Limited impact of El Niño–Southern Oscillation on variability and growth rate of atmospheric methane

Abstract

Abstract. The El Niño–Southern Oscillation (ENSO) has been suggested as a strong forcing in the methane cycle and as a driver of recent trends in global atmospheric methane mole fractions [CH4]. Such a sensitivity of the global CH4 budget to climate events would have important repercussions for climate change mitigation strategies and the accuracy of projections for future greenhouse forcing. Here, we test the impact of ENSO on atmospheric CH4 in a correlation analysis. We use local and global records of [CH4], as well as stable carbon isotopic records of atmospheric CH4 (δ13CH4), which are particularly sensitive to the combined ENSO effects on CH4 production from wetlands and biomass burning. We use a variety of nominal, smoothed, and detrended time series including growth rate records. We find that at most 36 % of the variability in [CH4] and δ13CH4 is attributable to ENSO, but only for detrended records in the southern tropics. Trend-bearing records from the southern tropics, as well as all studied hemispheric and global records, show a minor impact of ENSO, i.e. < 24 % of variability explained. Additional analyses using hydrogen cyanide (HCN) records show a detectable ENSO influence on biomass burning (up to 51 %–55 %), suggesting that it is wetland CH4 production that responds less to ENSO than previously suggested. Dynamics of the removal by hydroxyl likely counteract the variation in emissions, but the expected isotope signal is not evident. It is possible that other processes obscure the ENSO signal, which itself indicates a minor influence of the latter on global CH4 emissions. Trends like the recent rise in atmospheric [CH4] can therefore not be attributed to ENSO. This leaves anthropogenic methane sources as the likely driver, which must be mitigated to reduce anthropogenic climate change.

On the Causes and Consequences of Recent Trends in Atmospheric Methane

“…The drop could be caused by a shift from pre-dominant El Niños, which enhance global biomass burning [88], to predominant La Niñas. However, an influence of ENSO throughout the atmospheric δ 13 CH 4 record is not evident [4], suggesting that it is unlikely that the ENSO shift can explain the trends in both [CH 4 ] and δ 13 CH 4 as hypothesised [63].…”

“…However, in most models, precipitation and wetland area are the main control of emissions. ENSO switched from drier El Niño predominance to wetter La Niñas around the start of the renewed rise but plays a minor role in global [CH 4 ] and δ 13 CH 4 trends [4]. The correlation between tropical precipitation and TD wetland emissions is strong until 2005, but weak thereafter [65], negating a clear link between the main control and reconstructed fluxes of at least that wetland model.…”

“…All reconstructions of the methane history are fundamentally underconstrained by observations, due to the multitude of sources and sinks. ] growth rate; c δ 13 CH 4 records from Baring Head, New Zealand, and global average [4]; d global BU fossil fuel CH 4 emissions from oil and natural gas production (O-NG) [5,6] and total values, hatched area indicates emissions from coal mining [5]; e ethane total column observations and trends from Jungfraujoch, Switzerland [7]; f global yearly [8] and decadal [9] BU estimates of CH 4 emissions from ruminant livestock; g global BU estimates of biomass burning CH 4 emissions [10, 11••, 12]; h global multi-model wetland CH 4 emissions for identical forcing [13•], individual model runs are shown as anomalies relative to the ensemble average over the study period; i global wetland CH 4 emissions modelled for different climate reconstructions [14•] (red lines in panels h and i indicate results for the same model and climate data set); k global OH anomalies as reconstructed using MCF records from AGAGE and NOAA [15•, 16, 17] and from an atmospheric process model [18], anomalies are reported relative to varying reference periods. Vertical blue dashed lines indicate the start and end of the source-sink imbalance anomaly.…”

Impact of El Niño–Southern Oscillation on the interannual variability of methane and tropospheric ozone

“…The control run (CTRL) allows all emissions and meteorology to vary throughout the modelled period. GFED biomass burning emission inventories began in 1997; therefore, the 1993-1996 spin-up simulation uses repeating 1999 emissions instead, as the closest year of "average" emissions, having excluded 1997 and 1998 due to the exceptionally high emissions in those years (Schultz et al, 2008).…”

FTIR time series of tropospheric HCN in eastern China: seasonality, interannual variability and source attribution

“…In addition, the El Niño Southern Oscillation (ENSO) can cause large scale variations in the convection, circulation, and air temperature of the global atmosphere-ocean system Zhao et al, 2002), which could affect the distribution, frequency, and intensity of biomass burning emissions (Schaefer et al, 2018). Furthermore, ENSO could also alter the destruction processes of tropospheric species through their photochemical reactions with tropospheric OH (Zhao et al, 2002).…”