A recently published study found that human emissions (from coal mines in China and oil and gas fields in North America) are responsible for the recent uptick in methane emissions (since 2007). They found "There is no evidence of emission enhancement due to climate warming, including the boreal regions, during our analysis period."
https://www.jstage.jst.go.jp/article/jmsj/advpub/0/advpub_2021-015/_articleEmissions from the Oil and Gas Sectors, Coal Mining and Ruminant Farming Drive Methane Growth over the Past Three Decades
Naveen CHANDRA, Prabir K. PATRA, Jagat S. H. BISHT, Akihiko ITO, Taku UMEZAWA, Nobuko SAIGUSA, Shinji MORIMOTO, Shuji AOKI, Greet JANSSENS-MAENHOUT, Ryo FUJITA, Masayuki TAKIGAWA, Shingo WATANABE, Naoko SAITOH, Josep G. CANADELL
Abstract
Methane (CH4) is an important greenhouse gas and plays a significant role in tropospheric and stratospheric chemistry. Despite the relevance of methane (CH4) in human-induced climate change and air pollution chemistry, there is no scientific consensus on the causes of changes in its growth rates and variability over the past three decades. We use a well-validated chemistry-transport model for simulating CH4 concentration and estimation of regional CH4 emissions by inverse modelling for the period of 1988-2016. The control simulations are performed using a seasonally varying hydroxyl (OH) concentrations and assumed no interannual variability. Using inverse modelling of atmospheric observations, emission inventories, a wetland model, and a δ13C-CH4 box model, we show that reductions in emissions from Europe and Russia since 1988, particularly from oil-gas exploitation and enteric fermentation, led to decreased CH4 growth rates in the 1990s. This period was followed by a quasi-stationary state of CH4 in the atmosphere during the early 2000s. CH4 resumed growth from 2007, which we attribute to increases in emissions from coal mining mainly in China and intensification of ruminant farming in tropical regions. A sensitivity simulation using interannually varying OH shows that regional emission estimates by inversion are unaffected for the mid- and high latitude areas. We show that meridional shift in CH4 emissions toward the lower latitudes and the increase in CH4 loss by hydroxyl (OH) over the tropics finely balance out, which keep the CH4 gradients between the southern hemispheric tropical and polar sites relatively unchanged during 1988-2016. The latitudinal emissions shift is confirmed using the global distributions of the total column CH4 observations by satellite remote sensing. There is no evidence of emission enhancement due to climate warming, including the boreal regions, during our analysis period. These findings highlight key sectors for effective emission reduction strategies toward climate change mitigation.
Between the period 1999-2006 and 2007-2016, the a posteriori emission shows ~30 Tg yr-1 increase in the global emissions (Fig. 5c and Table 1), and ~24Tg yr-1 of the global increase occurred in Asia (Fig. 12).The East Asia, West Asia, South Asia, and Southeast Asia regions accounted for 28%, 19%, 18%, and 19% of the global a posteriori emission increase from 1999-2006 to 2007-2016, respectively (Fig. 12f,c,h,j).The EDGARv4.3.2 inventory suggests that the coal sector over East Asia, West Asia, Southeast Asia, and ruminant farming sector over Southeast Asia drive a large part of emission increase from 1999-2006 to 2007-2016 (Fig. 5f,c,j,h).The a posteriori emission trend over East Asia shows an excursion from the continuous (extrapolated) increase of that of the a priori after 2012 (Fig. 12f). One possible explanation is that emissions from abandoned coal mines in East Asia may have ceased, in agreement with that is suggested by the d13C-CH4 model.Themost recent inventory (EDGARv5.0) emissions from coal mining indeed show a slowdown in the rate of increase, at 2.6% yr-1during 2012-2014 and by 3% yr-1 during 2014-2015, relative to a rate of increase of 5.7 % yr-1during 2003-2011 (Crippa et al. 2020).
The sources of the North American increase are uncertain and may be due to increased emissions from wetlands. The huge increases in fracked oil and gas wells and the documented problems they have with fugitive emissions may indicate that the oil and gas industry is responsible.
More recently, a posterioriemission over Temperate North America(Fig. 12e)shows 9.8Tg yr-1 increase (from 30 Tg yr-1 in 2010 to 39.8 Tg yr-1in 2016) between 2010 and 2016, which is consistentwith the increase in wetland emission by VISIT model and fugitive emissionsinEDGARv4.3.2inventory during the same period (Fig. 1e). This emission increase outside fast growing economies may have helped to sustain the regrowth in the 2010s. However, the increase in emissions from fugitive sources from the United States are uncertain (Sheng et al. 2018; Lan et al. 2019), and a broader assessment is needed for the wetland emission increase.
The higher northern regions show a systematic decrease in a posteriori emission. The European emissions gradually decreased between 1988 and 2010(Fig. 12b) due to the decline in ruminant farming emissions (Fig. 1b), and Russian emission decreased between 1988 and 2000 (Fig. 12d) due to the decrease infugitive emissions (Fig. 1d). TheFAOSTAT statistics suggest that the cow and cattle population decreased largely over the western Europe (FAOSTAT 2018). As reported by some previous studies (Sweeney et al. 2016; Thompson et al. 2018), we did not find any detectable increase in natural CH4 emissions from the wetlands and other ecosystems in the northern high latitudes, e.g., the Boreal North America and Russia, despite the increase in annual mean temperature by ~1.2oC/decade for the period of 1985-2015 over the Arctic region (Sweeney et al. 2016). These regions are expected to release CH4 in response to future global warming in the boreal zones (Anthony et al. 2018).
