Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment

Abstract

The continental shelves of the Arctic Ocean and surrounding seas contain large stocks of organic matter (OM) and methane (CH4), representing a potential ecosystem feedback to climate change not included in international climate agreements. We performed a structured expert assessment with 25 permafrost researchers to combine quantitative estimates of the stocks and sensitivity of organic carbon in the subsea permafrost domain (i.e. unglaciated portions of the continental shelves exposed during the last glacial period). Experts estimated that the subsea permafrost domain contains ∼560 gigatons carbon (GtC; 170–740, 90% confidence interval) in OM and 45 GtC (10–110) in CH4. Current fluxes of CH4 and carbon dioxide (CO2) to the water column were estimated at 18 (2–34) and 38 (13–110) megatons C yr−1, respectively. Under Representative Concentration Pathway (RCP) RCP8.5, the subsea permafrost domain could release 43 Gt CO2-equivalent (CO2e) by 2100 (14–110) and 190 Gt CO2e by 2300 (45–590), with ∼30% fewer emissions under RCP2.6. The range of uncertainty demonstrates a serious knowledge gap but provides initial estimates of the magnitude and timing of the subsea permafrost climate feedback.

Strong Increase of Thawing of Subsea Permafrost in the 22nd Century Caused by Anthropogenic Climate Change

“…Our present day (2020) SSPF ice area, ≈ 2.74 • 10 6 km 2 , is larger than the SSPF area reported by Overduin et al (2019), 2.48 • 10 6 km 2 which in turn is somewhat larger than the ≈ 2 • 10 6 km 2 presented in Sayedi et al (2020). Part of this difference may be explained by Overduin et al (2019) presenting a preindustrial estimate while Sayedi et al (2020) refers to presentday. In our model ≈ 0.15 • 10 6 km 2 of SSPF ice area disappears during the 170 year period in between.…”

Impacts of Permafrost Degradation on Carbon Stocks and Emissions under a Warming Climate: A Review

“…As a result of submerging extensive LGM permafrost, a large stock of carbon is stored in the subsea permafrost domains. Sayedi et al (2020) reported an organic carbon pool of 560 Pg C (170-740, 90% confidence level) in subsea permafrost and of 45 Pg C (10-110) as methane hydrates [49]. The subsea permafrost has undergone degradation, transferring POC into both the atmosphere and the Arctic Ocean by coastal erosion [2].…”

“…An annual CO 2 emission of ~29 Tg C from the subsea permafrost in Arctic Siberia (9.87 × 10 5 km 2 ) [50] and 38 Tg C (13-110) from the subsea permafrost has been estimated. Annual CH 4 emission ranges from 3-18 Tg C from the subsea permafrost [49,51,52]. Insufficient measurements of subsea permafrost largely increase uncertainties of estimates in subsea carbon storage and a wide range of carbon emission rates.…”

“…Later, a more conservative estimation yielded a stock of 20 Pg C sequestered in Arctic permafrost regions [55]. Methane hydrates in subsea permafrost domains are 45 Pg C [49]. Methane hydrates on the QTP amount to 175.4 ± 39.2 Pg C [56].…”

Reply on RC1

“…Most studies have been focused on the thawing of the terrestrial per-mafrost (Koven et al, 2015;Kleinen and Brovkin, 2018;Turetsky et al, 2020). Since the Last Glacial Maximum (LGM) about 3.5•10 6 km 2 (Sayedi et al, 2020) of permafrost soils were submerged by the sea level rising by about 120 m. These submerged permafrost sediments (sub-sea permafrost, SSPF) now form the major part of the Arctic Shelf. Since the benthic temperatures are above the freezing point and their interannual variability small, the submergence below sea water causes a slow but continuous thawing of the submerged permafrost sediments from the top.…”