The growing role of methane in anthropogenic climate change

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 benefits and economic, health and agricultural co-benefits that are highly complementary to CO2 mitigation.


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/landfill, 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 atmosphere as well as methanotrophy in soils. Based on a new ensemble of atmospheric studies, global emissions are estimated at 559 [540-568] Tg CH 4 .yr −1 for the 2003-2012 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 (figure 2). Approximately two-thirds of global emissions are also attributable to anthropogenic activities, including those from both mid-latitudes and the tropics (e.g., agriculture and waste, figure 2).

Changes in the methane budget since 2007
Despite substantial knowledge about the location, size and trends of methane sources and sinks, the relative contributions explaining the recent atmospheric increase remain uncertain (e.g. Nisbet et al 2014Nisbet et al , 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 3-4 Tg each year since 2007 (EPA 2012; EDGAR 2014). Using ethane measurements and methane-to-ethane ratios, Haussmann et al (2016) also suggest a substantial contribution of fossil-related emissions (18%-73% of the total increase in atmospheric methane). 13 CH 4 isotopic observations show a significant depletion of 13 C in the atmosphere (∼−0.12‰ in seven years), suggesting that increases in methane emissions after 2006 are primarily biogenic and are more consistent with sources from agriculture than natural wetlands (Nisbet Sinks may also be playing a role in the rapid rise in atmospheric methane over the last decade (figure 1). Using a chemistry-transport model run over 40 years, Dalsøren et al (2016) infer a stabilization of OH concentrations 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 may help explain the atmospheric methane increase as well, as a decrease of 1% in atmospheric OH concentrations is roughly equivalent to ∼5 Tg yr −1 of increased methane emissions (e.g. Saunois et al 2016).
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 observations suggests three main contributors for emission changes. The first element is an increase in biogenic emissions, mostly from agriculture ( 13 C compatible, Schaefer et al 2016). The second is an increase of fossil- Strategies to reduce uncertainties on the methane budget Scientific breakthroughs are needed to predict methane emissions today and in the future, particularly with a changing climate. First, annual to decadal CH 4 emissions from natural wetlands and other inland water systems are highly uncertain. The sum of all natural methane sources as inferred by process-based bottomup modelling is too large by about 30% compared to the constraint provided by methane atmospheric mixing ratios. The strategy to address this issue requires developing and synthesizing (i) direct methane flux measurements in the field to constrain the parametrizations of land surface models similarly to Fluxnet-CO 2 , (ii) process-based models for lakes, rivers, and permafrost methane emissions (e.g., Tan and Zhuang 2015 for lakes), and (iii) dynamic global high resolution maps (50-100 m) with all inland water surfaces consistently categorized to avoid double counting emitting surfaces (Yamazaki et al 2015).
Second, the partitioning of CH 4 emissions and sinks by region and process needs to be better constrained by atmospheric observations and process-based models. Beyond the recurring need for a broader network of methane observations, it is essential (i) to extend observations of tracers more specific to individual methane sources and sinks such as methane isotope concentra-  Climate Model Initiative (CCMI) update of Lamarque et al 2013) and CMIP6 simulations scheduled for the next IPCC report. Breakthrough technologies already allow high precision measurements of methane and its isotopes at the surface, for instance using cavity ring down spectrometers such as in Maher et al (2014). Future LIDAR measurements from space will provide the first low-bias global estimate of methane atmospheric columns all year round beginning in ∼2020 (Kiemle et al 2014). The partitioning of emissions will also benefit from efforts to improve and regularly update anthropogenic inventories.
Third, uncertainties in the modelling of atmospheric transport and chemistry limit the optimal assimilation of atmospheric observations and increase the uncertainties of the inversion-derived flux estimates. Key steps should include the improvement of OH fields and other methane sinks (e.g.,

Mitigation opportunities
Despite important uncertainties in methane sources and sinks, the recent increase in methane concentrations suggests a dominant anthropogenic contribution (either biogenic or thermogenic). Methane therefore offers growing opportunities for climate change mitigation that could allow a return to lower emission trajectories such as RCP6 or RCP4.5. Because of methane's high global warming potential and short lifetime in the atmosphere compared to CO 2 , its mitigation offers the possibility to slow climate change efficiently in a shorter time horizon. In addition to climate benefits, reducing methane emissions could help improve human health and crop production through simultaneous reductions in ozone production (West et al 2013; Shindell 2016) and provide business and employment opportunities. A diverse set of strategies already exists, as proposed by multilateral partnerships such as the Global Methane Initiative (www.globalmethane.org) and the Climate and the Clean Air Coalition (www.ccacoalition.org), and supported further by the G7 Leaders Declaration in May 2016 (www.whitehouse.gov/the-press-office/2016/ 05/27/g7-ise-shima-leaders-declaration) to 'recognize the importance of mitigating emissions of shortlived climate pollutants'. These opportunities include (i) venting and flaring of methane in coal-mines, while also improving worker safety, (ii) detecting and removing natural gas leaks, from wellpads upstream through the distribution chain downstream (e.g., McKain et al 2015), (iii) covering landfills, which reduces methane emissions while producing biogas for energy and transport usage, and (iv) developing farm bio-digesters, which has been extensively applied in Germany and is spreading to other European countries (e.g., Lebuhn et al 2014). Other strategies are being developed but need more research on potential unintended consequences. For example, modifying ruminants' diet (e.g., linseed fed) to limit methane emissions is currently being examined but needs evaluation against the quality of meat and milk (e.g., Marette and Millet 2014) and against emissions of other greenhouse gases such as N 2 O. Modification of rice agriculture practices (e.g., semi-inundated paddies, dry cultivation) is well tested and promising, assuming yield and quality of the staple food for more than 3 billion people can be guaranteed (e.g., Sun et al 2016). Such mitigation policies in the agriculture and waste sectors are key to reducing methane emissions in most of the high emitting regions (figure 2).

Conclusions
Methane appears to play an increasing role in on-going anthropogenic climate change, particularly in light of the slowdown of CO 2 fossil fuel emissions over the past three years ( figure 1, bottom right). Methane emissions from increasing agricultural activities seem to be a major, possibly dominant, cause of the atmospheric growth trends of the past decade (e.g., Herrero et al 2016). The rapid increase in methane concentrations offers a growing mitigation opportunity, acknowledging the need to balance food security and environmental protection (Wollenberg et al 2016). Keeping global warming below 2°C is already a challenging target, with most of the attention placed primarily on CO 2 emissions. Such a target will become increasingly difficult if reductions in methane emissions are not also addressed strongly and rapidly.