Soil Moisture (SM) is a key parameter in northern Arctic and sub-Arctic (A-SA) environments that are highly vulnerable to climate change. We evaluated six SM satellite passive microwave datasets using thirteen ground-based SM stations across Northwestern America. The best agreement was obtained with SMAP (Soil Moisture Active Passive) products with the lowest RMSD (Root Mean Square Difference) (0.07 m$3$3 m${-3}$-3) and the highest R (0.55). ESA CCI (European Space Agency Climate Change Initiative) also performed well in terms of correlation with a similar R (0.55) but showed a strong variation among sites. Weak results were obtained over sites with high water body fractions. This study also details and evaluates a dedicated retrieval of SM from SMOS (Soil Moisture and Ocean Salinity) brightness temperatures based on the $\tau -\omega$tau-omega model. Two soil dielectric models (Mironov and Bircher) and a dedicated soil roughness and single scattering albedo parameterization were tested. Water body correction in the retrieval shows limited improvement. The metrics of our retrievals (RMSD = 0.08 m$3$3 m${-3}$-3 and R = 0.41) are better than SMOS but outperformed by SMAP. Passive microwave satellite remote sensing is suitable for SM retrieval in the A-SA region, but a dedicated approach should be considered.

Increased permafrost temperatures have been reported in the circum-Arctic, and widespread degradation of permafrost peatlands has occurred in recent decades. The timing of permafrost aggradation in these ecosystems could have implications for the soil carbon lability upon thawing, and an increased understanding of the permafrost history is therefore needed to better project future carbon feedbacks. In this study, we have conducted high-resolution plant macrofossil and geochemical analyses and accelerator mass spectrometry radiocarbon dating of active layer cores from four permafrost peatlands in northern Sweden and Norway. In the mid-Holocene, all four sites were wet fens, and at least three of them remained permafrost-free until a shift in vegetation toward bog species was recorded around 800 to 400 cal. BP, suggesting permafrost aggradation during the Little Ice Age. At one site, Karlebotn, the plant macrofossil record also indicated a period of dry bog conditions between 3300 and 2900 cal. BP, followed by a rapid shift toward species growing in waterlogged fens or open pools, suggesting that permafrost possibly was present around 3000 cal. BP but thawed and was replaced by thermokarst.

Climate warming in northern high latitudes has progressed twice as fast as the global average, leading to prominent but puzzling changes in vegetation structure and functioning of tundra and boreal ecosystems. While some regions are becoming greener, others have lost or shifted vegetation condition as indicated by a browning signal. The mechanisms underlying this 'greening or browning enigma' remain poorly understood. Here we use multi-sourced time-series of satellite-derived vegetation indices to reveal that spectral greening is associated with reductions in surface water cover (i.e. fraction of surface water bodies), whereas spectral browning is linked to increases in surface water cover. These patterns are consistently observed from both 30 m resolution Landsat data and 250 m resolution MODIS data on the basis of grid cells sized of 1, 2 and 4 km. Our study provides, to our knowledge, the first biome-scale demonstration that interactions between vegetation condition and water cover change can explain the contrasting trajectories of ecosystem dynamics across the northern high latitudes in response to climate warming. These divergent trajectories we identified have major implications for ecosystem functioning, carbon sequestration and feedbacks to the climate system. Further unraveling the interaction between vegetation and surface water will be essential if we are to understand the fate of tundra and boreal biomes in a warming climate.

Layers of permafrost developed during the 1950s and 1960s incorporated tritium from the atmosphere that originated from global nuclear weapons testing. In regions underlain by substantial permafrost, this tritium has been effectively trapped in ice since it was deposited and subject to radioactive decay alone, which has substantially lengthened its environmental half-life compared to areas with little or no permafrost where the weapons-test era precipitation has been subject to both decay and hydrodynamic dispersion. The Arctic is warming three times faster than other parts of the world, with northern regions incurring some of the most pronounced effects of climate change, resulting in permafrost degradation. A series of 23 waterbodies across the Canadian sub-Arctic spanning the continuous, discontinuous and isolated patches permafrost zones in northern Manitoba, Northwest Territories and Labrador were sampled. Surface water and groundwater seepage samples were collected from each lake and analyzed for tritium, stable isotopes (delta O-18 and delta H-2) and general water chemistry characteristics. Measured tritium was significantly higher in surface waters (SW) and groundwater seepage (GW) in water bodies located in the sporadic discontinuous (64 +/- 15 T U. in SW and 52 +/- 9 T U. in GW) and extensive discontinuous (53 +/- 7 T U. in SW and 61 +/- 7 T U. in GW) permafrost regions of the Northwest Territories than in regions underlain by continuous permafrost in northern Manitoba (< 12 T U. in both SW and GW) or those within isolated patches of permafrost in Labrador (16 +/- 2 T U. in SW and 21 +/- 4 T U. in GW). The greatest tritium enrichment (up to 128 T U.) was observed in lakes near Jean Marie River in the Mackenzie River valley, a region known to be experiencing extensive permafrost degradation. These results demonstrate significant permafrost degradation in the central Mackenzie River basin and show that tritium is becoming increasingly mobile in the sub-Arctic environment-at concentrations higher than expected-as a result of a warming climate. A better understanding of the cycling of tritium in the environment will improve our understanding of Arctic radioecology under changing environmental conditions.

The Kobuk River runs west along the southern Brooks Range from Gates of the Arctic National Park in Alaska, USA, to the Chukchi Sea. It is highly vulnerable to changes in climate due to its sub-Arctic location, unique geography, and permafrost foundation. Combined with its pristine condition, these qualities make the Kobuk an ideal system upon which to build a conceptual model for predicting ecosystem effects of climate change. We constructed a conceptual ecosystem model for the Kobuk River synthesizing surveyed baseline hydrologic, geomorphic and biotic conditions with literature on Arctic rivers. While the mainstem Kobuk has limited biological productivity, it provides spawning habitat and connectivity for large resident and migratory fish that rely upon off-channel habitat for food resources. System function is dependent largely on intermittent pulse flows that connect riverine habitats, allowing periods of late summer high productivity in off-channel habitat. Spring break-up and hill slope processes are critically important for maintaining habitat complexity and inter-connectivity. Climate change models predict the region will experience a disproportionate increase in average winter air temperature relative to summer temperatures, in the number of ice-free days, and in annual rainfall. Our conceptual model predicts that changes to fish and invertebrate populations on the Kobuk River will result not from physiological responses to increased temperatures, but rather to shifts in two main physical drivers: 1) spring break-up intensity, resulting in changes to scour rate and sediment deposition; and 2) discontinuous permafrost melt, resulting in widespread heterogeneous zones of active layer thickening and thermokarsting. The interaction of these two drivers offers four potential scenarios of geomorphic change in the system and four dramatically different biological outcomes. This model should help managers and scientists evaluate the magnitude and direction of ecosystem changes as they occur within the Kobuk system and potentially other sub-Arctic river systems.