PerspectiveClimate Change

Rising methane: A new climate challenge

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Science  07 Jun 2019:
Vol. 364, Issue 6444, pp. 932-933
DOI: 10.1126/science.aax1828

In 2007, the amount of methane in the atmosphere (CH4) began to rise after a 7-year period of near-zero growth (1). Recent research shows that a second step change occurred in 2014 (2). From 2014 to at least the end of 2018, the amount of CH4 in the atmosphere increased at nearly double the rate observed since 2007 (see the figure). Because CH4 is a potent greenhouse gas, rising atmospheric CH4 presents a major challenge to achieving the goals set out in the Paris Agreement, an international consensus to limit temperature increase to 2°C or, if possible, to 1.5°C above preindustrial levels.

Methane trends

Data from U.S. National Oceanic and Atmospheric Administration observing stations show that global mean atmospheric CH4 started to rise in 2007, with a sharper increase beginning in 2014 (2).

GRAPHIC: N. CARY/SCIENCE; (DATA) HTTPS://WWW.ESRL.NOAA.GOV/GMD/DV/DATA

The causes for the recent rise in atmospheric CH4 remain a subject of scientific debate, even for the initial period of increase from 2007 to 2014 (18). Process-based estimates of CH4 emissions from inventory data, wetland models, and other information offer conflicting explanations, but measurements of the distribution of CH4 in the atmosphere and its 13C/12C isotopic ratio at a global network of stations hold clues.

Although CH4 has been rising across the globe, this growth has been largest in the midlatitudes and tropics of the Northern Hemisphere (2, 3). Further, the proportion of 13C in atmospheric CH4 has declined as atmospheric CH4 has risen (see the figure) (1, 2). The 13C/12C ratio in CH4 depends on the sources of the CH4 emissions. Release from biogenic sources (such as wetlands and agriculture) reduces the proportion of 13C in atmospheric CH4, whereas fossil emissions slightly increase this proportion and biomass burning emissions increase it strongly (1, 2). On the basis of selected CH4 and 13C/12C time series from four latitudinal bands, a multibox atmospheric model, and a running-budget analysis, Nisbet et al. (2) identified three potential pathways consistent with both the CH4 and isotope data: a surge in biogenic emissions, a decrease in the amount of CH4 destroyed in the atmosphere through CH4 oxidation, and an increase in fossil fuel emissions if balanced by a decrease in biomass burning.

Recent studies have identified source and sink processes that can explain part of the rise, but no single process can simultaneously account for the sudden onset of the rise and the steadiness of the increase, while remaining consistent with other available data. The most likely scenario is a combination of processes.

Biogenic emissions mainly come from wetlands and agriculture, particularly ruminant livestock. Multimodel wetland studies do not confirm an emission increase since 2007 (3, 9). However, livestock inventories show that ruminant emissions began to rise steeply around 2002 and can account for about half of the CH4 increase since 2007 (4).

CH4 is destroyed in the atmosphere by reaction with hydroxyl radicals (OH) and other atmospheric constituents. Reduced chemical destruction of CH4 could both increase atmospheric CH4 and decrease its proportion of 13C. Actual OH changes over the past years are controversial (5, 6), as is the role of sinks in global inversion studies (7, 8). Only extreme changes in all major sinks can cause the observed CH4 rise and still do not explain observed short-term variability (2), limiting the contribution that sinks are likely to make.

Increasing fossil emissions could explain the change, but a simultaneous reduction in 13C-rich emissions from biomass burning is required to balance the 13C/12C trends. Fire reconstructions using satellite observations support such a decline with an emissions drop around 2006 (10). The resulting 13C/12C balance restricts fossil fuels to half of total additional emissions since 2007. Coal mining in East Asia is universally recognized to contribute to the CH4 increase (2, 7, 8), whereas fossil CH4 emissions from North America remained flat despite a nearly 50% increase in natural gas production (11).

Coincident with the 2014 acceleration, Nisbet et al. find a source shift to the southern tropics, where wetlands are concentrated (2). They hypothesize that record high temperatures in 2014 and the following years spiked wetland CH4 production. Such a wetland climate feedback challenges the commonly held view that wetland area rather than temperature is the main control of wetland CH4 emissions (although some wetland CH4 models are more temperature driven) (10). If natural wetlands, or changes in atmospheric chemistry, indeed accelerated the CH4 rise, it may be a climate feedback that humans have little hope of slowing. Although studies have demonstrated the potential for substantial CH4-climate feedbacks, they were expected to occur gradually, not reaching the magnitude observed by Nisbet et al. for decades (12).


Embedded Image

Agriculture is thought to be responsible for over half of all anthropogenic CH4 emissions and may have contributed to the rise in CH4 since 2007.

PHOTO: SERGIO AZENHA/ALAMY STOCK PHOTO

While the scientific community continues to debate the causes of the CH4 surge, the consequences are clear. The latest Intergovernmental Panel on Climate Change (IPCC) emission scenarios that limit warming to 1.5°C assume that the amount of CH4 in the atmosphere will decrease by 35% between 2010 and 2050 (13). Yet, between 2007 and 2014, the amount has risen by an average of 5.7 parts per billion (ppb) per year, and by an average of 9.7 ppb per year since 2014. If this rise continues unabated, cuts to carbon dioxide and other greenhouse gases will need to be even steeper to achieve the Paris goal.

Atmospheric greenhouse gas measurements remain the fastest way to assess progress toward slowing climate change. More atmospheric observations are essential to understand the sources of rising CH4, particularly in the tropics, which appear to be the engines of this change. Atmospheric models informed by CH4 data will incorrectly attribute emission changes to regions poorly constrained by data, like equatorial ones (3).

Together with satellite observations and time series of additional tracers (14), a comprehensive global measurements network will be crucial to understand changes in CH4. Ascension Island in the South Atlantic is currently the only tropical site with observations of CH4, its 13C/12C ratio, and column CH4 measurements, which are indispensable for validating satellite observations. Yet, this site is in danger of being discontinued. Ongoing support for vitally important sites like Ascension Island, and establishing similar ones in other parts of the tropics, will be crucial for studies of CH4 trends.

Close integration between atmospheric observations, process-based studies, and policy is urgently needed to provide meaningful answers about the real emission reductions needed to meet the climate goals of the Paris Agreement. The World Meteorological Organization established the Integrated Global Greenhouse Gas Information System (IG3IS) to address this issue. IG3IS provides a bridge between the atmospheric greenhouse gas community and decisionmakers. Timely dialogue between these groups has never been more essential, as the window for achieving the goals of the Paris Agreement is rapidly closing.

References and Notes

Acknowledgments: This work was funded by the New Zealand Ministry of Business, Innovation, and Employment through contract C01X1817 and by NIWA through the Greenhouse Gases, Emissions and Carbon Cycle Science Programme. S.M.F. serves on the Scientific Steering Committee for the IG3IS. We thank E. Dlugokencky (NOAA) for post-2017 CH4 data and S. Michel and B. Vaughn (CU/INSTAAR) for post-2017 isotope data.

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