Abstract
Atmospheric methane (CH4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions, but the precise magnitude of these contributions is a subject of debate2,3. Carbon-14 in CH4 (14CH4) can be used to distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century4,5. Moreover, the partitioning of total fossil CH4 emissions (presently 172 to 195 teragrams CH4 per year)2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH4 per year6,7. Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene, about 11,600 years ago8, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions9,10.
Generated Summary
This research article investigates the preindustrial atmospheric methane (CH4) levels using ice core 14CH4 measurements to estimate natural geological CH4 emissions. The study employs a combination of ice core and firn air data from various locations, reconstructing the atmospheric 14CH4 history. The research aims to distinguish between fossil and biogenic CH4 sources and to refine the understanding of anthropogenic fossil CH4 emissions. The methodology involves analyzing 14CH4 in ice core samples, correcting for in situ cosmogenic 14CH4 production, and using a one-box atmospheric model to calculate fossil CH4 emissions. The study’s scope encompasses the preindustrial era, offering insights into natural geological CH4 emissions and the human impact on the atmosphere. The findings provide a target for global CH4 budget inventories and inform strategies for emission reductions.
Key Findings & Statistics
- Atmospheric methane (CH4) has more than doubled since the preindustrial era.
- Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions.
- Total fossil CH4 emissions presently account for 172 to 195 teragrams CH4 per year.
- Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene.
- The study estimates that natural geological CH4 emissions were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95% confidence limit).
- Anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates.
- The fossil fraction of the contemporary CH4 budget is 30±2.3%.
- The study finds an increase in total (geological plus anthropogenic) fossil emissions from negligible CH4 emissions in the mid-19th century to 64.8 teragrams CH4 per year in 1940.
- The 95% confidence interval (CI) maximum for natural geological CH4 emissions is 5.4 Tg CH4 yr-1.
- Natural geological CH4 emissions are suggested to be ~40-60 Tg CH4 yr-1.
- The preindustrial-era emission estimates can be applied to the modern CH4 budget with confidence.
- The 95% confidence limit of 5.4 Tg CH4 yr-1 agrees with and provides a tighter constraint than the only other published 14CH4-based estimate of natural geological CH4 emissions from ice cores, which sampled air from the most recent deglaciation (0 to 15.4 Tg CH4 yr-1, 95% CI range).
- The study’s results indicate that bottom-up inventories strongly underestimate CH4 emissions from fossil fuel extraction, distribution, and use.
- Anthropogenic fossil CH4 emissions now account for about 30% of the global CH4 source and for nearly half of anthropogenic emissions.
- The study estimates recent anthropogenic fossil CH4 emissions to be 177 ± 37 Tg CH4 yr-1 for 2003-2012.
- Our estimate is 22% higher than the previous estimate of 145 ± 23 Tg CH4 yr-1 over the same interval, and 33-55% higher than the range of bottom-up estimates (114-133 Tg CH4 yr-1).
- A study using both ground-based facility-scale measurements and verification from aircraft sampling found that US oil and natural gas CH4 emissions are ~60% higher than those reported by the US Environmental Protection Agency.
- If global bottom-up emissions of fossil CH4 from the oil and natural-gas industries are similarly underreported by 60%, this would amount to unreported emissions of ~47 Tg CH4 yr-1.
Other Important Findings
- Radiocarbon (14C) is an ideal tracer for quantifying the fossil component of the atmospheric CH4 budget because all 14C in fossil CH4 has decayed.
- The preindustrial-era emission estimates can be applied to the modern CH4 budget with confidence, as geological CH4 emissions have likely been constant over the past few centuries.
- The timing of the 14CH4 minimum (in the 1940s in the reconstruction) is difficult to establish due to the broad age distributions of individual firn air and ice core samples.
- The study’s results suggest that anthropogenic fossil CH4 emissions are underestimated, providing a firm target for inventories of the global CH4 budget.
- Measurements of ethane in ice cores have also been used to suggest considerable emissions of fossil CH4 during the preindustrial era, but this is an ambiguous constraint.
- The study’s findings imply that anthropogenic fossil CH4 emissions now account for about 30% of the global CH4 source and for nearly half of anthropogenic emissions.
- The study uses the contemporaneous biogenic 14CH4 source signature to calculate the natural geological CH4 emissions during the preindustrial era.
Limitations Noted in the Document
- The broad age distributions of the air samples result in a smoothed representation of the atmospheric 14CH4 history, which cannot capture the abrupt increase of bomb 14CO2.
- The precise timing of the 14CH4 minimum (in the 1940s in the reconstruction) is difficult to establish owing to the broad age distributions of individual firn air and ice core samples, as well as the smoothing applied by the matrix inversion technique.
- The study acknowledges uncertainties associated with global upscaling of geological emissions from discrete measurements.
- Ice core measurements of ethane cannot discriminate between contributions from biomass burning and natural geological emissions.
- The study cannot accurately quantify the post-1950 fossil CH4 budget due to poor constraints on the interfering nuclear 14CH4 sources.
Conclusion
The study’s findings emphasize the significant underestimation of anthropogenic fossil CH4 emissions in current inventories. The research demonstrates that natural geological CH4 emissions were considerably lower during the preindustrial era than previously estimated, challenging the accuracy of existing global CH4 budgets. The study highlights the human impact on the atmosphere and climate, as the increase in atmospheric CH4 levels closely correlates with the use of fossil fuels. “Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions.” The research underscores the importance of improving emission inventories, particularly for fossil fuel extraction and distribution. The results indicate that anthropogenic fossil CH4 emissions now account for a substantial portion of the global CH4 source. The study’s findings have implications for understanding and mitigating climate change. The study concludes that the current generation of bottom-up inventories underestimates CH4 emissions from fossil fuel extraction, distribution, and use. “Our results imply that anthropogenic fossil CH4 emissions now account for about 30% of the global CH4 source and for nearly half of anthropogenic emissions, highlighting the critical role of emission reductions in mitigating climate change.”