The permafrost carbon inventory on the Tibetan Plateau: a new evaluation using deep sediment cores

Abstract

The permafrost organic carbon (OC) stock is of global significance because of its large pool size and the potential positive feedback to climate warming. However, due to the lack of systematic field observations and appropriate upscaling methodologies, substantial uncertainties exist in the permafrost OC budget, which limits our understanding of the fate of frozen carbon in a warming world. In particular, the lack of comprehensive estimates of OC stocks across alpine permafrost means that current knowledge on this issue remains incomplete. Here, we evaluated the pool size and spatial variations of permafrost OC stock to 3 m depth on the Tibetan Plateau by combining systematic measurements from a substantial number of pedons (i.e. 342 three-metre-deep cores and 177 50-cm-deep pits) with a machine learning technique (i.e. support vector machine, SVM). We also quantified uncertainties in permafrost carbon budget by conducting Monte Carlo simulations. Our results revealed that the combination of systematic measurements with the SVM model allowed spatially explicit estimates to be made. The OC density (OC amount per unit area, OCD) exhibited a decreasing trend from the south-eastern to the north-western plateau, with the exception that OCD in the swamp meadow was substantially higher than that in surrounding regions. Our results also demonstrated that Tibetan permafrost stored a large amount of OC in the top 3 m, with the median OC pool size being 15.31 Pg C (interquartile range: 13.03-17.77 Pg C). 44% of OC occurred in deep layers (i.e. 100-300 cm), close to the proportion observed across the northern circumpolar permafrost region. The large carbon pool size together with significant permafrost thawing suggests a risk of carbon emissions and positive climate feedback across the Tibetan alpine permafrost region.

State of the Climate in 2016

“…The amount of organic carbon in the Tibetan and North China region is currently estimated at 35.7 Pg (Luo et al 2000;Mu et al 2015;Ding et al 2016). Simple calculations based on extremely limited data suggest that another ~350-465 Pg C can be found in additional deep terrestrial deposits, although more sampling and data synthesis need to be done to verify or revise these potential deep permafrost carbon deposits (Schuur et al 2015).…”

A conceptual model of the controlling factors of soil organic carbon and nitrogen densities in a permafrost-affected region on the eastern Qinghai-Tibetan Plateau

“…Based on these data, the carbon pool in the upper 3 m was estimated to be about 33 Pg by upscaling the field observation data to the regional scale data according to land cover types [ Mu et al ., ]. A machine learning technique was used to estimate the SOC pools on the QTP, and it was reported that the SOC storage in the upper 3 m depth was 15 Pg [ Ding et al ., ]. The discrepancy between these studies was attributed to two factors.…”

Warming effects on permafrost ecosystem carbon fluxes associated with plant nutrients

“…, Ding et al. ). A warmer climate may promote microbial breakdown of organic C and increase soil‐respired carbon dioxide (CO 2 ) emissions into the atmosphere, which will lead to a positive C feedback to climate change (Schädel et al.…”

“…, Ding et al. ). Therefore, a comprehensive analysis from Tibetan alpine permafrost regions may provide novel insights for understanding the interactions of N and P availabilities with C cycling under climate warming.…”

“…The alpine swamp meadow is one of the three most common grassland types on the Tibetan Plateau and contains the highest soil C density (C amount per area, 65 kg C/m 2 in top 3 m of soil; Ding et al. ) of these grassland ecosystems, similar in magnitude to C densities from soils across northern circumpolar permafrost region (Hugelius et al. ).…”