Methane increase: a new climate challenge

In 2007, the amount of methane in the atmosphere (CH₄) began to increase after a seven-year period of almost zero growth (1). Recent research shows that a second change took place in 2014 (2). From 2014 to at least the end of 2018, the amount of CH₄ in the atmosphere has almost doubled compared to the rate observed since 2007 (see figure). Because ch₄ is a potent greenhouse gas and a rise in the atmospheric CH₄ represents a major challenge to the achievement of the goals set out in the Paris Agreement, an international consensus to limit the increase in temperature to 2 ° C or, if possible, to 1.5 ° C above pre-industrial levels .

Causes of the recent rise in atmospheric CH₄ remains a topic of scientific debate, even for the initial period of increase from 2007 to 2014 (1–8). CH estimates based on processes₄ emissions from inventory data, wetland models and other information offer contradictory explanations, but measurements of the CH distribution₄ in the atmosphere and its ¹³C /¹²The isotopic ratio of C on a global network of stations is a clue.

Although CH₄ has increased worldwide, this growth has been greatest in the mid-latitudes and tropics of the northern hemisphere (2, 3). In addition, the proportion of ¹³C in atmospheric CH₄ has declined as atmospheric CH₄ has increased (see figure) (1, 2). the ¹³C /¹²Ratio C in CH₄ depends on the sources of the CH₄ shows. Discharges from biogenic sources (such as wetlands and agriculture) reduce the proportion of ¹³C in atmospheric CH₄, while fossil-based emissions slightly increase this proportion and that biomass-related emissions increase sharply (1, 2). Based on selected CH₄ and ¹³C /¹²C time series from four latitudinal bands, an atmospheric multibox model and a budget analysis being run, Nisbet et al. (2) identified three potential pathways compatible with both₄ and isotopic data: increase in biogenic emissions, decrease in the amount of CH₄ destroyed in the atmosphere by CH₄ oxidation and an increase in fossil fuel emissions if offset by a decrease in biomass combustion.

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

Biogenic emissions come mainly from wetlands and agriculture, particularly ruminant livestock. Multimodal studies on wetlands do not confirm an increase in emissions since 2007 (3, 9). However, livestock inventories show that ruminant emissions began to increase sharply around 2002 and can account for about half of chlorine production.₄ increase since 2007 (4).

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

The increase in fossil emissions could explain the change, but a simultaneous reduction in ¹³C-rich emissions from biomass combustion are needed to balance the ¹³C /¹²C trends. The reconstructions of fires using satellite observations corroborate this decline, with a reduction in emissions by 2006 (ten). The resultant ¹³C /¹²Balance C limits fossil fuels to half of the total additional emissions since 2007. Coal mining in East Asia is universally recognized for its contribution to CH₄ increase (2, 7, 8), while the fossil CH₄ North American emissions remained stable despite a nearly 50% increase in natural gas production (11).

Coinciding with the 2014 acceleration, Nisbet et al. find a source shift towards the southern tropics, where wetlands are concentrated (2). They hypothesize that record temperatures in 2014 and subsequent years have driven wetland CHs upwards₄ production. Such a return to the climate of wetlands calls into question the generally accepted view that wetlands, and not temperature, constitute the main control of wetland wetlands.₄ emissions (although some CH₄ the models are more focused on temperature) (ten). If natural wetlands or changes in atmospheric chemistry have actually accelerated the₄ rising, it may be a climate feedback that humans have little hope of slowing down. Although studies have shown the significant production potential of CH₄climate feedbacks, they should occur gradually, not reaching the magnitude observed by Nisbet et al. for decades (12).

Agriculture would be responsible for more than half of all human-induced CH emissions₄ emissions and could have contributed to the rise in CH emissions₄ since 2007.

PHOTO: SERGIO AZENHA / ALAMY PHOTO STOCK

As the scientific community continues to debate the causes of CH₄ outbreak, the consequences are clear. The latest emissions scenarios from the Intergovernmental Panel on Climate Change (IPCC) 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). However, between 2007 and 2014, the amount has increased on average by 5.7 parts per billion (ppb) per year and 9.7 ppb on average per year since 2014. If this increase continues unabated, the reductions of dioxide Carbon and other gases will have to be even steeper to achieve the Paris goal.

Measurements of greenhouse gases in the atmosphere remain the fastest way to assess progress towards a slowdown in climate change. More atmospheric observations are essential to understand the sources of the rise of the CH₄especially in the tropics, which seem to be driving this change. Atmospheric models informed by CH₄ Data will incorrectly attribute emission changes to regions that are weakly constrained by data, such as equatorial regions (3).

With satellite observations and time series of additional tracers (14), a global network of global measures will be crucial to understand the changes in the CH₄. Ascension Island in the South Atlantic is currently the only tropical site where₄, his ¹³C /¹²Ratio C and CH column₄ essential measures for validation of satellite observations. Still, this site is about to be interrupted. Ongoing support for sites of vital importance such as the Ascension Island and the creation of similar sites in other tropical regions will be essential for cultural heritage studies.₄ tendencies.

Close integration between atmospheric observations, process-oriented studies and policies is urgently needed to provide useful answers on the actual emission reductions needed to achieve the Paris Agreement's objectives in the area of ​​climate change. fight against climate change. The World Meteorological Organization has put in place the Global Integrated Greenhouse Gas Information System (IGG).³IS) to solve this problem. IG³The SI provides a bridge between the atmospheric greenhouse gas community and policy makers. Timely dialogue between these groups has never been so essential as the window to achieve the goals of the Paris Agreement is closing rapidly.

Thanks: This work was funded by the New Zealand Ministry of Enterprise, Innovation and Employment under contract C01X1817 and NIWA through the Science Program on Greenhouse Gases. emissions and the carbon cycle. S.M.F. sits on the Scientific Steering Committee of the IG³IS. We thank E. Dlugokencky (NOAA) for the CH after 2017₄ data and S. Michel and B. Vaughn (CU / INSTAAR) for post-2017 isotope data.