Abstract

Atmospheric methane began rising rapidly and unexpectedly in 2007, after the global burden had been stable for some years. Sustained methane growth took place between 2007-2013 at about 6 ppb yr-1. Then, for the three years 2014, 2015 and 2016, growth increased again at over 10 ppb yr-1. This growth continues in 2017. Total growth from 2007-present has been about 70 ppb (Nisbet et al., 2016 and submitted). Methane growth has been accompanied by a significant negative shift in the δ13CCH4 ratio of the methane burden, reversing the positive trend that has been sustained through the 20th century. The causes of the rise and shift are not understood. The isotopic shift may indicate an increase in biogenic emissions, especially in the tropics and subtropics (Schaefer et al, 2016, Nisbet et al. 2016), or perhaps a decline in methane destruction by OH (Turner et al., 2017, Rigby et al. 2017). Biomass burning may have declined, or the proportion of fossil fuel emissions may have dropped. Possible factors driving increased biogenic emissions are warmer, wetter tropical wetlands and increases in emissions from cattle, perhaps as the inter-tropical convergence zone expands its range. However, models suggest methane’s lifetime may be changing also: the ‘sources vs sinks’ debate focuses on the time-scale of the isotopic shift.

The 2015 UNFCCC Paris Agreement ambitiously sought to hold the increase in the global average temperature below 2°C above pre-industrial levels, and, going beyond that, to strive for 1.5°C. There has been optimism that this very ambitious goal may be feasible, though the roadmap has steep hills and depends heavily on a sharp near-future cut in the methane burden. But the recent methane increase makes this increasingly difficult. Methane growth, if sustained at current rates, would lead to warming well above the Paris target (Nisbet et al., submitted). However, although methane’s recent rise appears to be from factors that are not directly anthropogenic, most methane emissions do indeed come directly from human activities, including leaks from the energy industry (Schwietzke et al., 2017), emissions from landfills, and output from agricultural ruminants, especially cows. Thus for Paris targets to be achieved, anthropogenic emissions must be cut back urgently, and more importance given to proactive removal from the atmosphere.

The debate about the causes of methane’s rise cannot be solved until better tropical data, especially isotopic measurements, are available. The ongoing changes in the methane budget are the focus of project MOYA (Methane Observations and Yearly Assessments), which is a multi-partner UK consortium supported from 2016-2020 by the Natural Environment Research Council. The project is global in scope and includes measurement, field campaigns, and modelling work: http://moya.blogs.bris.ac.uk/project-moya-nercs-study-of-the-global-methane-budget/ . At the core of MOYA is a network of in situ observations, including planned continuous CO2 and CH4 measurement on Ascension Is., E. Falkland Is. Halley Bay, Antarctica, and the Atlantic-crossing ships RRS JC Ross and Cap San Lorenzo, as well as flask or bag sampling from many other locations. Ascension is the only remote equatorial station worldwide with continuous measurement of greenhouse gases, but unfortunately the collapse of the Ascension runway has made it difficult to sustain measurement.

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