PDF《EDITORIAL》

Environ.

Res. Lett.

11 (2016)

120207 doi:10.1088/1748-9326/11/12/120207 EDITORIAL The growing role of methane in anthropogenic climate change M Saunois1 , R B Jackson2 , P Bousquet1 , B Poulter3 and J G Canadell4

1 Laboratoire des Sciences du Climat et de l'

Environnement, LSCE-IPSL (CEA-CNRS-UVSQ), Université Paris-Saclay, F-91191 Gif-sur- Yvette, France

2 Department of Earth System Science, Woods Institute for the Environment, and Precourt Institute for Energy, Stanford University, Stanford, CA 94305-2210, USA

3 Institute on Ecosystems and Department of Ecology, Montana State University, Bozeman, MT 59717, USA

4 Global Carbon Project, CSIRO Oceans and Atmosphere, Canberra, ACT 2601, Australia E-mail: [email protected] Abstract Unlike CO2, atmospheric methane concentrations are rising faster than at any time in the past two decades and, since 2014, are now approaching the most greenhouse-gas-intensive scenarios. The reasons for this renewed growth are still unclear, primarily because of uncertainties in the global methane budget. New analysis suggests that the recent rapid rise in global methane concentrations is predominantly biogenic-most likely from agriculture-with smaller contributions from fossil fuel use and possibly wetlands. Additional attention is urgently needed to quantify and reduce methane emissions. Methane mitigation offers rapid climate bene?ts and economic, health and agricultural co- bene?ts that are highly complementary to CO2 mitigation. Introduction Atmospheric methane (CH4) has experienced puz- zling dynamics over the past

15 years. After a period of relative stagnation in the early

2000 s (+0.5?± 3.1 ppb yr?1 increase on average for 2000C2006), atmospheric methane concentrations have increased rapidly since

2007 at more than ten times this rate (+6.9?±?2.7 ppb yr?1 for 2007C2015;

?gure

1 top left;

Dlugokencky 2016). The atmospheric growth rate of methane accelerated to?+12.5 ppb in

2014 and?+9.9 ppb in 2015, reaching an annual average concentration of

1834 ppb in

2015 (Dlugokencky 2016). Because of this acceleration, the evolution of atmospheric methane over the last three years is inconsistent with the mitigation required in the Representative Concentration Pathways (RCP) of 2.5,

4 and

6 W m?2 and now most closely aligns with the RCP 8.5 W m?2 (?gure

1 top left) (Fujino et al 2006, Clarke et al 2007, Riahi et al 2007, van Vuuren et al 2007). This emerging dynamic highlights methane'

s growing contribution to global warming relative to the observed slower growth rates of CO2 over the past three years (Le Quéré et al 2016, ESSD;

?gure

1 top right, Jackson et al 2016) and a relatively constant growth rate of nitrous oxide (N2O) (Hartmann et al 2013). The global methane budget The balance of surface sources and sinks determines the global methane budget. Surface sources include methane originating from biogenic (wetlands, lakes, agriculture, waste/land?ll, permafrost), thermogenic (fossil fuel usage and natural seeps), pyrogenic (biomass and biofuel burning) or mixed (hydrates, geological) sources. Dominant sinks include methane oxidation by the hydroxyl radical (OH) and other radicals in the atmosphereaswellasmethanotrophyindrysoils.Based on a new ensemble of atmospheric studies, global emissions are estimated at

559 [540C568] Tg CH4.yr?1 for the 2003C2012 decade (Saunois et al 2016). Tropical sources, including both natural and anthropogenic sources represent two-thirds of total global emissions and are dominated by emissions from wetlands (?gure 2). Approximately two-thirds of global emis- sions are also attributable to anthropogenic activities, includingthose fromboth mid-latitudesand thetropics (e.g.,agriculture andwaste, ?gure 2). Changes in the methane budget since

2007 Despite substantial knowledge about the location, size and trends of methane sources and sinks, the relative OPEN ACCESS PUBLISHED

12 December

2016 Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. ?

2016 IOP Publishing Ltd contributions explaining the recent atmospheric increase remain uncertain (e.g. Nisbet et al 2014, 2016). Based on activity data and emission factors from various anthropogenic sectors, bottom-up inventories of anthropogenic emissions estimate an increase of fossil-related emissions of 3C4 Tg each year since

2007 (EPA 2012;

EDGAR 2014). Using ethane measurements and methane-to-ethane ratios, Hauss- mann et al (2016) also suggest a substantial contrib- ution of fossil-related emissions (18%C73% of the total increase in atmospheric methane).

13 CH4 iso- topic observations show a signi?cant depletion of

13 C in the atmosphere (??0.12‰ in seven years), suggest- ing that increases in methane emissions after

2006 are primarily biogenic and are more consistent with sources from agriculture than natural wetlands (Nisbet et al 2016;

Schaefer et al 2016). Recent bottom-up inventories estimate an increase in agricultural annual emissions of 3C5 Tg between

2006 and 2012, mostly from Africa and Asia, whereas wetland emissions were estimated to be mostly unchanged between

2006 and

2012 (Poulter et al 2016). Meanwhile, biomass burning emissions decreased by 2C3 Tg yr?1 between

2007 and

2013 compared to

2000 and 2006, although the recent El Ni?o conditions have lead to abnormally large peat ?res in Indonesia (Van der Werf et al 2016). Not accounting for this long-term decrease in the

13 C- heavy methane source from biomass burning, and based on

13 C atmospheric observations and on an enriched database for isotopic source signatures, Schwietzke et al (2016) even ?nd decreasing fossil fuel emissions since 2000, a different conclusion than in most other recent studies. Sinks may also be playing a role in the rapid rise in atmospheric methane over the last decade (?gure 1). Using a chemistry-transport model run over

40 years, Dals?ren et al (2016) infer a stabilization of OH con- centrations after 2006, in contrast to a total 3% increase since the late 1990s (8% since the 1970s). Stabilized OH concentrations can increase methane lifetimes and likely help explain the atmospheric methane increase as well, as a decrease of 1% in atmospheric OH concentra- tions is roughly equivalent to ?5 Tg yr?1 of increased methaneemissions (e.g.Saunoisetal2016). These various factors notwithstanding, there is no consensus scenario of methane sources and sinks that explains the atmospheric increase since

2007 (Kirschke et al 2013). Recent evidence from atmospheric observa- tions suggests three main contributors for emission changes. The ?rst element is an increase in biogenic emissions, mostly from agriculture (13 C compatible, Schaefer et al 2016). The second is an increase of fossil- Figure 1. Top: projections of atmospheric methane concentrations (left, ppb) and carbon dioxide concentrations (right, ppm) for the four Representative Concentration Pathway (RCP) scenarios and observed globally averaged atmospheric abundance at marine boundary layer sites from the NOAA network (black, Dlugockenky 2016). Tropospheric concentrations from RCP model have been scaled to ?t surface observations. Bottom: projections of methane (left) and carbon dioxide (right) anthropogenic emissions. For methane, four harmonized RCP scenarios are plotted together with the EDGARv4.2FT2012, USEPA and GAINS-ECLIPSE5a inventories. For carbon dioxide, four harmonized RCP scenarios are plotted together with the recent EDGARv4.3FT2014, and CDIAC estimates for fossil and cement-production emissions. RCP concentration data are from Meinshausen et al (2011). Concentrations and emissions from RCP4.5 are above those of RCP6 before 2030.

2 Environ. Res. Lett.

11 (2016)

120207 M Saunois et al related emissions (ethane-compatible, Haussman et al 2016). The third is a decrease of biomass burning emis- sions (13 C compatible and cancelling a fossil fuel increase, Van der Werf et al 2016). The necessity of an anthropogenic emission increase can still be reduced by a possible stagnation of OH concentrations or by regio- nal contributions from wetland emissions, such as emis- sions ?uctuations resulting from drought conditions in SouthAmerica(e.g.in2010C2011,Bassoetal2016). At the regional scale, methane emissions contribut- ing to the observed atmospheric increase since

2006 are most likely tropical, although some mid-latitude regions, such as China, also appear to contribute to the increase (e.g. Bergamaschi et al 2013). To date, no sig- ni?cant contribution to the atmospheric increase from Arctic regions has been found, except in

2007 and attri- butable then to a relatively warm and late summer (Dlugokencky et al 2011). Contrary to a recent estimate based on three different atmospheric inversions (Turner et al 2016), no trend in US methane emissions is found in the ensemble of inversions gathered in Sau- nois et al (2016), and thus a substantial contribution of US shale gas industry to the recent methane atmo- sphericincrease seemsunlikely(Bruhwiler etal........