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Arctic research faces unprecedented disruptions due to COVID-19. This ‘pause’ gives an opportunity to reflect on the current state and the future of Arctic science and move towards a more resilient, thus equitable, coordinated, safe and locally-embedded Arctic research enterprise. Arctic science has been greatly affected by COVID-19. This comment looks forward to how Arctic science could be conducted in the future.

Modeling hydrological processes in the Alaskan sub-arctic is challenging because of the extreme spatial heterogeneity in soil properties and vegetation communities. Nevertheless, modeling and predicting hydrological processes is critical in this region due to its vulnerability to the effects of climate change. Coarse-spatial-resolution datasets used in land surface modeling pose a new challenge in simulating the spatially distributed and basin-integrated processes since these datasets do not adequately represent the small-scale hydrological, thermal, and ecological heterogeneity. The goal of this study is to improve the prediction capacity of mesoscale to large-scale hydrological models by introducing a small-scale parameterization scheme, which better represents the spatial heterogeneity of soil properties and vegetation cover in the Alaskan sub-arctic. The small-scale parameterization schemes are derived from observations and a sub-grid parameterization method in the two contrasting sub-basins of the Caribou Poker Creek Research Watershed (CPCRW) in Interior Alaska: one nearly permafrost-free (LowP) sub-basin and one permafrost-dominated (HighP) sub-basin. The sub-grid parameterization method used in the small-scale parameterization scheme is derived from the watershed topography. We found that observed soil thermal and hydraulic properties – including the distribution of permafrost and vegetation cover heterogeneity – are better represented in the sub-grid parameterization method than the coarse-resolution datasets. Parameters derived from the coarse-resolution datasets and from the sub-grid parameterization method are implemented into the variable infiltration capacity (VIC) mesoscale hydrological model to simulate runoff, evapotranspiration (ET), and soil moisture in the two sub-basins of the CPCRW. Simulated hydrographs based on the small-scale parameterization capture most of the peak and low flows, with similar accuracy in both sub-basins, compared to simulated hydrographs based on the coarse-resolution datasets. On average, the small-scale parameterization scheme improves the total runoff simulation by up to 50 % in the LowP sub-basin and by up to 10 % in the HighP sub-basin from the large-scale parameterization. This study shows that the proposed sub-grid parameterization method can be used to improve the performance of mesoscale hydrological models in the Alaskan sub-arctic watersheds.

Difficulties in obtaining accurate precipitation measurements have limited meaningful hydrologic assessment for over a century due to performance challenges of conventional snowfall and rainfall gauges in windy environments. Here, we compare snowfall observations and bias adjusted snowfall to end‐of‐winter snow accumulation measurements on the ground for 16 years (1999–2014) and assess the implication of precipitation underestimation on the water balance for a low‐gradient tundra wetland near Utqiagvik (formerly Barrow), Alaska (2007–2009). In agreement with other studies, and not accounting for sublimation, conventional snowfall gauges captured 23–56% of end‐of‐winter snow accumulation. Once snowfall and rainfall are bias adjusted, long‐term annual precipitation estimates more than double (from 123 to 274 mm), highlighting the risk of studies using conventional or unadjusted precipitation that dramatically under‐represent water balance components. Applying conventional precipitation information to the water balance analysis produced consistent storage deficits (79 to 152 mm) that were all larger than the largest actual deficit (75 mm), which was observed in the unusually low rainfall summer of 2007. Year‐to‐year variability in adjusted rainfall (±33 mm) was larger than evapotranspiration (±13 mm). Measured interannual variability in partitioning of snow into runoff (29% in 2008 to 68% in 2009) in years with similar end‐of‐winter snow accumulation (180 and 164 mm, respectively) highlights the importance of the previous summer's rainfall (25 and 60 mm, respectively) on spring runoff production. Incorrect representation of precipitation can therefore have major implications for Arctic water budget descriptions that in turn can alter estimates of carbon and energy fluxes., Key Points Water storage deficits are consistently overestimated when precipitation is not adjusted for underestimationEnd‐of‐snowmelt through July rainfall is a more effective measure of drought conditions than total warm season rainfallPrecipitation bias lead to underestimated snowmelt runoff, due to overestimated antecedent water storage deficits and underestimated snowfall

The morphological and physicochemical traits of leaves are important in terms of plant adaptation to various growth environments, because such traits play central roles in various functions including photosynthesis. We measured the toughness, mass per unit area (LMA), nitrogen content, and δ15N levels of the leaves of different life-forms of 39 broadleaf woody plants in interior Alaska. The plants were divided into three life-forms based on the maximum height of adult plants: understory (15N values reflected the presence of root symbionts such as ericoid mycorrhiza or Frankia sp. Our results suggest that leaf traits may affect both the life-form of the host plants and leaf longevity in both evergreen and deciduous species.

Geomorphological and thermo-hydrological changes to tundra, caused by a wildfire in 2002 on the central Seward Peninsula of Alaska, were investigated as a case study for understanding the response from ice-rich permafrost terrain to surface disturbance. Frozen and unfrozen soil samples were collected at burned and unburned areas, and then water isotope geochemistry and cryostratigraphy of the active layer and near-surface permafrost were analyzed to investigate past hydrological and freeze/thaw conditions, and how this information could be recorded within the permafrost. The development of thermokarst subsidence due to ice-wedge melting after the fire was clear from analyses of historical sub-meter-resolution remote sensing imagery, long-term monitoring of thermo-hydrological conditions within the active layer, in-situ surveys of micro-relief, and geochemical signals recorded in the near-surface permafrost. The resulting polygonal relief coincided with depression lines along an underground ice-wedge network, and cumulative subsidence to 2013 was estimated as at least 10.1 to 12.1 cm (0.9-1.1 cm/year eleven-year average). Profiles of water geochemistry in the ground indicated mixing or replenishment of older permafrost water with newer meteoric water, as a consequence of the increase in active layer thickness due to wildfire or past thaw event. Our geocryological analysis of cores suggests that permafrost could be used to reconstruct the permafrost degradation history for the study site. Distinct hydrogen and oxygen isotopic compositions above the Global Meteoric Water Line were found for water from these sites where permafrost degradation with geomorphological change and prolonged surface inundation were suggested.

An open black spruce forest, the most common ecosystem in interior Alaska, is characterized by patchy canopy gaps where the forest understory is exposed. This study measured CO2, sensible heat, and latent heat fluxes with eddy covariance (EC) in one of those large canopy gaps, and estimated understory fluxes in a black spruce forest in 2011–2014. Then understory fluxes and ecosystem fluxes were compared. The understory fluxes during the snow-free seasons were determined by two approaches. The first approach determined understory fluxes as the fluxes from the canopy gap, assuming that fluxes under the canopy crown also had the same magnitude as the canopy gap fluxes. The second approach determined the understory fluxes by scaling canopy gap fluxes with a canopy gap fraction, assuming that only canopy gaps, which mostly constitutes the forest floor, contribute to fluxes. The true understory fluxes would be in between these two estimates. Overall, the understory accounted for 53% (39–66%), 61% (45–77%), 63% (45–80%), 73% (56–90%), and 79% (59–98%) of the total net ecosystem productivity (NEP), gross primary productivity (GPP), ecosystem respiration (RE), sensible heat flux (H), and latent heat flux (LE), respectively. The ratio of understory NEP (NEPU) to the ecosystem NEP (NEPE) and similarly calculated LEU/LEE during the daytime increased with vapor pressure deficit (VPD) at low VPD conditions (∼2000 Pa) at half-hourly temporal scale. At high VPD conditions, however, NEPU/NEPE decreased with VPD, whereas LEU/LEE was maintained at the high level even at high VPD conditions. Despite large ranges of the estimates for the understory contributions, we conclude that the understory plays an important role in the carbon and energy balances of the black spruce ecosystem, and their contribution highly depends on the level of VPD.

Summary Climate change will shift the frequency, intensity, duration and persistence of extreme hydroclimate events and have particularly disastrous consequences in vulnerable systems such as the warm permafrost-dominated Interior region of boreal Alaska. This work focuses on recent research results from nonparametric trends and nonstationary generalized extreme value (GEV) analyses at eight Interior Alaskan river basins for the past 50/60 years (1954/64–2013). Trends analysis of maximum and minimum streamflow indicates a strong (>+50%) and statistically significant increase in 11-day flow events during the late fall/winter and during the snowmelt period (late April/mid-May), followed by a significant decrease in the 11-day flow events during the post-snowmelt period (late May and into the summer). The April–May–June seasonal trends show significant decreases in maximum streamflow for snowmelt dominated systems (

In black spruce stands on permafrost, trees and understory plants showed higher biomass allocation especially to ‘thin’ fine roots (diam.

The Arctic Science Summit Week, Arctic Observing Summit, Arctic Council Senior Arctic Officials, Model Arctic Council, and International Arctic Assembly were convened on the campus of the University of Alaska Fairbanks with great productivity and satisfaction of the participants. We were pleased to welcome over 1000 participants from 30 different nations and over 130 different institutions. The organization and execution of these meetings was extensive and complex involving more than 250 coordinators, volunteers and contributors from across Alaska. The participants were enthusiastic in their praise of the content and accomplishments of the meeting, but they were equally happy about the genuine welcome offered to our guests by the people of Alaska. Hosting a complex event such as this summit required an army of supporting services and we were blessed to have volunteers from Fairbanks, North Pole, Anchorage and other communities throughout Alaska helping us meet these needs. This truly was an event hosted by the people of Alaska. The significance of these events cannot be overstated. The US and global communities are finally coming to the realization of the important role that the Arctic plays in international politics, economics, and science. The Arctic has experienced tremendous changes in recent years,more » offering new opportunities that may be addressed through international collaborations, and serious challenges that must be addressed through active investment, adaptation and national and international coordination. Over 10% of the meeting participants were indigenous peoples, from indigenous organizations or hailed from small remote communities. This is still lower than we had hoped, but it is greater participation than similar meetings have experienced in the past. It is through such engagement that we can attack problems related to the changing environment, stagnant economies, and social ills.« less

This synthesis paper provides a summary of the major components of the physical terrestrial Arctic and the influences of their changes upon the larger eco-climate system. Foci here are snow cover, permafrost, and land hydrology. During the last century, snow cover duration has shortened in a large portion of the circum-Arctic, mainly because of its early northward retreat in spring due to warming. Winter precipitation has generally increased, resulting in an increase in maximum snow depth over large areas. This is consistent with the increase in river discharge over large Russian watersheds. Soil temperature has also increased, and the active layer has deepened in most of the permafrost regions, whereas thinning of the seasonally frozen layer has been observed in areas not underlain by permafrost. These active components are mutually interrelated, conditioned by ambient micro- to landscape-level topography and local surface and subsurface conditions, and they are closely related with vegetation and ecology, as evidenced by evolution in the late Quaternary. Further, we provide examples and arguments for discussions on the pathways through which changes in the Arctic terrestrial system can affect or propagate to remote areas beyond the Arctic, reaching to the extratropics in the larger climate system. These considerations include dynamical and thermodynamical responses and feedbacks,'modification of hemisphere-scale atmospheric circulation associated with troposphere-stratosphere couplings, and moisture intrusion at a continental scale.

Although much remains to be learned about the Arctic and its component processes, many of the most urgent scientific, engineering, and social questions can only be approached through a broader system perspective. Here, we address interactions between components of the Arctic system and assess feedbacks and the extent to which feedbacks (1) are now underway in the Arctic and (2) will shape the future trajectory of the Arctic system. We examine interdependent connections among atmospheric processes, oceanic processes, sea-ice dynamics, marine and terrestrial ecosystems, land surface stocks of carbon and water, glaciers and ice caps, and the Greenland ice sheet. Our emphasis on the interactions between components, both historical and anticipated, is targeted on the feedbacks, pathways, and processes that link these different components of the Arctic system. We present evidence that the physical components of the Arctic climate system are currently in extreme states, and that there is no indication that the system will deviate from this anomalous trajectory in the foreseeable future. The feedback for which the evidence of ongoing changes is most compelling is the surface albedo-temperature feedback, which is amplifying temperature changes over land (primarily in spring) and ocean (primarily in autumn-winter). Other feedbacks likely to emerge are those in which key processes include surface fluxes of trace gases, changes in the distribution of vegetation, changes in surface soil moisture, changes in atmospheric water vapor arising from higher temperatures and greater areas of open ocean, impacts of Arctic freshwater fluxes on the meridional overturning circulation of the ocean, and changes in Arctic clouds resulting from changes in water vapor content.

Collaborations amongst researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan and the International Arctic research Center (IARC), University of Alaska Fairbanks (UAF), U.S., have been on-going since 1998 and resulted in a great number and magnitude of accomplishments that could not have been achieved without this close partnership. The Arctic represents an important region for Japan, the U.S. and the world, and many opportunities and challenges press for immediate understanding to enable wise decisions and policy making. We have many common interests and our countries face many common problems and goals. Addressing the tremendous scientific challenges of the Arctic requires such massive investment of manpower and resources that sharing efforts, data and working together on expeditions are in our mutual best interests. This issue presents a compilation of selected results on recent analyses conducted in the five-year (2009–2014) research term related to observational studies, model development and remote sensing applications of the Arctic Ocean, adjacent marginal seas, and the surrounding terrestrial regions. All of these studies are intended to provide a better understanding of how individual components and processes interact to form a complex and dynamic arctic system. Through these collaborations, Japanese and UAF Arctic researchers can achieve our goals of developing a quantitative understanding of the Arctic System.

Almost half a century ago, Williams (1970) compiled an annotated bibliography of published materials (up to 1960) on permafrost groundwater in the former Soviet Union, United States and Canada, and Scandinavia. That comprehensive survey encompassed literature on permafrost and related temperature and hydrological phenomena, and scientific and applied aspects of groundwater in permafrost terrain. In the intervening years, there have been major advances in permafrost hydrogeology, accompanied by shifts in research emphasis driven by scientific, environmental and societal demands. The present theme issue of Hydrogeology Journal presents a collection of articles that reflect the current status of progress in research on the hydrogeology of cold regions. Several of the papers in this theme issue describe current conditions of resources and processes in different specific regions of the world. Overall, the studies address conditions across different parts of Alaska (USA), Canada, Siberia, China, Fennoscandia and other parts of Europe, and Antarctica. Although snow and seasonally or permanently frozen ground recurs as a common consideration, each paper presents a unique perspective on widely varying conditions. Greater understanding of polar and sub-polar processes may be gleaned from a comparison among these results. With the focus of this thematic issue being on the hydrogeology of cold regions, all the papers in the issue discuss important elements of subsurface hydrology, and there is particular emphasis on ground ice and permafrost interactions with groundwater. However, the studies also deal with externalities that impact the subsurface such as surface energy budget, snow hydrology or glaciers, and groundwater below ice sheets, and some contributions address subsurface controls on the surface conditions via interaction with surface water or ecology. With regard to investigation approaches, the studies deal with interpretation of field or remotely sensed data, theory of ice-impacted hydrologic processes, field techniques, and modeling of groundwater systems in cold regions. Some of themodeling papers combine in pointing at safety assessment of nuclear waste repositories under climate-cooling scenarios as an important motivation for development and application of numerical simulation approaches for the hydrogeology of cold regions. Some articles are about management of groundwater resources, and there is also some discussion on societal aspects of cold-regions hydrology, for example, impacts of climate warming in these regions. In summary, the range of investigations presented in this thematic issue constitutes a valuable compilation of the state of knowledge of cold region hydrogeology. This contribution is timely, with groundwater resources in cold regions facing immediate or impending changes as a consequence of the ongoing climate change, the intensification of the hydrologic cycle, and in response to increased demands from communities and industry.

Thermokarst is the process of ground subsidence caused by either the thawing of ice-rich permafrost or the melting of massive ground ice. The consequences of permafrost degradation associated with thermokarst for surface ecology, landscape evolution, and hydrological processes have been of great scientific interest and social concern. Part of a tundra patch affected by wildfire in northern Alaska (27.5 km2) was investigated here, using remote sensing and in situ surveys to quantify and understand permafrost thaw dynamics after surface disturbances. A two-pass differential InSAR technique using L-band ALOS-PALSAR has been shown capable of capturing thermokarst subsidence triggered by a tundra fire at a spatial resolution of tens of meters, with supporting evidence from field data and optical satellite images. We have introduced a calibration procedure, comparing burned and unburned areas for InSAR subsidence signals, to remove the noise due to seasonal surface movement. In the first year after the fire, an average subsidence rate of 6.2 cm/year (vertical) was measured. Subsidence in the burned area continued over the following two years, with decreased rates. The mean rate of subsidence observed in our interferograms (from 24 July 2008 to 14 September 2010) was 3.3 cm/year, a value comparable to that estimated from field surveys at two plots on average (2.2 cm/year) for the six years after the fire. These results suggest that this InSAR-measured ground subsidence is caused by the development of thermokarst, a thawing process supported by surface change observations from high-resolution optical images and in situ ground level surveys.

Ice wedges are common features of the subsurface in permafrost regions. They develop by repeated frost cracking and ice vein growth over hundreds to thousands of years. Ice-wedge formation causes the archetypal polygonal patterns seen in tundra across the Arctic landscape. Here we use field and remote sensing observations to document polygon succession due to ice-wedge degradation and trough development in ten Arctic localities over sub-decadal timescales. Initial thaw drains polygon centres and forms disconnected troughs that hold isolated ponds. Continued ice-wedge melting leads to increased trough connectivity and an overall draining of the landscape. We find that melting at the tops of ice wedges over recent decades and subsequent decimetre-scale ground subsidence is a widespread Arctic phenomenon. Although permafrost temperatures have been increasing gradually, we find that ice-wedge degradation is occurring on sub-decadal timescales. Our hydrological model simulations show that advanced ice-wedge degradation can significantly alter the water balance of lowland tundra by reducing inundation and increasing runoff, in particular due to changes in snow distribution as troughs form. We predict that ice-wedge degradation and the hydrological changes associated with the resulting differential ground subsidence will expand and amplify in rapidly warming permafrost regions. The polygonal patterns in permafrost regions are caused by the formation of ice wedges. Observations of polygon evolution reveal that rapid ice-wedge melting has occurred across the Arctic since 1950, altering tundra hydrology.

Terrestrial hydrology is central to the Arctic system and its freshwater circulation. Water transport and water constituents vary, however, across a very diverse geography. In this paper, which is a component of the Arctic Freshwater Synthesis, we review the central freshwater processes in the terrestrial Arctic drainage and how they function and change across seven hydrophysiographical regions (Arctic tundra, boreal plains, shield, mountains, grasslands, glaciers/ice caps, and wetlands). We also highlight links between terrestrial hydrology and other components of the Arctic freshwater system. In terms of key processes, snow cover extent and duration is generally decreasing on a pan-Arctic scale, but snow depth is likely to increase in the Arctic tundra. Evapotranspiration will likely increase overall, but as it is coupled to shifts in landscape characteristics, regional changes are uncertain and may vary over time. Streamflow will generally increase with increasing precipitation, but high and low flows may decrease in some regions. Continued permafrost thaw will trigger hydrological change in multiple ways, particularly through increasing connectivity between groundwater and surface water and changing water storage in lakes and soils, which will influence exchange of moisture with the atmosphere. Other effects of hydrological change include increased risks to infrastructure and water resource planning, ecosystem shifts, and growing flows of water, nutrients, sediment, and carbon to the ocean. Coordinated efforts in monitoring, modeling, and processing studies at various scales are required to improve the understanding of change, in particular at the interfaces between hydrology, atmosphere, ecology, resources, and oceans.

The Arctic and the surrounding region of the sub-Arctic represent a key area for the study of global change because the anthropogenic impact, particularly the rate of warming, is projected to be the greatest in any part of the world due to the complicated feedback processes which occur. This Arctic region has undergone very large changes in recent years due to global warming, and accelerated change is predicted. The rapid changes that are occurring in the Arctic, and that have been the topic of the ISAR-1 and ISAR-2 conferences, manifest themselves at a number of scales. The large scales are Arctic-wide changes in key environmental parameters, which are described in a series of papers in this issue. On a more subtle scale we see changes to species and to biological processes in the Arctic. We hope that readers will enjoy the range of papers published in this issue, and will appreciate that phenomena ranging in scale from global radiation balance to clutch size of birds' eggs are actually all related via the central fact of the present-day Arctic, its high rate of warming.

Projected increases in air temperature and precipitation due to climate change in Arctic wetlands could dramatically affect ecosystem functioning. As a consequence, it is important to define the controls on evapotranspiration, which is the major pathway of water loss from these systems. We quantified the multi-year controls on midday arctic coastal wetland evapotranspiration measured with the eddy covariance method at two vegetated drained thaw lake basins near Barrow, Alaska. Variations in near-surface soil moisture and atmospheric vapor pressure deficits were found to have nonlinear effects on midday evapotranspiration rates. Vapor pressure deficits near and above 0.3 kPa appeared to be an important hydrological threshold, allowing latent heat fluxes to persistently exceed sensible heat fluxes. Dry soils increased the bulk surface resistance (water-limited). Wet soils favored ground heat flux and therefore limited the energy available to sensible and latent heat fluxes (energy-limited). Thus, midday evapotranspiration was suppressed on both dry and wet soils through different mechanisms. We also found that wet soils (ponding excluded) combined with large atmospheric demands resulted in an increased bulk surface resistance and therefore suppressing the evapotranspiration to below its potential rate (Priestley-Taylor α < 1.26). This is likely caused by the limited ability of mosses to transfer moisture during large atmospheric demands. Ultimately, in addition to net radiation, the various controlling factors on midday evapotranspiration (near-surface soil moisture, atmospheric vapor pressure, and the limited ability of mosses that are saturated at depth to transfer water during high atmospheric vapor demands) resulted in an average evapotranspiration rate of up to 75 % of the potential evapotranspiration rate. The multiple limitations on midday evapotranspiration rates have the potential to moderate interannual variation of total evapotranspiration and dampen excessive water loss during a warmer climate. Combined with the prevailing maritime winds and the projected increase in precipitation, these dampening mechanisms will likely prevent extensive future soil drying and hence maintain the presence of coastal wetlands.

A supersite has been established and is operated under the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and the International Arctic Research Center (IARC) Collaboration Study (JICS), with special emphasis on activities in Research Area 2 (Terrestrial Processes and Variation), which aims understanding multi-scale interactions in the Arctic system, quantifying the impact of observing system components, and utilizing observations in tests and validation experiments for modeling and remote sensing. We have shown that the formation of a supersite under JICS can promote the effective integration of observational studies, modeling, and satellite remote sensing research targeting various components of the processes occurring in the Arctic system. Through the study of these processes, we can enhance our understanding of the nature of the Arctic and the regional aspects of global environmental change, and improve the predictability of climatic fluctuations. We plan to maintain the supersite for the five years of JICS between 2009 JFY and 2013 JFY, as well as to improve the operation of the supersite.

As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets developed by 2.6 Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From not, vert, similar2.6 to not, vert, similar1.0 Ma ago, ice sheets came and went about every 41 ka, in pace with cycles in the tilt of Earth’s axis, but for the past 700 ka, glacial cycles have been longer, lasting not, vert, similar100 ka, separated by brief, warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from 41 ka to 100 ka glacial cycles is still debated. During the penultimate interglaciation, not, vert, similar130 to not, vert, similar120 ka ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence, Arctic summers were not, vert, similar5 °C warmer than at present, and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent. With the loss of land ice, sea level was about 5 m higher than present, with the extra melt coming from both Greenland and Antarctica as well as small glaciers. The Last Glacial Maximum (LGM) peaked not, vert, similar21 ka ago, when mean annual temperatures over parts of the Arctic were as much as 20 °C lower than at present. Ice recession was well underway 16 ka ago, and most of the Northern Hemisphere ice sheets had melted by 6 ka ago. Solar energy reached a summer maximum (9% higher than at present) not, vert, similar11 ka ago and has been decreasing since then, primarily in response to the precession of the equinoxes. The extra energy elevated early Holocene summer temperatures throughout the Arctic 1–3 °C above 20th century averages, enough to completely melt many small glaciers throughout the Arctic, although the Greenland Ice Sheet was only slightly smaller than at present. Early Holocene summer sea ice limits were substantially smaller than their 20th century average, and the flow of Atlantic water into the Arctic Ocean was substantially greater. As summer solar energy decreased in the second half of the Holocene, glaciers re-established or advanced, sea ice expanded, and the flow of warm Atlantic water into the Arctic Ocean diminished. Late Holocene cooling reached its nadir during the Little Ice Age (about 1250–1850 AD), when sun-blocking volcanic eruptions and perhaps other causes added to the orbital cooling, allowing most Arctic glaciers to reach their maximum Holocene extent. During the warming of the past century, glaciers have receded throughout the Arctic, terrestrial ecosystems have advanced northward, and perennial Arctic Ocean sea ice has diminished. Here we review the proxies that allow reconstruction of Quaternary climates and the feedbacks that amplify climate change across the Arctic. We provide an overview of the evolution of climate from the hot-house of the early Cenozoic through its transition to the ice-house of the Quaternary, with special emphasis on the anomalous warmth of the middle Pliocene, early Quaternary warm times, the Mid Pleistocene transition, warm interglaciations of marine isotope stages 11, 5e, and 1, the stage 3 interstadial, and the peak cold of the last glacial maximum.

Landscape modifications in Arctic and subarctic regions due to warming climate conditions have been reported in recent years. This work describes the rapid development of a thermokarst in Interior Alaska over a two-year period. The research was conducted in the Caribou-Poker Creeks Research Watershed, the only research watershed in the United States underlain by discontinuous permafrost. The basin's coordinates are 65°10' N latitude and 147°30' W longitude. Within the short span of two warm summers, a portion of the study area progressed from a hummocky terrain to a well-defined channel configuration. Suspended sediment concentration and discharge measurements indicated high sediment loads following rainfall events, which play a major role in the study area's sediment transport process. Fluvio-thermal erosion detected in late spring of 2004 triggered block failures on the thermokarst's sides. The spatio-temporal evolution of a cryogenic pipe was documented during the second summer. Water flow inside the pipe caused high erosion in the soil matrix. Eroded soil sediment was transported and deposited downstream, creating a fairly smooth bed slope. Topographical surveys conducted during the two field seasons revealed an average erosion rate of 3.5 m/year.

Spring snowmelt floods in the Arctic are common and can be expected every year, mainly because of the extensive snow cover that ablates relatively quickly. However, documentation of extreme flows (both low and high) in the Arctic is lacking in part because extreme flows are relatively rare and gauging sites are very sparse, with most of short duration. In the nested Kuparuk River research watersheds on the North Slope of Alaska, two large summer floods have been observed (July 1999 and August 2002) in the headwaters; these high flows are contrasted to the low flows (drought conditions) observed in the summers of 2005 and 2007. It is clear that the continuous permafrost and the limited near-surface storage in the shallow active layer are responsible for both the high and low flow responses. Or, stated another way, the active layer is a poor buffer to both floods and droughts. When contrasting summer floods with snowmelt floods, it is clear from flood frequency analyses that the smaller, high-gradient headwater basins will be dominated by summer floods while those watersheds draining the low gradient coastal plain will be dominated by snowmelt floods. The two summer floods in the headwaters had flows that were three to four times greater than the largest measured snowmelt flood, while on the coastal plain the 2002 summer storm for the whole of the Kuparuk River only produced the maximum summer runoff of record that was about 1/4 of the maximum snowmelt flood. So, on the coastal plain and even for the Greater Kuparuk River that drains across the coastal plain, snowmelt floods dominate. Drought conditions prevail in summers when the limited surface water storage in the active layer and surface water bodies is depleted because evapotranspiration exceeds precipitation.

The relative importance of snowmelt versus rain-generated floods on sediment transport in arctic streams is largely unknown because studies documenting either event-type are rare. An August 2002 precipitation event produced the largest discharge level (snowmelt or rain) over the previous ten-year period of hydrologic and geomorphologic monitoring in the Upper Kuparuk River, Alaska providing an opportunity to document the geomorphologic response to of an arctic stream to an extreme event. In this study we document the geomorphologic response and estimate the bedload transport rate using the virtual velocity method. This flood mobilized virtually the entire bed, with the exception of random boulders greater than 0.5 m. The channel cross-section and water edge survey data illustrate the considerable morphologic response generated by the flood. The magnitude of this response resulted in only a 13% tracer rock recovery rate. The total bedload transport was estimated to be 870 m3 of bed material through the study cross sections. Channel morphology, and therefore habitat, is maintained by large and infrequent summer rain events. These events, particularly when they occur in the late summer months, when active layer depth is at its greatest, have the potential to generate orders of magnitude more bedload transport than a snowmelt runoff event. It is unclear, however, if the lack of significant bedload transport during snowmelt is due to protection by bedfast ice or if flows are insufficient.

Climate change exerts influence on the globe over relatively long temporal and at broad spatial scales. However, at the local scale in which communities undertake their daily activities, changes in land-use may result in changes that accumulate and manifest more quickly in the landscape. In this paper we set out a methodology for identifying social–ecological system (SES) vulnerabilities in the landscape with respect to freshwater resources for Arctic communities. A multiple-scale approach is used at regional and watershed scales and is demonstrated for the Seward Peninsula region, Alaska and the Fish River watershed on Seward Peninsula but may be applied elsewhere. The approach includes change in permafrost distribution as an important effect of climate change, and change in mining activity as an important land-use effect. Vulnerability in the SES is identified as a consequence of spatially coinciding values. The resulting patterns of vulnerability highlight the interaction between changes, which act on slower temporal scales (e.g., permafrost distribution) and changes which act more quickly (e.g., downstream aggregation of mining activity). These results are discussed in the context of using the integration approach outlined in this paper to better enable communities’ responses to change at local scales in such a way that they are both adaptive and resilient.

Lakes are important water resources on the North Slope of Alaska. Freshwater is required for oilfield production as well as exploration, which occurs largely on ice roads and pads. Since most North Slope lakes are shallow, the quantity and quality of the water under ice at the end of winter are important environmental management issues. Currently, water-use permits are a function of the presence of overwintering fish populations, and their sensitivity to low oxygen concentrations. Sampling of five North Slope lakes during the winter of 2004-2005 shed some light on the winter chemistry of four lakes that were used as water supplies and one undisturbed lake. Field analysis was conducted for oxygen, conductivity, pH, and temperature throughout the lake depth, as well as ice thickness and water depth. Water samples were retrieved from the lakes and analyzed for Na, Ca, K, Mg, Fe, dissolved-organic carbon, and alkalinity in the laboratory. Lake properties, rather than pumping, were the best predictors of oxygen depletion, with the highest dissolved-oxygen levels maintained in the lake with the lowest concentration of constituents. Volume weighted mean dissolved-oxygen concentrations ranged from 4 to 94% of saturation in March. Dissolved oxygen and specific conductance data suggested that the lakes began to refresh in May.

Human activities in the Arctic are often mentioned as recipients of climate-change impacts. In this paper we consider the more complicated but more likely possibility that human activities themselves can interact with climate or environmental change in ways that either mitigate or exacerbate the human impacts. Although human activities in the Arctic are generally assumed to be modest, our analysis suggests that those activities may have larger influences on the arctic system than previously thought. Moreover, human influences could increase substantially in the near future. First, we illustrate how past human activities in the Arctic have combined with climatic variations to alter biophysical systems upon which fisheries and livestock depend. Second, we describe how current and future human activities could precipitate or affect the timing of major transitions in the arctic system. Past and future analyses both point to ways in which human activities in the Arctic can substantially influence the trajectory of arctic system change.

Permafrost and fire are important regulators of hydrochemistry and landscape structure in the discontinuous permafrost region of interior Alaska. We examined the influence of permafrost and a prescribed burn on concentrations of dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and other solutes (, Ca2+, K+, Mg2+, Na+) in streams of an experimentally burned watershed and two reference watersheds with varying extents of permafrost in the Caribou–Poker Creeks Research Watershed in interior Alaska. The low-permafrost watershed has limited permafrost (3%), the high-permafrost watershed has extensive permafrost (53%), and the burn watershed has intermediate permafrost coverage (18%). A three end-member mixing model revealed fundamental hydrologic and chemical differences between watersheds due to the presence of permafrost. Stormflow in the low-permafrost watershed was dominated by precipitation and overland flow, whereas the high-permafrost watershed was dominated by flow through the active layer. In all watersheds, organic and groundwater flow paths controlled stream chemistry: DOC and DON increased with discharge (organic source) and base cations and (from weathering processes) decreased. Thawing of the active layer increased soil water storage in the high-permafrost watershed from July to September, and attenuated the hydrologic response and solute flux to the stream. The FROSTFIRE prescribed burn, initiated on 8 July 1999, elevated nitrate concentrations for a short period after the first post-fire storm on 25 July, but there was no increase after a second storm in September. During the July storm, nitrate export lagged behind the storm discharge peak, indicating a flushing of soluble nitrate that likely originated from burned soils. Copyright © 2006 John Wiley & Sons, Ltd.

The integration of local knowledge and science represents an opportunity to enhance the understanding of interrelations among climate, hydrology, and socioeconomic systems while providing mutual benefits to scientists and rural communities. Insight from rural Alaskans helped to identify a social-ecological threshold used to model potential driftwood harvest from the Yukon River. Information from residents of Tanana, Alaska, was combined with scientific data to model driftwood harvest rates. Modeling results estimated that between 1980 and 2010, hydrologic factors alone were responsible for a 29% decrease in the annual wood harvest, which approximately balanced a 23% reduction in wood demand because of a decline in number of households. The community's installation of wood-fired boilers in 2007 created a threshold increase (76%) in wood demand that is not met by driftwood harvest. Modeling analyses of numerous climatic scenarios illustrated that increases in hydrologic variability would decrease the reliability of future driftwood harvest. Economic analyses demonstrated that increased climatic variability could have serious economic consequences for subsistence users while demanding more of their time. Lost time is important because it reduces their availability for performing other subsistence activities and learning to adapt to climate-related challenges. Our research may benefit communities by providing a tool that can be used to predict the timing and duration of driftwood runs. Information gathered from discussions with local stakeholders provided critical information for model development and thus provided a better understanding of regional social-ecological dynamics. Our research also illustrates the potential for regional-scale adaptations to limit the social-ecological impacts of environmental change, while providing economic opportunities and energy independence that reduce their vulnerability to variations in climate.

Evaporation (ET) from land surfaces in high latitudes is examined on a circumpolar perspective based upon the study results obtained in various environments, from boreal forest (taiga) to the high Arctic desert. Direct and indirect methods of evaporation measurement are reviewed, as well as numerous computational techniques. We have focused upon methods conveniently adopted for calculating evaporation when detailed information on meteorological conditions within the surface boundary layer is not available. These methods range from complicated ones, such as eddy correlation, energy balance and Penman equations, to empirical relationships between ET and incoming solar radiation. Great attention was paid to the principles of each method, especially those developed in Russia as they differ from most of the methods utilized internationally. For example, the Budyko–Zubenok empirical scheme is based upon the principle of potential evaporation, which is affected by soil moisture (SM). This relationship between ET and SM, expressed in terms of the field capacity, has been found to be non-linear; a complication that is not typically accounted for in traditional approaches. This paper also contains a brief review of a number of evaporation case studies including Alaska (USA), north-western Russia and Siberian taiga, Yukon basin (Canada), mountainous forest on Hokkaido Island (Japan), Canadian Arctic and glacierized basins of Greenland.

The Arctic climate is changing. Permafrost is warming, hydrological processes are chang- ing and biological and social systems are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates con- vincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and pro- cesses influenced by the threshold and phase change of freezing point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to isolate due to compounding factors. In almost every discipline represented, we show

A long history of research documents that McCall Glacier, Arctic Alaska, USA, continues to lose mass at a rate that is likely increasing with time. We present a photo comparison (1958-2003) that visually documents these volume changes, along with survey measurements that quantify these losses. Measurements of longitudinal profiles initially acquired from airborne laser altimetry, and repeated by ground-based surveys, indicate that the areally averaged rate of thinning increased between 1956-93 and 1993-2002, from 0.35 ± 0.07 m a-1 to 0.47 ± 0.03 m a-1, respectively; total volume loss was (8.3 × 107) ± (1.7 × 107) m3 and (2.7 × 107) ± (0.2 × 107) m3 (all in water equivalent) for these two time periods. These profiles also indicate that a 1 km stretch of the mid-ablation area is behaving differently from this trend, with a rate of thinning that is not changing with time. Lastly we present a comparison of several methods for calculating volume change and assess their relative errors.

The objective of this paper is to review research conducted over the past decade on the application of multi-temporal remote sensing for monitoring changes of Arctic tundra lands. Emphasis is placed on results from the National Science Foundation Land–Air–Ice Interactions (LAII) program and on optical remote sensing techniques. Case studies demonstrate that ground-level sensors on stationary or moving track platforms and wide-swath imaging sensors on polar orbiting satellites are particularly useful for capturing optical remote sensing data at sufficient frequency to study tundra vegetation dynamics and changes for the cloud prone Arctic. Less frequent imaging with high spatial resolution instruments on aircraft and lower orbiting satellites enable more detailed analyses of land cover change and calibration/validation of coarser resolution observations. The strongest signals of ecosystem change detected thus far appear to correspond to expansion of tundra shrubs and changes in the amount and extent of thaw lakes and ponds. Changes in shrub cover and extent have been documented by modern repeat imaging that matches archived historical aerial photography. NOAA Advanced Very High Resolution Radiometer (AVHRR) time series provide a 20-year

Differential interferometric sythetic aperture radar (DInSAR) measurements using the European Remote Sensing 2 (ERS-2) satellite in a high-plains region of Colorado show intriguing spatial variations in millimeter-scale path-length change that may correspond to variations in soil moisture of a few percent by volume, in both farm fields and uncultivated terrain. The observed signal is hypothesized to result from both changes in penetration depth and the swelling of clay-rich soils, both due to changes in soil moisture. Comparisons with our field measurements of soil moisture cannot conclusively verify this, but strong support is found from prior and complementary research as well as the visual correlation with hydrological features such as stream channels and watershed boundaries on a 50-m scale. Detection of these subtle signals was facilitated using a digital elevation model with high vertical accuracy. If our interpretations are correct, C-band DInSAR is a promising new tool for the remote sensing of soil moisture in a variety of terrain.

The concentration of dissolved organic carbon (DOC) and the contents of water-extractable organic carbon (WOC) and minerals were measured in moss and soils before and after a prescribed fire in interior Alaska to clarify the effect of the fire on the DOC dynamics in soil with discontinuous permafrost. The DOC concentration in the soil solution retrieved from suction cup lysimeters in the moss layer decreased from 30 (± 8.7 SD) mg C L -1 before the fire to 11-14 (± 7.2-8.9 SD) mg C L -1 1 month after the fire. The WOC content in the moss layer (including black carbon) 1 year after the fire was significantly lower in the burned plots (0.2-0.7 mg C g -1 ) compared to the controls (2.8-4.7 mg C g -1 ). The water extractable inorganic mineral contents in the moss layer were also lower in the burned plots, especially for iron, aluminum, potassium, and sodium. Our results indicated that the fire significantly decreased the DOC concentration in the surface moss layer compared to that in the unburned site about 1 month after the fire and also decreased the contents of WOC and water-extractable inorganic cations in the moss layer compared to the unburned site even 1 year after the fire.

Spatial and temporal variations of surface albedo on the North Slope of Alaska were investigated using both ground-based tower measurements and satellite remote sensing data. Ground-based measurements of incident and reflected solar radiation at several stations along the Dalton Highway over the period 1985–98 are used to determine in situ surface albedo. Advanced Very High Resolution Radiometer (AVHRR)-derived surface albedo were obtained from AVHRR Polar Pathfinder products, available from the National Snow and Ice Data Center, using a modified cloud mask. AVHRR-derived surface albedo agrees closely with in situ measurements. Results from this study indicate that surface albedo varies from greater than 0.9 for a snow-covered land surface under overcast conditions to less than 0.1 for a wet tundra land surface. Five distinct temporal periods are discerned, based on seasonal variations of surface albedo: winter stationary, spring snowmelt, postsnowmelt, summer stationary, and autumn freeze-up per...

Permafrost degradation leads to substantial changes in soil thermal and hydrologic characteristics. We investigated the effects of changes in active layer thickness and soil drainage on vegetation distribution near the arctic treeline on the Seward Peninsula, Alaska. We measured active layer thickness, soil moisture, density of tall shrub species, cover of low shrub species, and reconstructed white spruce establishment history along transects across the banks of a network of thaw ponds. We found that active layer thickness did not vary along our transects, but soils on thaw pond banks were significantly drier than those on level tundra or in thaw-pond channels. Thaw-pond banks were the only sites in which trees successfully established, and shrub communities on thaw-pond banks were taller and more dominated by tall shrub species like willow and shrub birch. The data suggest that the establishment of tree and tall shrub species at the arctic treeline can be limited by the availability of well-drained microsites, and the response of these species to regional climatic changes will be constrained by the availability of such microsites and thus contingent upon further degradation of the permafrost. Copyright © 2003 John Wiley & Sons, Ltd.

Differences in the summer insulative value of the zonal vegetation mat affect the depth of thaw along the Arctic bioclimate gradient. Toward the south, taller, denser plant canopies and thicker organic horizons counter the effects of warmer temperatures, so that there is little correspondence between active layer depths and summer air temperature. We examined the interactions between summer warmth, vegetation (biomass, Leaf Area Index, Normalized Difference Vegetation Index), soil (texture and pH), and thaw depths at 17 sites in three bioclimate subzones of the Arctic Slope and Seward Peninsula, Alaska. Total plant biomass in subzones C, D, and E averaged 421 g m � 2 , 503 g m � 2 , and 1178 g m � 2 respectively. Soil organic horizons averaged 4 cm in subzone C, 8 cm in subzone D, and 14 cm in subzone E. The average late-August thaw depths in subzones C, D, and E were 44 cm, 55 cm, and 47 cm respectively. Non-acidic soils in equivalent climates generally have shorter-stature sedge-dominated canopies and many frost boils, and consequently have thicker active layers than acidic soils. The trends reported here are useful for palaeo-ecological reconstructions and predictions of future ecosystem changes in the Low Arctic. Climate change will not lead to uniform thickening of the active layer, and could lead to shallower active layers in some presently dry areas due to paludification. Copyright  2003 John Wiley & Sons, Ltd.

The purpose of this study was to characterize the geomorphological processes controlling the dynamics of ponds and to identify and characterize groundwater infiltration and surface water dynamics for a tundra terrain located in discontinuous permafrost near Council, Alaska. Thermokarst processes and permafrost degradation were studied, focusing upon the interaction between surface and groundwater systems via an open talik. Synthetic aperture radar (SAR) data were used for classification of terrain units and surface water properties, while historical aerial photographs and satellite images (IKONOS) were used for assessment of pond shrinking and recent thermokarst progression. Geophysical surveys (ground penetrating radar and DC resistivity) were conducted to detect permafrost thickness and talik formations. Temperature boreholes and hydrological observation wells were monitored throughout the year and provided ground truth for validation of remotely-sensed data and geophysical surveys. Field and laboratory analyses enabled quantitative determination of subsurface hydrologic and thermal properties. We found many areas where alluvium deposits and ice-wedge polygonal terrain had developed thermokarst features within the last 20 years. Thermokarst ponds located over ice-wedge terrain have decreased in surface area since at least the early 20th Century. Small thermokarst features initially developed into tundra ponds perched over permafrost in response to some local disturbance to the surface. These thermokarst ponds grew larger and initiated large taliks that completely penetrated the permafrost. These taliks allowed internal drainage throughout the year causing the ponds to shrink under recent climatic conditions. Shrinking pond surface areas may become a common feature in the discontinuous permafrost regions as a consequence of warming climate and thawing permafrost. Copyright © 2003 John Wiley & Sons, Ltd.

The responses of high latitude ecosystems to global change involve complex interactions among environmental variables, vegetation distribution, carbon dynamics, and water and energy exchange. These responses may have important consequences for the earth system. In this study, we evaluated how vegetation distribution, carbon stocks and turnover, and water and energy exchange are related to environmental variation spanned by the network of the IGBP high latitude transects. While the most notable feature of the high latitude transects is that they generally span temperature gradients from southern to northern latitudes, there are substantial differences in temperature among the transects. Also, along each transect temperature co-varies with precipitation and photosynthetically active radiation, which are also variable among the transects. Both climate and disturbance interact to influence latitudinal patterns of vegetation and soil carbon storage among the transects, and vegetation distribution appears to interact with climate to determine exchanges of heat and moisture in high latitudes. Despite limitations imposed by the data we assembled, the analyses in this study have taken an important step toward clarifying the complexity of interactions among environmental variables, vegetation distribution, carbon stocks and turnover, and water and energy exchange in high latitude regions. This study reveals the need to conduct coordinated global change studies in high latitudes to further elucidate how interactions among climate, disturbance, and vegetation distribution influence carbon dynamics and water and energy exchange in high latitudes.

The subarctic region is highly responsive and vulnerable to climate change. Understanding the structure of subarctic soil microbial communities is essential for predicting the response of the subarctic soil environment to climate change. To determine the composition of the bacterial community and its relationship with soil properties, we investigated the bacterial community structure and properties of surface soil from the moist acidic tussock tundra in Council, Alaska. We collected 70 soil samples with 25-m intervals between sampling points from 0-10 cm to 10-20 cm depths. The bacterial community was analyzed by pyrosequencing of 16S rRNA genes, and the following soil properties were analyzed: soil moisture content (MC), pH, total carbon (TC), total nitrogen (TN), and inorganic nitrogen (NH4+ and NO3-). The community compositions of the two different depths showed that Alphaproteobacteria decreased with soil depth. Among the soil properties measured, soil pH was the most significant factor correlating with bacterial community in both upper and lower-layer soils. Bacterial community similarity based on jackknifed unweighted unifrac distance showed greater similarity across horizontal layers than through the vertical depth. This study showed that soil depth and pH were the most important soil properties determining bacterial community structure of the subarctic tundra soil in Council, Alaska.