HCFC-22 produced).

We compiled a developing country no abatement bottom-up HFC-23 emissions estimate (Supplementary Table 1) based on HCFC-22 production data obtained from the UNEP HCFC database (https://ozone.unep.org/countries) (Supplementary Table 2) and available time-varying emissions factors6,9,10,11 for developing nations (defined here as Article 5 countries under the Montreal Protocol and non-Annex I under the UNFCCC, including Israel). For developed countries (non-Article 5 countries under the Montreal Protocol and Annex I under the UNFCCC, including Turkey but excluding Israel), HFC-23 emission estimates were taken from the 2019 National Inventory Reports (NIR) submitted to the UNFCCC12 (Supplementary Table 1). Next, we compiled a developing country with abatement bottom-up estimate (Supplementary Table 1), which also includes reported HFC-23 emission reductions from the UNFCCC CDM (compiled data in Supplementary Table 3) between 2006 and 2014, whereby developing nations could provide Certified Emission Reduction (CER) credits for the destruction of HFC-23 by-product, which were then traded with developed countries to meet their emission reduction targets1,6. The level of abatement of HFC-23 reported under the CDM dropped and reached zero by the end of 2014. After 2015, expected abatement from developing countries is dominated by reported emissions reduction by China and India9,10,13. Under the Chinese HCFC production phase-out management plan (HPPMP), China reported a reduction of 45, 93 and 98% of total HFC-23 emissions in 2015, 2016 and 2017, respectively10. In India, an executive order (Control of emission/venting of Hydrofluorocarbon (HFC)-23, produced as by product while manufacturing of Hydrochlorofluorocarbon (HCFC)-22, in the atmosphere)14 issued by the Indian government on 13 October 2016 required all producers of HCFC-22 to destroy by-product HFC-23 via incineration using efficient and proven technologies9. For simplicity, we collectively referred to these measures as post-2015 abatement.

Total HCFC-22 production has increased substantially between 1990 (65 Gg yr\({}^{-1}\)) and 2017 (947 Gg yr\({}^{-1}\); Fig. 1b). Since 2005, HCFC-22 production has been dominated by developing countries. From 2013, a freeze in HCFC production for dispersive use has been in place under the Montreal Protocol15. However, a small increase from developing countries has been reported, from 675 Gg yr\({}^{-1}\) in 2013 to 725 Gg yr\({}^{-1}\) in 2017. For developed countries, HCFC production and consumption are due to be completely phased-out by 2030, with a 99.5% reduction expected by 2020 from the baseline year (1989)16. Notwithstanding this schedule, a small increase in total production has been reported from developed countries, from 208 Gg yr\({}^{-1}\) in 2016 to 222 Gg yr\({}^{-1}\) in 2017. Similarly, HFC-23 emissions reported to the UNFCCC (2019 NIR; aggregated values in Supplementary Table 1) show increases from 1.0 Gg yr\({}^{-1}\) in 2016 to 1.8 Gg yr\({}^{-1}\) in 2017. This increase in emissions is driven by fluorochemical production, predominantly from Russia (0.6 Gg yr\({}^{-1}\) in 2016 to 1.2 Gg yr\({}^{-1}\) in 2017) and the USA (0.2 Gg yr\({}^{-1}\) in 2016 to 0.4 Gg yr\({}^{-1}\) in 2017), possibly signalling a recent increase in production for non-dispersive HCFC-22 uses.

Fig. 1: Global HFC-23 emissions and HCFC-22 production. a Top-down global HFC-23 emissions (blue line) and uncertainties (blue shaded area; 1\(\sigma\), incorporating uncertainties due to the prior constraints, measurements, model representations of the data, calibration scale and HFC-23 lifetime, see Methods section) derived from Advanced Global Atmospheric Gases Experiment (AGAGE) data and the 12-box model (inferred emissions available in Supplementary Table 4). All other lines represent global (dashed) or sub-global (dotted) bottom-up estimates: developing countries emissions estimates (HCFC-22 production multiplied by an emissions factor (EF)) are shown in dark green; developed countries emissions obtained from United Nations Framework Convention on Climate Change (UNFCCC) reports are shown in black; the developing countries no abatement global total emissions estimates (sum of emissions from the unabated developing countries estimates and developed countries reports) are shown in orange; the developing countries with abatement global estimates are shown in green (equivalent to the developing countries no abatement estimates but with clean development mechanism (CDM) and post-2015 reports subtracted); the red dotted line is equivalent to the developing countries no abatement estimates, but with the maximum reported CDM abatement amount subtracted each year after 2011 (9.0 Gg yr\({}^{-1}\)), thus estimating the maximum possible legacy of the CDM. The hatched and cross-hatched areas represent the CDM and post-2015 abatement (Chinese and Indian reported emissions reductions combined), respectively. b Total HCFC-22 production (dashed orange line) from developing countries (dark green dotted line) and developed countries (black dotted line). All data points are shown as the mid-point of the respective year. Full size image

China and India, the two largest current producers of HCFC-22, with 2017 reported production of 645 Gg (68% of the global total) and 65 Gg (7%), respectively, have reported actions to dramatically reduce their emissions9,10,14. In the period between the cessation of Indian CDM projects (end of 2013) and 13 October 2016, Indian producers of HCFC-22 were allowed to vent HFC-23 by-product to the atmosphere. Indeed, emissions inferred from aircraft observations during the summer of 2016 suggest that a substantial fraction of India’s HCFC-22 production-related emissions were likely unabated at that time13. Collectively, the post-2015 abatement reported by both countries is shown in the cross-hatched area in Fig. 1a. We estimate that the HFC-23 emissions reductions reported under China’s HPPMP are 6.1, 13.0 and 15.2 Gg yr\({}^{-1}\) in 2015, 2016 and 2017, respectively, whilst India’s reductions correspond to 0, 0.4 and 1.9 Gg yr\({}^{-1}\) for these years (Fig. 1a). Therefore, combined emission reductions should have totalled 6.1, 13.4 and 17.1 Gg yr\({}^{-1}\) in 2015, 2016 and 2017, respectively, leading to a global total emission rate of 2.4 Gg yr\({}^{-1}\) by 2017.

Evaluating global emissions using atmospheric observations

We provide an update on global HFC-23 emissions through to the end of 2018, based on in situ HFC-23 measurements from the five core Advanced Global Atmospheric Gases Experiment (AGAGE) stations17,18, located in remote sampling locations at Mace Head (Ireland), Trinidad Head (California, USA), Ragged Point (Barbados), Cape Matatula (American Samoa) and Cape Grim (Tasmania, Australia), in conjunction with the AGAGE 2-D atmospheric 12-box model and a Bayesian inversion methodology (see Methods section)6. Our estimates are consistent with the most recent prior study, which covered the period until 20156, showing an overall increase in HFC-23 emissions over the previous three decades, but with a substantial decline during the CDM period and a small drop between 2014 and 2015. The AGAGE data show a renewed increase in the HFC-23 growth rate from 2016, which reached 1.1 \(\pm\) 0.05 pmol mol\({}^{-1}\) yr\({}^{-1}\) in 2018, when global annual mean mole fractions were 31.1 parts per trillion (pmol mol\({}^{-1}\); Fig. 2). In contrast, a forward model run using our developing country with abatement estimate suggests that the global mean mole fraction growth rate should have declined to less than zero after around 2016 (Supplementary Fig. 1). Our top-down emissions estimate shows that the observed growth has been driven by an increase in HFC-23 emissions from 2016 to a new maximum of 15.9 \(\pm\) 0.9 Gg yr\({}^{-1}\) in 2018 (Fig. 1a), despite the aforementioned reported emission reductions post-2015.

Fig. 2: HFC-23 observations, model-derived mole fractions and growth rates. a Modelled atmospheric HFC-23 mole fractions in pmol mol\({}^{-1}\) for the four equal-mass latitudinal subdivisions in the 12-box model based on in situ measurements at the core background measurement sites (points), northern hemispheric flask samples (blue circles; only shown prior to 2007) and Cape Grim Air Archive data (orange circle, only shown prior to 2007). b Model-derived annual HFC-23 growth rates (global - blue solid line with 1-sigma uncertainty indicated by shading; dashed lines show semi-hemispheric growth rates) in pmol mol\({}^{-1}\) yr\({}^{-1}\). Full size image

Discrepancies between top-down and bottom-up estimates

In common with previous studies, our bottom-up HFC-23 estimate, based on HCFC-22 production and UNFCCC reports, is in good agreement with emissions inferred from atmospheric observations prior to the CDM period (2006)6,8. During the CDM period, the measurement-derived emissions show a decline to a minimum in 2009, as expected from CDM reports. Between 2009 and 2012, both top-down and bottom up (with developing country abatement) HFC-23 emissions estimates increased and were in good agreement, within the uncertainty of the top-down estimate.

After 2012, our top-down estimates grow more slowly than would be expected from HCFC-22 production and the decline in reported CDM abatement (Fig. 1a); CDM abatement declines to zero by the end of 2014, resulting in our abatement and no-abatement bottom-up estimates nearly converging at 20.8 Gg yr\({}^{-1}\), compared to the top-down estimate of 14.5 Gg yr\({}^{-1}\). This corresponds to a maximum discrepancy between top-down and bottom-up with abatement of 6.3 Gg yr\({}^{-1}\) in 2014. If the full abatement capacity installed during the CDM had continued, our bottom-up estimate would be substantially lower in 2014; if we assume that the maximum amount abated during the CDM (9.0 Gg yr\({}^{-1}\) in 2011) had continued (red dotted line in Fig. 1a), our bottom-up estimate would be around 11.8 Gg yr\({}^{-1}\) in 2014. Based on these considerations, we suggest that the growth in top-down emissions between 2012 and 2014 can be explained by new emissions from newly installed, at least partly unabated, HCFC-22 production capacity, combined with the switching off of some, but not all, abatement that was installed during the CDM period (consistent with observations from one previous study in India13).

A previous study suggested that the decline in HFC-23 emissions between 2014 and 2016 was consistent with the timing of the reductions undertaken by China and reported under its HPPMP6. However, here we show that the magnitude of this decline in top-down emissions between 2014 and 2016 (around \(1.6\pm 1.6\) Gg (2\(\sigma\) uncertainty), or 11%) is substantially smaller than the anticipated emission reductions (around 13 Gg from China, which would represent around 72.4% of the 2014 top-down value, Fig. 3). We note that this period coincided with a drop in HCFC-22 production of 6.3%, and therefore, we propose that HFC-23 emissions may have largely followed this trend (Fig. 1a). Indeed, when HCFC-22 production increased substantially in 2017, global top-down HFC-23 emissions also re-bounded. In contrast, when reported reductions from China and India are considered together with developed country reports to the UNFCCC (hatched area in Fig. 1a), our bottom-up estimate of global total HFC-23 emissions for 2017, 2.4 Gg yr\({}^{-1}\), is 12.5 \(\pm\) 0.7 Gg yr\({}^{-1}\) lower than our top-down value. Global emissions continued to grow in 2018. Therefore, it seems likely that the HFC-23 emission reductions reported since 2015 have not been successfully implemented until at least the beginning of 2019, or there is substantial, unreported production of HCFC-22 from which HFC-23 is vented, or some combination thereof.