Abstract
It has been found that the concentration of atmospheric methane (CH4) has rapidly increased since 2007 after a decade of nearly constant concentration in the atmosphere. As an important greenhouse gas, such an increase could enhance the threat of global warming. To better quantify this increasing trend, a novel statistic method, i.e. the Ensemble Empirical Mode Decomposition (EEMD) method, was used to analyze the CH4 trends from three different measurements: the mid–upper tropospheric CH4 (MUT) from the space-borne measurements by the Atmospheric Infrared Sounder (AIRS), the CH4 in the marine boundary layer (MBL) from NOAA ground-based in-situ measurements, and the column-averaged CH4 in the atmosphere (XCH4) from the ground-based up-looking Fourier Transform Spectrometers at Total Carbon Column Observing Network (TCCON) and the Network for the Detection of Atmospheric Composition Change (NDACC). Comparison of the CH4 trends in the mid–upper troposphere, lower troposphere, and the column average from these three data sets shows that, overall, these trends agree well in capturing the abrupt CH4 increase in 2007 (the first peak) and an even faster increase after 2013 (the second peak) over the globe. The increased rates of CH4 in the MUT, as observed by AIRS, are overall smaller than CH4 in MBL and the column-average CH4. During 2009–2011, there was a dip in the increase rate for CH4 in MBL, and the MUT-CH4 increase rate was almost negligible in the mid-high latitude regions. The increase of the column-average CH4 also reached the minimum during 2009–2011 accordingly, suggesting that the trends of CH4 are not only impacted by the surface emission, however that they also may be impacted by other processes like transport and chemical reaction loss associated with [OH]. One advantage of the EEMD analysis is to derive the monthly rate and the results show that the frequency of the variability of CH4 increase rates in the mid–high northern latitude regions is larger than those in the tropics and southern hemisphere.
Highlights
As the third most important long life greenhouse gas after carbon dioxide (CO2) and water vapor, atmospheric methane (CH4) has a life time of about 12 years and its radiative forcing is about 26 times more than that of CO2 on a 100-year time horizon, accounting for 32% of the total anthropogenic well-mixed greenhouse gas radiative forcing [1]
Measurements from the ground-based networks operated by National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division (NOAA/ESRL/GMD) since 1983 show that the increase rate of atmospheric CH4 was less than 1 ppbv yr−1 from 1999 to 2006 [2], nearly reaching a constant state, a rapid increase was observed since 2007
A recent study by Bader et al [4] showed an increase of atmospheric CH4 total column of 0.31 ± 0.03% year−1 using 10 years of in-situ measurements from 2005 to 2014, and based on GEOS-Chem model simulations, Bader et al pointed out that anthropogenic emissions such as coal mining and gas and oil transport and exploration, which were mainly emitted in the Northern Hemisphere, had played a major role in the increase of atmospheric CH4 observed since 2005
Summary
As the third most important long life greenhouse gas after carbon dioxide (CO2) and water vapor, atmospheric methane (CH4) has a life time of about 12 years and its radiative forcing is about 26 times more than that of CO2 on a 100-year time horizon, accounting for 32% of the total anthropogenic well-mixed greenhouse gas radiative forcing [1]. We use the space-born AIRS CH4 products combined with the ground-based measurements from NDACC, TCCON, and NOAA/GMD to derive the changing trends of atmospheric CH4 over the globe and at different altitudes. This trend analysis aims to give a better 3-D picture of CH4 trends, helping us to better understand the increase trends of CH4 during recent decades, especially since 2007.
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