Abstract:Over the next century, near-surface permafrost across the circumpolar Arctic is expected to degrade significantly, particularly for land areas south of 70°N. This is likely to cause widespread impacts on arctic hydrology, ecology, and trace gas emissions. Here, we present a review of recent studies investigating linkages between permafrost dynamics and river biogeochemistry in the Arctic, including consideration of likely impacts that warming-induced changes in permafrost may be having (or will have in the future) on the delivery of organic matter, inorganic nutrients, and major ions to the Arctic Ocean. These interacting processes can be highly complex and undoubtedly exhibit spatial and temporal variabilities associated with current permafrost conditions, sensitivity to permafrost thaw, mode of permafrost degradation (overall permafrost thaw, active layer deepening, and/or thermokarst processes), and environmental characteristics of watersheds (e.g. land cover, soil type, and topography). One of the most profound consequences of permafrost thaw projected for the future is that the arctic terrestrial freshwater system is likely to experience a transition from a surface water-dominated system to a groundwater-dominated system. Along with many other cascading impacts from this transition, mineral-rich groundwater may become an important contributor to streamflow, in addition to the currently dominant contribution from mineral-poor surface water. Most studies observe or predict an increase in major ion, phosphate, and silicate export with this shift towards greater groundwater contributions. However, we see conflicting accounts of whether the delivery of inorganic nitrogen and organic matter will increase or decrease with warming and permafrost thaw. It is important to note that uncertainties in the predictions of the total flux of biogeochemical constituents are tightly linked to future uncertainties in discharge of rivers. Nonetheless, it is clear that over the next century there will be important shifts in the river transport of organic matter, inorganic nutrients, and major ions, which may in turn have critical implications for primary production and carbon cycling on arctic shelves and in the Arctic Ocean basin interior.

Phytoplankton blooms over Arctic Ocean continental shelves are thought to be restricted to waters free of sea ice. Here, we document a massive phytoplankton bloom beneath fully consolidated pack ice far from the ice edge in the Chukchi Sea, where light transmission has increased in recent decades because of thinning ice cover and proliferation of melt ponds. The bloom was characterized by high diatom biomass and rates of growth and primary production. Evidence suggests that under-ice phytoplankton blooms may be more widespread over nutrient-rich Arctic continental shelves and that satellite-based estimates of annual primary production in these waters may be underestimated by up to 10-fold.

Until recently, northern Bering Sea ecosystems were characterized by extensive seasonal sea ice cover, high water column and sediment carbon production, and tight pelagic-benthic coupling of organic production. Here, we show that these ecosystems are shifting away from these characteristics. Changes in biological communities are contemporaneous with shifts in regional atmospheric and hydrographic forcing. In the past decade, geographic displacement of marine mammal population distributions has coincided with a reduction of benthic prey populations, an increase in pelagic fish, a reduction in sea ice, and an increase in air and ocean temperatures. These changes now observed on the shallow shelf of the northern Bering Sea should be expected to affect a much broader portion of the Pacific-influenced sector of the Arctic Ocean.

[1] Extensive new data from previously unstudied Siberian streams and rivers suggest that mobilization of currently frozen, high-latitude soil carbon is likely over the next century in response to predicted Arctic warming. We present dissolved organic carbon (DOC) measurements from ninety-six watersheds in West Siberia, a region that contains the world's largest stores of peat carbon, exports massive volumes of freshwater and DOC to the Arctic Ocean, and is warming faster than the Arctic as a whole. The sample sites span $10 6 km 2 over a large climatic gradient ($55-68°N), providing data on a much broader spatial scale than previous studies and for the first time explicitly examining stream DOC in permafrost peatland environments. Our results show that cold, permafrostinfluenced watersheds release little DOC to streams, regardless of the extent of peatland cover. However, we find considerably higher concentrations in warm, permafrost-free watersheds, rising sharply as a function of peatland cover. The two regimes are demarcated by the position of the À2°C mean annual air temperature (MAAT) isotherm, which is also approximately coincident with the permafrost limit. Climate model simulations for the next century predict near-doubling of West Siberian land surface areas with a MAAT warmer than À2°C, suggesting up to $700% increases in stream DOC concentrations and $2.7-4.3 Tg yr À1 ($29 -46%) increases in DOC flux to the Arctic Ocean.

Ice shelves modulate Antarctic contributions to sea-level rise 1 and thereby represent a critical, climate-sensitive interface between the Antarctic ice sheet and the global ocean. Following rapid atmospheric warming over the past decades 2,3 , Antarctic Peninsula ice shelves have progressively retreated 4 , at times catastrophically 5 . This decay supports hypotheses of thermal limits of viability for ice shelves via surface melt forcing 3,5,6 . Here we use a polar-adapted regional climate model 7 and satellite observations 8 to quantify the nonlinear relationship between surface melting and summer air temperature. Combining observations and multimodel simulations, we examine melt evolution and intensification before observed ice shelf collapse on the Antarctic Peninsula. We then assess the twenty-first-century evolution of surface melt across Antarctica under intermediate and high emissions climate scenarios. Our projections reveal a scenario-independent doubling of Antarctic-wide melt by 2050. Between 2050 and 2100, however, significant divergence in melt occurs between the two climate scenarios. Under the high emissions pathway by 2100, melt on several ice shelves approaches or surpasses intensities that have historically been associated with ice shelf collapse, at least on the northeast Antarctic Peninsula.Antarctic ice shelves have undergone widespread and accelerated thinning and retreat in recent decades in response to coupled atmospheric and oceanic forcing [3][4][5]9,10 . On the Antarctic Peninsula (AP), this recession has been particularly pronounced and punctuated with near-uniform, abrupt collapses of Larsen A, Prince Gustav, and Larsen B ice shelves occurring since 1995 ( Fig. 1). Across this region, recent atmospheric warming has exceeded global average rates 2 and current surface melting levels are unprecedented over the past millennium on the northeast AP (ref. 11). This warming and melt intensification has directly led to an expansion of meltwater ponding, and the resultant hydrofracturing is considered a leading mechanism of AP ice shelf collapse 3,5,12 .All Antarctic ice shelves experience surface melting today 7,8 , yet ocean-induced basal melting at present dominates ice shelf mass losses, particularly outside of the AP (refs 9,10). Nevertheless, surface melt intensities approach those of the AP elsewhere in Antarctica (Fig. 1c), meltwater ponding exists beyond the AP (refs 13,14), and strong basal melting can hasten ice shelf destabilization 4,10 . The question therefore arises, are recent ice shelf dynamics on the AP indicative of forthcoming changes elsewhere in Antarctica? Understanding the present-day and future viability of all Antarctic ice shelves requires an improved characterization of the sensitivity of ice shelves to temperature change, a better historical context for AP melt acceleration and ice shelf collapse, and robust projections of future pan-Antarctic change.Air temperature is often used to parameterize surface melt owing to several important physical linkages with the sur...

Interpolar methane gradient (IPG) data from ice cores suggest the "switching on" of a major Northern Hemisphere methane source in the early Holocene. Extensive data from Russia's West Siberian Lowland show (i) explosive, widespread peatland establishment between 11.5 and 9 thousand years ago, predating comparable development in North America and synchronous with increased atmospheric methane concentrations and IPGs, (ii) larger carbon stocks than previously thought (70.2 Petagrams, up to approximately 26% of all terrestrial carbon accumulated since the Last Glacial Maximum), and (iii) little evidence for catastrophic oxidation, suggesting the region represents a long-term carbon dioxide sink and global methane source since the early Holocene.

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