Permafrost thaw can accelerate climate warming by releasing carbon from previously frozen soil in the form of greenhouse gases. Rainfall extremes have been proposed to increase permafrost thaw, but the magnitude and duration of this effect are poorly understood. Here we present empirical evidence showing that one extremely wet summer (+100 mm; 120% increase relative to average June-August rainfall) enhanced thaw depth by up to 35% in a controlled irrigation experiment in an ice-rich Siberian tundra site. The effect persisted over two subsequent summers, demonstrating a carry-over effect of extremely wet summers. Using soil thermal hydrological modelling, we show that rainfall extremes delayed autumn freeze-up and rainfall-induced increases in thaw were most pronounced for warm summers with mid-summer precipitation rainfall extremes. Our results suggest that, with rainfall and temperature both increasing in the Arctic, permafrost will likely degrade and disappear faster than is currently anticipated based on rising air temperatures alone. Thawing permafrost releases carbon that serves as a positive feedback on climate warming. Here the authors experimentally demonstrate that rainfall extremes in the Siberian tundra increase permafrost thaw for multiple years, especially if rainfall coincides with warm periods.

Permafrost thaw can cause an intensification of climate change through the release of carbon as greenhouse gases. While the effect of air temperature on permafrost thaw is well quantified, the effect of rainfall is highly variable and not well understood. Here, we provide a literature review of studies reporting on effects of rainfall on ground temperatures in permafrost environments and use a numerical model to explore the underlying physical mechanisms under different climatic conditions. Both the evaluated body of literature and the model simulations indicate that continental climates are likely to show a warming of the subsoil and hence increased end of season active layer thickness, while maritime climates tend to respond with a slight cooling effect. This suggests that dry regions with warm summers are prone to more rapid permafrost degradation under increased occurrences of heavy rainfall events in the future, which can potentially accelerate the permafrost carbon feedback.

Modeling the physical state of permafrost landscapes is a crucial addition to field observations in order to understand the feedback mechanisms between permafrost and the atmosphere within a warming climate. A common hypothesis in permafrost modeling is that vertical heat conduction is most relevant to derive subsurface temperatures. While this approach is mostly applicable to flat landscapes with little topography, landscapes with more topography are subject to lateral flow processes as well. With our study, we contribute to the growing body of evidence that lateral surface and subsurface processes can have a significant impact on permafrost temperatures and active layer properties. We use a numerical model to simulate two idealized hillslopes (a steep and a medium case) with inclinations that can be found in Adventdalen, Svalbard, and compare them to a flat control case. We find that ground temperatures within the active layer uphill are generally warmer than downhill in both slopes (with a difference of up to ∼0.8 ∘C in the steep and ∼0.6 ∘C in the medium slope). Further, the slopes are found to be warmer in the uphill section and colder in the base of the slopes compared to the flat control case. As a result, maximum thaw depth increases by about 5 cm from the flat (0.98 m) to the medium (1.03 m) and the steep slope (1.03 m). Uphill warming on the slopes is explained by overall lower heat capacity, additional energy gain through infiltration, and lower evaporation rates due to drier conditions caused by subsurface runoff. The major governing process causing the cooling on the downslope side is heat loss to the atmosphere through evaporation in summer and enhanced heat loss in winter due to wetter conditions and resulting increased thermal conductivity. On a catchment scale, these results suggest that temperature distributions in sloped terrain can vary considerably compared to flat terrain, which might impact the response of subsurface hydrothermal conditions to ongoing climate change.

Permafrost carbon, stored in frozen organic matter across vast Arctic and sub-Arctic regions, representsa substantial and increasingly vulnerable carbon reservoir. As global temperatures rise, the acceleratedthawing of permafrost releases greenhouse gases (GHGs), exacerbating climate change. However, freshlythawed-out permafrost carbon may also experience lateral transport in the groundwater towards surface watersuch as rivers and lakes where it can be buried and removed from the active carbon cycle. The mobilizationand groundwater transport mechanisms are poorly understood and not yet accounted for in global climatemodels, leading to high uncertainties in the predictions of the permafrost carbon feedback. Here, we simulatecarbon transport in the form of a non-reactive tracer representing dissolved organic carbon (DOC) usinga physics-based numerical model in order to focus on the governing cryotic and hydrodynamic transportmechanisms relevant for permafrost regions. We analyze transport times for different carbon pools withinthe active layer under present-day climatic conditions as well as ancient carbon in the upper permafrostand thaw-out under different simulated future climate scenarios. We find that carbon at the bottom of theactive layer experiences rapid waterborne transport upon thaw due to highly saturated conditions, whilecarbon released close to the surface experiences slower waterborne transport in unsaturated soil. Ancientpermafrost carbon release exhibits vastly different transport behavior depending on the climate warmingrate. Gradual warming leads to small fractions of carbon being released every year, while the majority getsmoved vertically downwards through percolation. Upon simulated abrupt thaw as a consequence of a singlevery warm year, transport times are more similar to the active layer carbon released in saturated conditions.The results highlight the importance to account for the potential lateral export of permafrost carbon and itspotential to buffer or attenuate the release of such carbon as GHGs.