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. Permafrost underlies one-quarter of the NorthernHemisphere but is at increasing risk of thaw from climate warming. Recentstudies across the Arctic have identified areas of rapid permafrostdegradation from both top-down and lateral thaw. Of particular concern isthawing syngenetic “yedoma” permafrost which is ice-rich and has a highcarbon content. This type of permafrost is common in the region around Fairbanks, Alaska, and across central Alaska expanding westward to the Seward Peninsula. A major knowledge gap is relating belowground measurements of seasonal thaw, permafrost characteristics, and residual thaw layer development with aboveground ecotype properties and thermokarst expansion that can readily quantify vegetation cover and track surface elevation changes over time. This study was conducted from 2013 to 2020 alongfour 400 to 500 m long transects near Fairbanks, Alaska. Repeat active layer depths, near-surface permafrost temperature measurements, electricalresistivity tomography (ERT), deep (&gt; 5 m) boreholes, and repeatairborne light detection and ranging (lidar) were used to measure top-downpermafrost thaw and map thermokarst development at the sites. Our studyconfirms previous work using ERT to map surface thawed zones; however, ourdeep boreholes confirm the boundaries between frozen and thawed zones thatare needed to model top-down, lateral, and bottom-up thaw. At disturbedsites seasonal thaw increased up to 25 % between mid-August and earlyOctober and suggests measurements to evaluate active layer depth must bemade as late in the fall season as possible because the projected increasein the summer season of just a few weeks could lead to significantadditional thaw. At our sites, tussock tundra and spruce forest areassociated with the lowest mean annual near-surface permafrost temperatureswhile mixed-forest ecotypes are the warmest and exhibit the highest degreeof recent temperature warming and thaw degradation. Thermokarst features,residual thaw layers, and taliks have been identified at all sites. Ourmeasurements, when combined with longer-term records from yedoma across the500 000 km2 area of central Alaska, show widespread near-surfacepermafrost thaw since 2010. Projecting our thaw depth increases, by ecotype,across the yedoma domain, we calculate a first-order estimate that 0.44 Pg of organic carbon in permafrost soil has thawed over the past 7 years, which,for perspective, is an amount of carbon nearly equal to the yearly CO2emissions of Australia. Since the yedoma permafrost and the variety ofecotypes at our sites represent much of the Arctic and subarctic land cover, this study shows remote sensing measurements, top-down and bottom-up thermal modeling, and ground-based surveys can be used predictively to identify areas of the highest risk for permafrost thaw from projected future climate warming.

Recent degradation of interior Alaska permafrost mapped with ground surveys, geophysics, deep drilling, and repeat airborne lidar