Constraining the evolution of the fossil component of the global methane budget since the pre-industrial using 14C measurements in firn air and ice cores

Radiocarbon of atmospheric methane (14CH4) is much less studied than radiocarbon of atmospheric carbon dioxide (14CO2) yet has potential to serve as an unambiguous indicator of the balance between fossil and contemporaneous sources of this important greenhouse gas. Few measurements of atmospheric 14CH4 exist before the late 20th century. We present measurements of past atmospheric 14CH4 in firn air and ice at Summit, Greenland. These data provide a record of atmospheric 14CH4 from 2013 back to ~1750 CE. Results have been corrected for a small amount of cosmogenic in-situ production of 14CH4 within the ice crystal lattice. A firn gas transport model was used to simulate the transport of gases through the porous firn column and into fully closed ice, and an inverse model reconstructed the firn air and ice 14CH4 data into an atmospheric history. Our results from the mid-late 20th century agree with the only previously published measurements of 14CH4 from firn air (at Law Dome, Antarctica). Pre-industrial 14CH4 samples agree with the INTCAL13 14CO2 history within uncertainties, indicating that natural geologic methane emissions are very low and have been commonly overestimated in the global methane budget. From ~1880 to ~1950 CE, the atmospheric 14CH4 activity decreased via the Suess effect, indicating a 14 ± 2% fossil CH4 source in the mid 1900’s. After mid-century, despite increasing anthropogenic fossil CH4 emissions, the 14CH4 activity began increasing due to atmospheric nuclear bomb testing and direct 14CH4 emissions from nuclear power plants.

Keywords

Fossils, Methane, Carbon 14, Drill cores, Ice, Controlled atmospheres, Greenland, Carbon dioxide, Antarctica, Greenhouse gases

Citation

Hmiel, B., Dyonisius, M., Petrenko. V. V., Buizert, C., Smith, A. M., Place, P., Etheridge, D. M., Harth, C. M., Beaudette, R., Hua, Q., Yang, B., Vimont, I., Brook, E. J., Weiss, R. F., & Severinghaus, J. P. (2018). Constraining the evolution of the fossil component of the global methane budget since the pre-industrial using 14C measurements in firn air and ice cores. Paper presented at the AGU Fall Meeting, Washington, D. C., 10 to 14 December 2018. Retrieved from: https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/438657