{"id":7216,"date":"2023-05-04T12:09:22","date_gmt":"2023-05-04T11:09:22","guid":{"rendered":"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/?p=7216"},"modified":"2023-05-04T12:09:22","modified_gmt":"2023-05-04T11:09:22","slug":"methane-origins-part-ii-the-millennium-methane-plateau-and-post-plateau-rise-2","status":"publish","type":"post","link":"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/2023\/05\/04\/methane-origins-part-ii-the-millennium-methane-plateau-and-post-plateau-rise-2\/","title":{"rendered":"Methane origins \u2013 part II: The millennium methane plateau and post-plateau rise"},"content":{"rendered":"<p>At the end of <a href=\"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/2023\/02\/24\/methane-an-origin-story\/\">part 1<\/a>, we touched upon how at the turn of the millennium and through the first half of the noughties, scientists working in emissions monitoring observed a plateau in what had been steadily increasing atmospheric methane concentrations. It was a phenomenon very much against the expected trend and the reasons behind it still generate scientific debate and controversy. So for part 2 of our methane origins story, Dr Semra Bakkaloglu turns ESI (Emissions Scene Investigator) to cast a magnifying glass over the evidence, and offers her perspective on what caused the plateau, and what\u2019s been happening since<strong>\u2026<\/strong><\/p>\n<hr \/>\n<h3><span class=\"NormalTextRun SCXW196952705 BCX9\">Since the beginning of the industrial revolution, the concentration of atmospheric methane has increased around threefold<\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">.\u00a0 <\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">We know this from a variety of scientific research efforts, including analysis of air trapped in ice cores which can be mapped to specific <\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">time-frames<\/span><span class=\"NormalTextRun SCXW196952705 BCX9\"> and the establishment, since <\/span><span class=\"NormalTextRun ContextualSpellingAndGrammarErrorV2Themed SCXW196952705 BCX9\">1983,<\/span> <span class=\"NormalTextRun SCXW196952705 BCX9\">of <\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">modern sampling and monitoring programmes for atmospheric methane concentration. This accumulation of data has corroborated an upward trend that was <\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">largely<\/span><span class=\"NormalTextRun SCXW196952705 BCX9\"> unabated<\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">.\u00a0 <\/span><span class=\"NormalTextRun SCXW196952705 BCX9\">That was until 1999 when the increase abruptly stopped.<\/span><\/h3>\n<p>It wasn\u2019t a short pause either. This plateau \u2013 where atmospheric methane concentrations more or less remained constant \u2013 went on for seven years. Only in 2006 was \u2018normal service resumed\u2019 and the relentless increase in concentration recommenced.<\/p>\n<figure id=\"attachment_7203\" aria-describedby=\"caption-attachment-7203\" style=\"width: 850px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" width=\"434\" height=\"317\" class=\"wp-image-7203\" src=\"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/files\/2023\/03\/Artboard-1.png\" alt=\"\" \/><figcaption id=\"caption-attachment-7203\" class=\"wp-caption-text\">Fig 1 &#8211; CH4 trend: This graph shows globally-averaged, monthly mean atmospheric methane abundance determined from marine surface sites since 1983. (NOAA Global Monitoring Laboratory)<\/figcaption><\/figure>\n<p>So was something, somewhere, absorbing the methane? Was it being degraded at a faster rate? Was it that we were producing less methane from anthropogenic sources such as agriculture or oil and gas production? Or had natural biogenic methane sources, for some reason, gone into decline?<\/p>\n<p>The 7-year plateau and renewed growth since have been the subject of conflicting explanations in the literature.<\/p>\n<p><a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/full\/10.1029\/2003GL018126\">Dlugokencky et al. (2003)<\/a> suggest that the stabilisation may be a new steady state for atmospheric methane, and it is the renewed growth that is anomalous.<\/p>\n<p>Others argue the increase in methane concentrations has continued for more than a decade, indicating that the plateau period from 1999 to 2006 could be seen as unusual (<a href=\"https:\/\/www.pnas.org\/doi\/full\/10.1073\/pnas.1814297116\">Turner et al. (2017)<\/a>).<\/p>\n<p>That leaves us with an intriguing question &#8211; which is the anomaly? The relentless growth since 2006 or the stable period of plateau.<\/p>\n<p><span class=\"TextRun SCXW150996878 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW150996878 BCX9\">So the focus of research should be on explaining the anomalous stabili<\/span><span class=\"NormalTextRun SCXW150996878 BCX9\">s<\/span><span class=\"NormalTextRun SCXW150996878 BCX9\">ation period and <\/span><span class=\"NormalTextRun SCXW150996878 BCX9\">identifying<\/span><span class=\"NormalTextRun SCXW150996878 BCX9\"> the sources of emissions that contribute to the continued rise of methane<\/span><span class=\"NormalTextRun SCXW150996878 BCX9\"> after stabilisation period<\/span><span class=\"NormalTextRun SCXW150996878 BCX9\">.<\/span><\/span><span class=\"EOP SCXW150996878 BCX9\" data-ccp-props=\"{&quot;201341983&quot;:0,&quot;335551550&quot;:6,&quot;335551620&quot;:6,&quot;335559739&quot;:160,&quot;335559740&quot;:259}\">\u00a0<\/span><\/p>\n<p>Scientists turned to carbon isotopes (which we explored in <a href=\"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/2023\/02\/24\/methane-an-origin-story\/\">part 1<\/a>) to try and understand exactly what had stopped the increasing concentration in its tracks. After all, if we can understand why, we might be able to put that to use in a technology that allows us to influence methane concentration levels.<\/p>\n<p>Let\u2019s start with the isotopic ratios. We know that different proportions of carbon isotopes provide a signature for the source of methane. The pattern of <sup>13<\/sup>C enrichment (less negative, heavier), along with a stable d<sup>13<\/sup>C, corresponds to the trends in methane concentration until the end of the 1999 and 2006 plateau (see Fig.2).<\/p>\n<figure id=\"attachment_7221\" aria-describedby=\"caption-attachment-7221\" style=\"width: 884px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" width=\"602\" height=\"220\" class=\"wp-image-7221\" src=\"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/files\/2023\/04\/methane_fig-2.jpg\" alt=\"\" \/><figcaption id=\"caption-attachment-7221\" class=\"wp-caption-text\">Fig 2. Global trends in methane mole fraction (A) and it isotopic signature (B) (Taken from Schaefar et al. (2016)<\/figcaption><\/figure>\n<p><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW248214523 BCX9\">Post-plateau, there is an increase in the concentration of methane, accompanied by a depletion of <\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun Superscript SCXW248214523 BCX9\" data-fontsize=\"12\">13<\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW248214523 BCX9\">C in the \u03b4<\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun Superscript SCXW248214523 BCX9\" data-fontsize=\"12\">13<\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW248214523 BCX9\">C. This implies that the emissions before and after the plateau have different \u03b4<\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun Superscript SCXW248214523 BCX9\" data-fontsize=\"12\">13<\/span><\/span><span class=\"TextRun SCXW248214523 BCX9\" lang=\"EN-GB\" xml:lang=\"EN-GB\" data-contrast=\"auto\"><span class=\"NormalTextRun SCXW248214523 BCX9\">C values.\u00a0<\/span><\/span><span class=\"EOP SCXW248214523 BCX9\" data-ccp-props=\"{&quot;201341983&quot;:0,&quot;335559739&quot;:160,&quot;335559740&quot;:259}\">\u00a0<\/span><\/p>\n<p>Various studies have suggested different explanations for the plateau in the 2000s and the growth of atmospheric methane since.<\/p>\n<p style=\"padding-left: 40px\"><strong>Decreased microbial methane production?<\/strong><br \/>\n<a href=\"https:\/\/www.nature.com\/articles\/nature10259\">Kai et al. (2011)<\/a> suggest that reduced growth rate can be explained by decreased microbial sources based on isotopic analysis.<\/p>\n<p style=\"padding-left: 40px\"><strong>A more active methane sink?<\/strong><br \/>\nOn the other hand, studies including methyl chloroform measurements indicate that changes in the methane sink played a role in both stabilisation and subsequent (<a href=\"https:\/\/www.pnas.org\/doi\/abs\/10.1073\/pnas.1616020114?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%20%200pubmed\">Turner et al. 2017<\/a>; <a href=\"https:\/\/acp.copernicus.org\/articles\/16\/7943\/2016\/\">McNorton et al. 2016<\/a>). <a href=\"https:\/\/www.pnas.org\/doi\/full\/10.1073\/pnas.1616020114\">Turner et al. (2017)<\/a> suggest that an explanation for the stabilisation is that the OH sink has increased, which has offset the increase in methane emissions.<\/p>\n<p style=\"padding-left: 40px\"><strong>A reduction in biomass burning?<\/strong><br \/>\n<a href=\"https:\/\/www.nature.com\/articles\/s41467-017-02246-0\">Worden et al. (2017)<\/a> used carbon monoxide measurements to suggest a decrease in biomass burning emissions (more enriched in <sup>13<\/sup>C), which could explain the potential increase in both fossil fuel and microbial emissions.<\/p>\n<p style=\"padding-left: 40px\"><strong>A reduction in emissions from Oil and Gas?<br \/>\n<\/strong><a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/full\/10.1029\/2005JD006058\">Chen and Prinn et al. (2006)<\/a> argue that fossil fuel sources decreased, a view backed up by <a href=\"https:\/\/www.science.org\/doi\/10.1126\/science.aad2705?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%20%200pubmed\">Schaefar et al<\/a>. (2016) 10 years later &#8211; that the plateau in methane levels can be attributed to reduced oil and gas production and investment into updating oil and gas infrastructure \u2013 largely a result of the collapse of the Soviet Union in 1992. This study qualified that \u00a0The plateau in methane\u00a0 mole fraction could be due to variations in the hydroxyl methane sink alone, or it could be a combination of variations in the hydroxyl methane sink and reduced fossil fuel emissions.<\/p>\n<p style=\"padding-left: 40px\">There\u2019s some important corroborating evidence here too; measurements of atmospheric ethane (almost solely a biproduct of the oil and gas sector) also showed a decline. <a href=\"https:\/\/www.nature.com\/articles\/nature11342\">Simpson et al. (2012)<\/a> showed that global ethane emission rates declined from 1984 to 2010.<\/p>\n<p>It can now be affirmed that the reduction in natural gas venting and flaring in oil fields contributed to this decrease, rather than a reduction in biofuel usage or biomass burning.<\/p>\n<p><a href=\"https:\/\/agupubs.onlinelibrary.wiley.com\/doi\/full\/10.1029\/2005JD006058\">Chen and Prinn et al. (2006)<\/a> reported an increase in emissions from rice and biomass burning, and suggested that a decrease in the energy sector could be the reason for the plateau. However, the evidence suggests that the reduction in fossil fuel emissions had a greater impact. It\u2019s an observation that also clarifies why the isotopic signature remained unchanged during this interval &#8211; since rice emissions are depleted, and biomass burning emissions are more enriched than fossil fuel emissions.<\/p>\n<p>Whilst isotopes are useful, the scientific toolkit for measuring slight and long-term changes in individual methane source strengths doesn\u2019t yet allow us to accurately forecast future atmospheric methane levels with certainty.\u00a0 New tools could help us improve our understanding in the sharp growth in methane concentrations since 2006 &#8211; and to examine that &#8211; you\u2019ve guessed it \u2013 we\u2019re going to need another blog!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>At the end of part 1, we touched upon how at the turn of the millennium and through the first half of the noughties, scientists working in emissions monitoring observed a plateau in what had been steadily increasing atmospheric methane concentrations. It was a phenomenon very much against the expected trend and the reasons behind [&hellip;]<\/p>\n","protected":false},"author":1649,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[305212,305211,305177],"tags":[],"class_list":["post-7216","post","type-post","status-publish","format-standard","hentry","category-carbon-isotopes","category-emissions","category-greenhouse-gas-emissions"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Methane origins \u2013 part II: The millennium methane plateau and post-plateau rise - Sustainable Gas Institute Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blogs.imperial.ac.uk\/sustainable-gas-institute\/2023\/05\/04\/methane-origins-part-ii-the-millennium-methane-plateau-and-post-plateau-rise-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Methane origins \u2013 part II: The millennium methane plateau and post-plateau rise - Sustainable Gas Institute Blog\" \/>\n<meta property=\"og:description\" content=\"At the end of part 1, we touched upon how at the turn of the millennium and through the first half of the noughties, scientists working in emissions monitoring observed a plateau in what had been steadily increasing atmospheric methane concentrations. 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