An increase in the abundance of open vertical taliks, particularly beneath water bodies and disturbed zones, may be expected in some areas and has been inferred from observed drainage of thermokarst lakes ( 176 ; 143 ). However, lack of adequate historical context and limited current data presents a challenge in distinguishing between the classic evolution of thermokarst lakes and an actual trajectory of climate‐induced widespread change. An increase in suprapermafrost and intrapermafrost taliks is expected and modeled (e.g., 181 ), yet observational support for this contention is also limited. A study by 63 provides indirect support for the growth of supra‐ and intrapermafrost taliks in the Yukon Flats of Alaska over the past several decades based on a large‐scale examination of lake area dynamics and subsurface permafrost characterization.

Although large‐scale permafrost models generally agree that permafrost spatial extent has decreased in the past several decades with southern boundaries moving northward and altitudinal boundaries moving upward ( 181 ; 51 ), observational support is limited by the lack of baseline information and established methods for effectively mapping large‐scale permafrost at sufficient resolution to detect change. As an alternative, landscape and vegetation changes strongly linked to permafrost degradation and detectable via aerial or satellite imagery have been used to infer reductions in permafrost spatial extent in study areas of interior Alaska, USA ( 67 ) and Northwest Territories, Canada ( 120 ).

The most readily observed mode of permafrost thaw is increasing active layer thickness (ALT). Many studies report large interannual ALT variability and note a surprising lack of consistent observed ALT increases (see references in Table 1 ), despite ubiquitous positive air temperature trends. This apparent discrepancy underscores the fact that the thermal regime in permafrost soils is additionally mediated by interactions among soil properties, moisture, snow, and vegetation that positively or negatively influence permafrost stability ( 68 ). For example, pore ice has a higher thermal conductivity than pore water, and thus soil heat transfer is typically more efficient in winter than summer. This leads to a “thermal offset” between the mean annual temperatures at the permafrost table and the ground surface ( 141 ). A pan‐Arctic assessment by 112 , using ALT observational data and a land surface model, highlighted the importance of hydrologic variables, including snow depth and summer soil moisture, which counterbalance or amplify ALT increases expected from atmospheric warming alone. As an example, a thinner snowpack may impede soil warming by offering weaker winter insulation to cold atmospheric air, whereas a thicker snowpack may enhance soil warming through the opposing effect. Variability of snow depth and summer soil moisture may help explain positive ALT trends in Eurasia and the contrasting lack of consistent trends observed in North America.

There is ample evidence that permafrost is warming and thawing in the pan‐Arctic basin ( 110 ; 51 ; 127 ). It is projected that these trends will continue and lead to large‐scale losses of near‐surface permafrost ( 139 ; 76 ; 114 ). However, nonuniform rates of permafrost degradation and irregular spatial distribution of thaw are anticipated, thereby imposing important sources of uncertainty in estimates of future conditions.

Hydrologic Impacts of Permafrost Thaw

Permafrost degradation generated through the modes described above will likely produce large changes in surface and subsurface hydrology in some areas. The input of permafrost meltwater contributes a relatively small and transitory pulse of subsurface water that is unlikely to be a dominant, long‐term signal in streamflow records in the watershed or basin undergoing permafrost thaw. The more impactful hydrologic modification from permafrost thaw is in the concomitant change of the hydrogeologic framework, particularly in permeable settings that allow for the opening of previously blocked (permafrost‐limited) vertical and lateral flowpaths that can transmit large groundwater fluxes when thawed. Permafrost thaw can also generate rapid landscape changes in certain settings (e.g., thermokarsting and plateau subsidence; 120) that in turn influence surface water storage, routing, and runoff (26).

Changes in the three‐dimensional distribution of permafrost have the potential to influence surface and subsurface water fluxes and flowpaths (Fig. 4) at local, regional, and circum‐Arctic scales. Large‐scale Arctic assessments project a transition away from a surface‐water dominated system to a more groundwater‐dominated system. Hydrologic and geochemical support for this contention is derived primarily from sub‐Arctic and low Arctic basins (Table 1) with warm, discontinuous permafrost. Permafrost thaw may enhance subsurface fluxes, including soil drainage and recharge, suprapermafrost flow, groundwater–surface water exchange, subpermafrost flow, and baseflow. Direct and indirect evidence for such changes in water fluxes induced by permafrost thaw has been observed throughout northern regimes (Table 1). However, due to inherent geologic heterogeneity and the hydrodynamics (transient water table elevations and hydraulic gradients) associated with permafrost thaw, as well as additional hydroclimatic variables (i.e., changes in precipitation, air temperature, evapotranspiration, and snow), actual spatial and temporal changes in subsurface water fluxes present a challenge for prediction. For example, water tables in the active layer may decline with permafrost degradation, and the reduction in hydraulic gradient may or may not be countered by increases in transmissivity affected by permafrost thaw.

Figure 4 Open in figure viewer Overview of impacts and feedbacks of permafrost (PF) thaw on water fluxes and distribution. Indicators of the expected trajectory of each variable with continued permafrost thaw, derived from a combination of observed trends and modeling analyses, are denoted as follows: , increase; , decrease; , variable response; ?, uncertain response. GW, groundwater; ET, evapotranspiration; GHG, greenhouse gas.

Increased ALT resulting in enhanced flow through the suprapermafrost aquifer is often postulated to be the cause for the increased baseflow detected in the historical record in many permafrost basins (Table 1). However, the saturated K of a vertical soil profile typical of boreal forests with a thick organic layer overlying mineral soil is known to decrease exponentially with depth (119; 148). Thus, the thickening of the active layer can drive more water to be transmitted through a lower K zone (72). If water table elevation is maintained in the thicker active layer, then suprapermafrost flux will increase, but if water table elevation declines with an increase in soil drainage, then the enhancements in suprapermafrost transmissivity due to thickening may be countered by reductions in bulk K (Fig. 5). An alternative explanation for commonly observed positive trends in baseflow is wholesale permafrost loss spanning decades that leads to the enhancement of regional groundwater circulation and discharge to major rivers (161). However, the attendant increase in baseflow derived from the latter mechanism is also strongly influenced by potential and nonstraightforward changes in the water table elevation affected by thaw. Wholesale permafrost loss is more likely to contribute to baseflow increases in discontinuous permafrost than in continuous permafrost, where permafrost tends to be cold and thick.

Figure 5 Open in figure viewer Timing of spring infiltration and seasonal thaw penetration in a typical high over low hydraulic conductivity (K) soil profile for a (a) current and (b) warmer climate. Deeper active layers lead to potential increased subsurface hydrologic storage, particularly when the active layer penetrates into the lower K zone.

Concurrent changes in water fluxes and flowpaths resulting from permafrost thaw influence the distribution and volume of water as soil moisture, lakes, wetlands, groundwater storage, and, in the winter, as aufeis and river ice. The projected trajectories of change in water distribution are less uniform than those of water flux (Fig. 4). Observational support for permafrost thaw–related changes in soil moisture, the distribution of lakes and wetlands, and groundwater storage is often inconsistent in terms of the magnitude and even direction of changes (trajectory information and supporting references in Table 1). This observed variability underscores the importance of location and scale of observation, as well as complexities in feedbacks and interactions between climatologic, thermal, ecologic, and hydrologic processes. A relevant research question currently posed at regional to pan‐Arctic scales relates to identifying the areas that will become drier or wetter.

In addition to potential large‐scale redistribution of near‐surface water, changes in water stored in the form of ice during winter months are also expected. Aufeis, or icing, is layered ice that accumulates along streams and rivers as a result of groundwater discharge during freezing temperatures (168). Aufeis volume and spatial distribution may be expected to increase concomitantly with observed increases in baseflow, yet warming winter temperatures and the thermal energy from increased groundwater discharge may counteract, to some degree, expected aufeis build up. Observational studies of changes in aufeis distribution are limited, but show no substantial change (177). However, winter river ice thickness has been observed to be decreasing (Table 1), fueling concerns for winter transportation hazards. Also, river ice breakup, which normally occurs in the spring, can be triggered by mid‐winter warm periods. These mid‐winter warm periods have caused destructive ice jams and river flooding in temperate regions of Canada (e.g., 6), and such events may become increasingly common at higher latitudes in the coming decades. Evaluating how much river ice thinning is due to warming from below via enhanced groundwater temperature and flux vs. increased heat exchange from above due to warming winter air temperature remains untested. Less is known about historical trends in river ice thickness than in river ice phenology because the latter is more readily derived from remote sensing techniques.