Interpreting contemporary trends in atmospheric methane

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Abstract

Atmospheric methane plays a major role in controlling climate, yet contemporary methane trends (1982–2017) have defied explanation with numerous, often conflicting, hypotheses proposed in the literature. Specifically, atmospheric observations of methane from 1982 to 2017 have exhibited periods of both increasing concentrations (from 1982 to 2000 and from 2007 to 2017) and stabilization (from 2000 to 2007). Explanations for the increases and stabilization have invoked changes in tropical wetlands, livestock, fossil fuels, biomass burning, and the methane sink. Contradictions in these hypotheses arise because our current observational network cannot unambiguously link recent methane variations to specific sources. This raises some fundamental questions: ( i) What do we know about sources, sinks, and underlying processes driving observed trends in atmospheric methane? ( ii) How will global methane respond to changes in anthropogenic emissions? And ( iii), What future observations could help resolve changes in the methane budget? To address these questions, we discuss potential drivers of atmospheric methane abundances over the last four decades in light of various observational constraints as well as process-based knowledge. While uncertainties in the methane budget exist, they should not detract from the potential of methane emissions mitigation strategies. We show that net-zero cost emission reductions can lead to a declining atmospheric burden, but can take three decades to stabilize. Moving forward, we make recommendations for observations to better constrain contemporary trends in atmospheric methane and to provide mitigation support.

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Simultaneously mitigating near-term climate change and improving human health and food security.

Drew Shindell, Johan Kuylenstierna, Elisabetta Vignati … (2012)

Tropospheric ozone and black carbon (BC) contribute to both degraded air quality and global warming. We considered ~400 emission control measures to reduce these pollutants by using current technology and experience. We identified 14 measures targeting methane and BC emissions that reduce projected global mean warming ~0.5°C by 2050. This strategy avoids 0.7 to 4.7 million annual premature deaths from outdoor air pollution and increases annual crop yields by 30 to 135 million metric tons due to ozone reductions in 2030 and beyond. Benefits of methane emissions reductions are valued at $700 to $5000 per metric ton, which is well above typical marginal abatement costs (less than $250). The selected controls target different sources and influence climate on shorter time scales than those of carbon dioxide-reduction measures. Implementing both substantially reduces the risks of crossing the 2°C threshold.

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Three-dimensional model synthesis of the global methane cycle

L. P. Steele, I Fung, J. John … (1991)

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Is Open Access

The global methane budget 2000–2012

Marielle Saunois, Philippe Jean Bousquet, Ben Poulter … (2016)

The global methane (CH 4 ) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH 4 over the past decade. Emissions and concentrations of CH 4 are continuing to increase, making CH 4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH 4 sources that overlap geographically, and from the destruction of CH 4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (∼ biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003–2012 decade, global methane emissions are estimated by top-down inversions at 558 Tg CH 4  yr −1 , range 540–568. About 60 % of global emissions are anthropogenic (range 50–65 %). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher than all other RCP scenarios. Bottom-up approaches suggest larger global emissions (736 Tg CH 4  yr −1 , range 596–884) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the top-down budget, it is likely that some of the individual emissions reported by the bottom-up approaches are overestimated, leading to too large global emissions. Latitudinal data from top-down emissions indicate a predominance of tropical emissions (∼ 64 % of the global budget, < 30° N) as compared to mid (∼ 32 %, 30–60° N) and high northern latitudes (∼ 4 %, 60–90° N). Top-down inversions consistently infer lower emissions in China (∼ 58 Tg CH 4  yr −1 , range 51–72, −14 %) and higher emissions in Africa (86 Tg CH 4  yr −1 , range 73–108, +19 %) than bottom-up values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for top-down inversions than for bottom-up inventories and models. The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute 30–40 % on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center ( http://doi.org/10.3334/CDIAC/GLOBAL_METHANE_BUDGET_2016_V1.1 ) and the Global Carbon Project.

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Journal

Journal ID (nlm-ta): Proc Natl Acad Sci U S A

Journal ID (iso-abbrev): Proc. Natl. Acad. Sci. U.S.A

Journal ID (hwp): pnas

Journal ID (pmc): pnas

Journal ID (publisher-id): PNAS

Title: Proceedings of the National Academy of Sciences of the United States of America

Publisher: National Academy of Sciences

ISSN (Print): 0027-8424

ISSN (Electronic): 1091-6490

Publication date (Print): 19 February 2019

Publication date (Electronic): 07 February 2019

Publication date PMC-release: 07 February 2019

Volume: 116

Issue: 8

Pages: 2805-2813

Affiliations

[1] ^aDepartment of Earth and Planetary Sciences, University of California, Berkeley, CA 94720;

[2] ^bDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91226;

[3] ^cJet Propulsion Laboratory, California Institute of Technology , Pasadena, CA 91109;

[4] ^dClimate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109

Author notes

²To whom correspondence may be addressed. Email: alexjturner@ 123456berkeley.edu , cfranken@ 123456caltech.edu , or eakort@ 123456umich.edu .

Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved January 9, 2019 (received for review August 18, 2018)

Author contributions: A.J.T., C.F., and E.A.K. designed research, performed research, analyzed data, and wrote the paper.

¹A.J.T., C.F., and E.A.K. contributed equally to this work.

Author information

Alexander J. Turner http://orcid.org/0000-0003-1406-7372

Christian Frankenberg http://orcid.org/0000-0002-0546-5857

Article

Publisher ID: 201814297

DOI: 10.1073/pnas.1814297116

PMC ID: 6386658

PubMed ID: 30733299

SO-VID: 389139fc-60d7-4e68-a733-8e02789b2b82

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This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

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Pages: 9

Funding

Funded by: Adolph C. and Mary Sprague Miller Institute for Basic Research in Science, University of California Berkeley (MIBRS) 100007247

Interpreting contemporary trends in atmospheric methane

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Most cited references 106

Simultaneously mitigating near-term climate change and improving human health and food security.

Three-dimensional model synthesis of the global methane cycle

The global methane budget 2000–2012

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