Global emissions of fossil methane are higher than previously predicted. While the share of natural geological emissions from the earth's crust is exactly that estimated by INGV over the last 15 years. The important role of geological phenomena in the contribution of greenhouse gases to the atmosphere is confirmed by a study by INGV and the National Oceanic and Atmospheric Administration, published in Nature
Understanding the dynamics of climate change through the analysis of global greenhouse gas emissions. This is the aim of the Upward revision of global fossil fuel methane emissions based on isotopic database study, coordinated by the National Oceanic and Atmospheric Administration (NOAA) of the United States, in collaboration with the National Institute of Geophysics and Volcanology (INGV) of Rome. The research, published in Nature, demonstrates that emissions of fossil methane (CH4), the result of both emissions produced by human activity and natural geological emissions from the earth's crust, are higher than expected so far in the global greenhouse gas budget. In particular, the share of fossil methane deriving from the production and distribution of natural gas and oil (anthropic) is greater than that indicated in the global inventories, while the share of natural geological emissions (emissions relating to the natural outgassing of the earth's crust) is exactly that estimated by INGV over the last 15 years.
"Methane is the most important greenhouse gas after water vapor (H2O) and CO2, and knowing its global emissions is of fundamental importance for understanding the dynamics of climate change", says Giuseppe Etiope, INGV researcher and scientific director of Labour. “Geological emissions are due to degassing processes, or natural exhalation of gas from the ground, along faults and fractures of the earth's crust which often give rise to gaseous manifestations on the surface, known as seeps and mud volcanoes. The geological emission represents the second natural source of methane for the atmosphere after wetlands (the large swamps and marshes of the tropical and boreal areas)”.
For some time, INGV had provided global estimates of methane flows from these natural phenomena, equal to approximately 60 million tons per year, based on direct measurements on the ground and so-called bottom-up extrapolations. These estimates were also reported in the latest report of the Intergovernmental Panel on Climate Change (IPCC) of 2013. Using inverse techniques, called top-down techniques, the new NOAA study confirms the numbers provided by INGV.
“The natural share of fossil methane emissions, the geological seepage studied by INGV, which does not depend on human activity, is considerable and varies from one third to more than half the value of the anthropogenic ones”, continues Etiope.
Furthermore, for anthropogenic sources, the study suggests a decrease in the last few decades of emissions from petroleum activity. Understanding the causes of this decrease is of fundamental importance for defining future climate change mitigation strategies. But if natural outgassing of methane from the earth's crust increases in the future, man-made emissions mitigation may not have the desired effects. "The study, concludes Ethiope, confirms the important role of geological phenomena in the contribution of greenhouse gases to the atmosphere".
Extended
Upward revision of global fossil fuel methane emissions based on isotopic database
Methane (CH4) has the second largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, and emissions from global fossil fuel (FF) industry activities (FFInd, ie vented and leaked CH4 from natural gas (NG), oil, and coal extraction, processing, transport, and use) are thought to contribute 15-22% to the total CH4 budget1 based on bottom-up2,3 (ie inventory-based) and top-down (ie atmospheric data-based) inversion4–9 and box-model10,11 studies. Significant questions remain about emission trends and about the contribution of largely co-located FFInd sources and natural geological FF seepage12,13 (FFGeo) to total FF (FFTot) CH4 emissions. This study re-evaluates the global CH4
budget and the contribution of FF based on long-term global CH4 and δ13C–CH4 records.
We have compiled the largest isotopic source signature (δ13CSource) database to-date, including those for FF, microbial and biomass burning (BB) sources. We find that FFTot CH4 emissions are not increasing over time, but are 60–110% greater than current estimates due to significant revisions in δ13CSource (especially isotopically lighter FF sources), and this is consistent with the observed global latitudinal CH4 gradient. Accounting for previously often neglected FFGeo (using ice core records and consistent with bottom-up and radiocarbon estimates12,13), CH4 emissions from NG, oil, and coal production and use are 20–60% greater than inventories. While this implies a greater potential for the FF industry
improvements to mitigate anthropogenic climate forcing, we also find that NG CH4 emissions as a fraction of production have declined from ~8% to ~2% over the past three decades. A better understanding of the industry processes that have led to this decline may help to develop strategies for
reducing CH4 emissions in the future.
Images:
Measurements of methane flux from natural gas emission from rocks along a tectonic fault in Azerbaijan (photo by L. Innocenzi, INGV, 2003)
Methane emission from a mud volcano in Italy (Regnano; photo by G. Etiope, INGV). Mud volcanoes are a major geological source of methane to the atmosphere. (Ethiope G., 2015. Natural Gas Seepage. The Earth's hydrocarbon degassing. Springer, pp. 199)
Link to the article
(http://www.nature.com/nature/journal/v538/n7623/full/nature19797.html)