Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity

摘要

High-latitude soils store vast amounts of perennially frozen and thereforeinert organic matter. With rising global temperatures and consequentpermafrost degradation, a part of this carbon stock will become availablefor microbial decay and eventual release to the atmosphere. We havedeveloped a simplified, two-dimensional multi-pool model to estimate thestrength and timing of future carbon dioxide (CO₂) and methane(CH₄) fluxes from newly thawed permafrost carbon (i.e. carbon thawedwhen temperatures rise above pre-industrial levels). We have especiallysimulated carbon release from deep deposits in Yedoma regions by describingabrupt thaw under newly formed thermokarst lakes. The computationalefficiency of our model allowed us to run large, multi-centennial ensemblesunder various scenarios of future warming to express uncertainty inherent tosimulations of the permafrost carbon feedback.Under moderate warming of the representative concentration pathway (RCP) 2.6scenario, cumulated CO₂ fluxes from newly thawed permafrost carbonamount to 20 to 58 petagrams of carbon (Pg-C) (68% range) by the year2100 and reach 40 to 98 Pg-C in 2300. The much larger permafrost degradationunder strong warming (RCP8.5) results in cumulated CO₂ release of 42 to141 Pg-C and 157 to 313 Pg-C (68% ranges) in the years 2100 and 2300,respectively. Our estimates only consider fluxes from newly thawedpermafrost, not from soils already part of the seasonally thawed activelayer under pre-industrial climate. Our simulated CH₄ fluxescontribute a few percent to total permafrost carbon release yet they cancause up to 40% of total permafrost-affected radiative forcing in the21st century (upper 68% range). We infer largest CH₄ emissionrates of about 50 Tg-CH₄ per year around the middle of the 21stcentury when simulated thermokarst lake extent is at its maximum and whenabrupt thaw under thermokarst lakes is taken into account. CH₄ releasefrom newly thawed carbon in wetland-affected deposits is only discernible inthe 22nd and 23rd century because of the absence of abrupt thawprocesses. We further show that release from organic matter stored in deepdeposits of Yedoma regions crucially affects our simulated circumpolarCH₄ fluxes. The additional warming through the release from newlythawed permafrost carbon proved only slightly dependent on the pathway ofanthropogenic emission and amounts to about 0.03–0.14 °C (68%ranges) by end of the century. The warming increased further in the22nd and 23rd century and was most pronounced under the RCP6.0scenario, adding 0.16 to 0.39 °C (68% range) to simulatedglobal mean surface air temperatures in the year 2300.