Co-evolution of Microtopography and Hydrology is Projected to Lead to Drier Polygonal Tundra Landscapes (INVITED)

Polygonal tundra, low-relief regions underlain by ice-wedge polygons, is common in continuous permafrost regions, occupying, for example, an estimated 65% of the Northern Alaska land surface. Shallow ice-wedge polygons in these regions can alter the microtopography and thus the landscape hydrology. Low-centered polygons with raised rims can store significant amounts of water in microtopographic depressions and impede runoff, whereas runoff may be more efficiently conveyed across the landscape in regions where uneven subsidence caused by melting ice wedges has lowered the edges of polygons to create high-centered polygons with connected networks of troughs. This interplay between thermal hydrology and dynamic microtopography has until recently been only crudely represented in models, which has created uncertainty about the trajectory of polygonal tundra in a warming Arctic. We used the integrated surface/subsurface cryohydrology simulator ATS with new capabilities to model subsidence and co-evolution of microtopography and hydrology in end-of-century projections for five polygonal tundra catchments ranging in size from a few hundred to a few thousand polygons. The ATS simulations for one catchment near Utqiaġvik, AK, build on fine-scale models that have been carefully evaluated against multiple types of observations and agree with observed landscape runoff and evapotranspiration, which improves confidence in our projections. Projections indicate significant increases in the active layer thickness and tens of centimeters of bulk subsidence in the coming decades from melting of excess ice in scenarios with moderate and high greenhouse gas emissions. Soil consolidation resulting from the melting of excess ice exposes more soil carbon to thawing in subsequent summers. Simulations with and without the effects of subsidence quantify the role of uneven subsidence. Specifically, projected transitions from low-centered to high-centered polygons decrease depression storage and increase landscape runoff efficiency, which will help maintain river flows but results in significantly drier tundra.