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Shrub expansion is among the most conspicuous of the warming-related phenomena occurring within tundra regions, but its effects on vertebrates are not well understood. Among passerines, the habitats of shrub- and tundra-adapted species are expected to increase and decrease, respectively, by roughly equivalent amounts. However, shrub expansion is highly discontinuous, and these predictions are typically based on coarse-scale models of vegetation that permit limited spatial heterogeneity. We used a hindcasting approach to investigate spatial and interspecific heterogeneity in passerine densities through time, as a function of changes in Normalized Difference Vegetation Index (NDVI). We conducted line transect surveys of passerines between 2015 and 2019 at four sites in arctic and subarctic Alaska. Collecting the encounter locations permitted fine-scale assessment of covariates of density. We fit spatial distance sampling models to these data and then used the predicted relationship between passerine density and NDVI to hindcast changes in density since 1999. Our hindcasts suggested limited areal changes in the habitat of tundra-adapted species over the last twenty years while the habitat corresponding to tall shrub–adapted species has likely expanded. Thus, our results contrast with previous predictions due partially to the finer spatial scale of our inference, which allowed for portions of the landscape to resist shrub expansion.

Rising temperatures in the Arctic can result in thaw of permafrost, with widespread implications for ecosystems and infrastructure. We analyzed mean annual air and ground temperatures in the eight northernmost national parks in Alaska using data from thirty-three National Park Service climate monitoring stations and eight National Weather Service stations. Mean annual air temperatures (MAATs) from 2014 to 2019 increased in a stepwise fashion relative to the preceding thirty-year period by at least 1°C at all locations in the study area; the increase was near 2°C in Denali National Park and most of the Arctic Alaska parks and 3°C in the far western coastal areas of the Arctic parks. The increase in mean annual ground temperatures (MAGT) was approximately equal to the increase in MAAT in windswept tundra areas with minimal snow, whereas under deeper taiga and alpine snowpacks the increase in MAGT was about half as large as the increase in MAAT. If the warm temperatures observed during 2014 to 2019 persist, there will be widespread degradation of permafrost in portions of these national parks and in similar environments across Alaska.

Lakes and ponds are important ecosystem components in arctic lowlands, and they are prone to rapid changes in surface area by thermokarst expansion and by sudden lake drainage. The 30 m resolution Landsat record (1984–2018) was used to derive a record of changes in the area of lakes and ponds in the five National Parks of northern Alaska. Surface-water area declined significantly in portions of the study area with ice-rich permafros t and water bodies of thermokarst origin. These declines were associated with rapid lake drainage events resulting from the thermoerosion of outlets. Thermoerosion was probably favored by the record warm mean annual temperatures in the study area, combined with precipitation that fluctuated near long-term normals. The rate of lake loss by rapid drainage was greatest in 2005–2007 and 2018. In landscapes with permafrost of lower ice content and water bodies in depressions of non-thermokarst origin, surface-water area generally fluctuated in response to year-to-year changes in precipitation, without a long-term trend, and lake drainage events were rare. Loss of surface water in ice-rich lowlands is likely to continue as the climate warms, with associated impacts on aquatic wildlife.