Recent and future trends in synthetic greenhouse gas radiative forcing

Atmospheric measurements show that emissions of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons are now the primary drivers of the positive growth in synthetic greenhouse gas (SGHG) radiative forcing. We infer recent SGHG emissions and examine the impact of future emissions scenarios, with a particular focus on proposals to reduce HFC use under the Montreal Protocol. If these proposals are implemented, overall SGHG radiative forcing could peak at around 355mWm!2 in 2020, before declining by approximately 26% by 2050, despite continued growth of fully fluorinated greenhouse gas emissions. Compared to “no HFC policy” projections, this amounts to a reduction in radiative forcing of between 50 and 240mWm!2 by 2050 or a cumulative emissions saving equivalent to 0.5 to 2.8 years of CO2 emissions at current levels. However, more complete reporting of global HFC emissions is required, as less than half of global emissions are currently accounted for. 1. Monitoring Global Trends in Synthetic Greenhouse Gases In 2012, the major long-lived synthetic greenhouse gases (gases with no significant natural sources and lifetimes of at least 1 year) were responsible for 350 ± 10mWm!2 of direct radiative forcing (RF), 19% as large as the increase in RF due to CO2 since the preindustrial era [Hall et al., 2012] (note that the uncertainty in SGHG RF excludes radiative transfer assumptions, which are estimated to be of the order of 10% [Forster et al., 2007]). This group consists of a few tens of compounds, which we break down into the following groups (in the order of their current contribution to RF): chlorofluorocarbons (CFCs) and other strongly ozone depleting substances (ODS) (which include CCl4 and CH3CCl3 here), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), fully fluorinated GHGs (FFGHGs) (consisting of perfluorocarbons, SF6, and NF3), and SO2F2. The CFCs and their replacements, the HCFCs and HFCs, are primarily used in refrigeration, air conditioning, and foam blowing [e.g., Montzka et al., 2011]. FFGHGs are emitted during aluminum manufacture (primarily CF4) and are used in a range of applications such as electrical insulation (e.g., SF6) or semiconductor manufacture (e.g., NF3 and C2F6) [Weiss et al., 2008; Mühle et al., 2010; Rigby et al., 2010; Arnold et al., 2013]. Some gases, such as HFC-23 (CHF3), have little practical use but are released to the atmosphere as unwanted by-products during certain industrial processes [Miller et al., 2010; Miller and Kuijpers, 2011]. Despite being present in the atmosphere at levels of only a few hundred parts per trillion or less, synthetic greenhouse gases (SGHGs) have a significant climate impact because of their very high radiative efficiencies and, in many cases, very long lifetimes (tens to thousands of years) [Ravishankara et al., 1993; Forster et al., 2007]. Here we examine recent trends in 25 of the most abundant SGHGs measured by the Advanced Global Atmospheric Gases Experiment (AGAGE) [Prinn et al., 2000]: CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CCl4, CH3CCl3, HCFC-22, HCFC-141b, HCFC-142b, HFC-23, HFC-32, HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC227ea, HFC-245fa, HFC-365mfc, CF4, C2F6, C3F8, SF6, NF3, and SO2F2. There are some SGHG with an RF known to be higher than some of the more minor members of this list, which are not included here because AGAGE measurements are not yet available for all or part of the time period investigated. Examples of these compounds include c-C4F8 [Oram et al., 2012], some halons [Fraser et al., 1999], and some minor CFCs and HCFCs [e.g., Culbertson et al., 2004; Maione et al., 2013]. Measurements have also been made of SGHGs RIGBY ET AL. ©2014. American Geophysical Union. All Rights Reserved. 2623 PUBLICATIONS Geophysical Research Letters