Threshold loss of discontinuous permafrost and landscape evolution

Abstract

This study demonstrates linkages between the 1997/1998 El Niño/Southern Oscillation index and a threshold shift to increased permafrost loss within a southern Taiga Plains watershed, Northwest Territories, Canada. Three-dimensional contraction of permafrost plateaus and changes in vegetation structural characteristics are determined from multitemporal airborne Light Detection And Ranging (LiDAR) surveys in 2008, 2011 and 2015. Morphological changes in permafrost cover are compared with optical image analogues from 1970, 1977, 2000 and 2008 and time-series hydro-climate data. Results demonstrate that significant changes in air temperature, precipitation, runoff and a shortening of the snow-covered season by 35 days (1998-2014) and 50 days (1998 only) occurred after 1997. The albedo reduction associated with 35 and 50 days less snow cover leads to increases in shortwave energy receipt during the active thaw period of ~12% (3% annually) and ~16% (5% annually), respectively. From 2000 to 2015, sporadic permafrost loss accelerated from 0.19% (of total basin area) per year between 1970 and 2000 to 0.58% per year from 2000 to 2015, with a projected total loss of permafrost by ~2044. From ~1997 to 2011, we observe a corresponding shift to increased runoff ratio. However, observed increases in the proportion of snow precipitation and the volumetric contribution of permafrost loss to runoff post-1997 (0.6-6.4% per year) cannot fully explain this shift. This suggests increases in drainage efficiency and possible losses from long-term groundwater storage as a result of subtle terrain morphological and soil zone hydraulic conductivity changes. These hydrological changes appear coincident with high vegetation mortality at plateau margins combined with succession-related canopy growth in some bog and fen areas, which are presumed to be drying. Similar changes in runoff response were observed at adjacent Birch, Trout and Jean Marie River watersheds indicating that observations are representative of northern Boreal sporadic permafrost/wetland watersheds in the Taiga Plains.

The Influence of Shallow Taliks on Permafrost Thaw and Active Layer Dynamics in Subarctic Canada

“…Chasmer and Hopkinson () postulate that the strong El Niño/Southern Oscillation (ENSO) event of 1997/1998 may also have been a tipping point for accelerated permafrost thaw in areas of discontinuous permafrost. They show that permafrost coverage and runoff ratios change significantly after this event; however, they do not identify a mechanism for this expedited thaw.…”

Direct and indirect climate change effects on carbon dioxide fluxes in a thawing boreal forest–wetland landscape

“…Additionally, transition zones were delineated based on aerial photographs as areas of wetland expansion (and thus of forest loss) since 1977 (see Chasmer et al, 2010). Similar to collapse-scar bogs, fens are expanding due to permafrost thaw (Chasmer & Hopkinson, 2016). Their flux footprint contributions were then separately derived for each half-hourly flux measurement.…”

Minor contribution of overstorey transpiration to landscape evapotranspiration in boreal permafrost peatlands

“…Along its southern limit, permafrost is relatively warm and thin (Smith, Burgess, Riseborough, & Mark Nixon, ). In recent decades, warming air temperatures have accelerated the rate of permafrost thaw (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, ; Chasmer & Hopkinson, ; Grosse et al, ; Quinton, Hayashi, & Chasmer, ), impacting forest composition, structure, and function (Baltzer et al, ; Jorgenson et al, ; Lara et al, ; Sniderhan & Baltzer, ), and thus carbon, water, and energy fluxes (Grosse et al, ; Helbig, Wischnewski, et al, ; Turetsky, Wieder, & Vitt, ). Thawing sporadic discontinuous permafrost, that is, permafrost occurring on <50% of the landscape, can induce surface subsidence, thereby replacing forests on peat plateaus with permafrost‐free peatlands (Baltzer et al, ; Jorgenson et al, ; Lara et al, ).…”