Fig. 4.

Circumpolar permafrost carbon emissions from gradual thaw (land) versus abrupt thaw (lakes) from 1950 to 2100. Permafrost soil carbon emissions, modeled according to representative concentration pathway (RCP) 4.5 and 8.5 scenarios, are distinguished for homogenous gradual thaw from CLM4.5BGC (brown) and heterogeneous abrupt thaw by AThaw thermokarst-lake formation (median values, blue). Cumulative emissions, including percent increase in emissions from old permafrost carbon by abrupt thaw (a, b). Annual CH₄ and CO₂ emissions expressed individually as Tg yr⁻¹ (c, d) and collectively as Tg C-CO₂e yr⁻¹ (e, f) based on a GWP₁₀₀ of 28 (ref. ¹²) and units conversions shown in Supplementary Table 2. Error bars (c, d) surrounding the median lake emissions are the 68% uncertainty range from a 500 member AThaw model ensemble. Radiative forcing (g, h) associated with fluxes in c and d. In g and h, the increase in circumpolar permafrost-carbon radiative effect (CPCRE) attributed to abrupt thaw lakes is shown only for years 2018–2100, when the AThaw modeled carbon release exceeds one standard deviation in simulated CLM permafrost carbon fluxes during the 1950– 2017 reference period