Emissions From The Oil And Gas Sectors, Coal Mining And Ruminant Farming Drive Methane Growth Over The Past Three Decades

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 modeling during 1988–2016. The control simulations are conducted using seasonally varying hydroxyl (OH) concentrations and assumed no interannual variability. Using inverse modeling of atmospheric observations, emission inventories, a wetland model, and a 8¹³C-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 the 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, keeping 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 via satellite remote sensing. During our analysis period, there is no evidence of emission enhancement due to climate warming, including the boreal regions. These findings highlight key sectors for effective emission reduction strategies toward climate change mitigation.

Generated Summary

This research investigates the drivers of methane (CH4) growth over the past three decades by analyzing emissions from the oil and gas sectors, coal mining, and ruminant farming. The study utilizes a chemistry-transport model (MIROC4-ACTM) to simulate CH4 concentrations and estimate regional CH4 emissions through inverse modeling from 1988 to 2016. The methodology involves the use of seasonally varying hydroxyl (OH) concentrations and the application of a two-box model to simulate hemispheric CH4 concentration and δ13C-CH4. The study incorporates various datasets, including atmospheric observations from surface sites, aircraft flights, and remote sensing satellites, to validate the emission trends. The research aims to identify key emission sectors and understand the factors contributing to the observed CH4 variability, providing insights into effective emission reduction strategies for climate change mitigation.

Key Findings & Statistics

• The global CH4 growth rate was 11.9 ± 0.9 ppb yr¯¹ during 1984-1989, declined to 6.8 ± 4.2 ppb yr¯¹ during 1990-1998, and then reached a quasi-stationary state (0.5 ± 3.3 ppb yr¯¹) during 1999–2005.
• The CH4 growth rate resumed to 7.0 ± 2.6 ppb yr¯¹ during 2007–2016, with further acceleration to 9.3 ± 2.2 ppb yr¯¹ since 2014.
• The study found 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.
• CH4 resumed growth from 2007, attributed to increases in emissions from coal mining mainly in China and the intensification of ruminant farming in tropical regions.
• The amount of excess CH4 burden in the atmosphere on yearly basis is a result of a fine balance between the large amounts of emission and loss of CH4.
• The global mean CH4 concentration in 1988-1989 was 1650.7 ± 7.4 ppb, and in 2010-2016 it was 1770.9 ± 15.8 ppb.
• The net a posteriori emission in 1988-1989 was 514 ± 1 Tg yr-1, and in 2010-2016 it was 549 ± 15 Tg yr-1.
• The chemical loss in 1988-1989 was 486 ± 0.3 Tg yr-1, and in 2010-2016 it was 522 ± 10 Tg yr-1.
• The burden in 1988-1989 was 4692 ± 26 Tg, and in 2010-2016 it was 5048 ± 77 Tg.
• The a posteriori emissions from 1999–2006 to 2007–2016 show an increase of ~ 30 Tg yr-1 in the global emissions, and ~ 24 Tg yr-1 of the global increase occurred in Asia.
• 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.
• The study found a substantial decrease in the CH4 growth rate (by ~ 12 ppb yr¯¹) during 1992–1993.
• The a posteriori emission over Temperate North America (Fig. 12e) shows a 9.8 Tg yr¯¹ increase (from 30 Tg yr¯¹ in 2010 to 39.8 Tg yr¯¹ in 2016) between 2010 and 2016, which is consistent with the increase in wetland emission by VISIT model and fugitive emissions in EDGARv4.3.2 inventory during the same period (Fig. 1e).
• The average ratio of a posteriori emissions at 30–60°N to those at 0–30°N was higher than 0.8 before 1996 but decreased to 0.75 during 2003-2016.
• The ISD between two polar sites (ALT-SYO) is observed to be greatest at 133.1 ± 5.6 ppb when averaged for 1988-2016.
• Mean ISD decreases sharply as the target site moved southward to the NH tropical latitudes, e.g., a value of 58.6 ± 4.5 ppb is observed for GMI-SYO (Fig. 9a).
• The average decreasing trend of the observed ALT-SYO ISD was calculated to be -0.37 ppb yr¯¹ for the whole period of 1988–2016, we see a substantially faster decrease during 1988-2002 (-1.51 ± 0.1 ppb yr¯¹) but a rapid increase during 2010–2016 (1.54 ± 0.2 ppb yr¯¹).

Other Important Findings

• The study highlights that the quasi-stationary state of CH4 in the atmosphere during the early 2000s was followed by resumed growth from 2007, attributed to emissions increases from coal mining in China and the intensification of ruminant farming in tropical regions.
• A sensitivity simulation using interannually varying OH shows that regional emission estimates by inversion are unaffected for mid- and high-latitude areas.
• The meridional shift in CH4 emissions towards the lower latitudes and the increase in CH4 loss by hydroxyl (OH) over the tropics finely balance out, keeping the CH4 gradients between the southern hemispheric tropical and polar sites relatively unchanged during 1988–2016.
• The study confirms the latitudinal emissions shift using the global distributions of total column CH4 observations via satellite remote sensing.
• There is no evidence of emission enhancement due to climate warming, including the boreal regions, during the analysis period.
• The research indicates that emissions from the oil and gas sectors, coal mining, and ruminant farming are key drivers of the observed methane growth over the past three decades.
• The revised case suggests that the renewed CH4 growth rate is caused by an increase in fugitive emissions during 2002-2010 (by ~ 10.9 Tg yr¯¹), whereas this emission increase is overestimated by the EDGARv4.3.2 inventory (at ~ 20.8 Tg yr¯¹).
• A better model-observation agreement in 813C-CH4 decrease (Fig. 7) for the revised case suggests that the renewed CH4 growth rate is caused by an increase in fugitive emissions during 2002-2010 (by ~ 10.9 Tg yr¯¹), whereas this emission increase is overestimated by the EDGARv4.3.2 inventory (at ~ 20.8 Tg yr¯¹).
• A total of 11.5% of the global emission increase (3.2 Tg yr¯¹) occurred over Brazil, and 10.4 % (2.9 Tg yr¯¹) over Northern Africa between 1999–2006 and 2007–2016.
• The European emissions gradually decreased between 1988 and 2010 due to the decline in ruminant farming emissions, and Russian emissions decreased between 1988 and 2000 due to the decrease in fugitive emissions.
• The comparison of CH4 emissions over India shows large discrepancies between our results and previous studies.
• The estimated emissions by Ganesan et al. (2017) and Miller et al. (2019) are 58 % (22 Tg yr¯¹) and 28% (35 Tg yr¯¹) respectively lower than that in this study (49 Tg yr¯¹).

Limitations Noted in the Document

• The study acknowledges that the uncertainties in representative 8¹³C-CH4 signatures make it difficult to pinpoint a unique solution for emission sources.
• The inversion model estimated emissions may represent a lower bound because all sources of error are not fully accounted for, particularly those arising from spatial and temporal variability of OH and a priori natural emissions (e.g., the fresh waters, geological seeps, and wetlands).
• The research notes that to provide robust estimates at national scales, a much denser observational network in space and time than the current network provides.
• The model representation error arises due to the inability of the coarse ACTM resolution to simulate site-specific concentration gradients.
• The study’s analysis period is limited to 1988–2016, and the results may not fully capture more recent trends.

Conclusion

The findings of this study underscore the critical role of specific emission sectors in driving the observed methane growth over the past three decades. The abstract emphasizes that emissions from the oil and gas sectors, coal mining, and ruminant farming are the primary contributors to this increase. The research provides a robust set of explanations about the processes and emissions sectors that led to changes in CH4 growth rates, covering the three distinct periods of decreasing (1988–1998), quasi-stationary (1999–2006), and renewed (2007–2016) phases. The study highlights that the quasi-stationary state of CH4 in the atmosphere during the early 2000s was followed by a resumed growth from 2007. The study also highlights the importance of understanding the regional contributions to CH4 emissions. A geographical shift of CH4 emissions from the regions of the north to south latitudes, explains well the interdecadal CH4 growth rate variabilities and concentration gradients. The study also suggests that emissions play a dominant role in driving the decadal CH4 growth rate changes over the past three decades. The loss of CH4 by reaction with OH played only a minor role. The study’s use of a well-validated three-dimensional chemistry-transport model, inverse modeling of regional emissions, and various datasets adds robustness to the conclusions. In conclusion, the study underscores the need for targeted mitigation strategies aimed at reducing anthropogenic CH4 emissions from these key sectors and regions, offering valuable insights for climate change mitigation efforts. The study’s results are consistent with the rate of emission changes in China due to coal mining. However, the assessment of emission increase from ruminant farming shows major discrepancies across the continents, which is a limitation in the study. The study’s results underscore the need and opportunity of reducing anthropogenic CH4 emissions from targeted sectors and regions for climate change mitigation.

DOI

10.2151/jmsj.2021-015