Historical archived satellite images were compared with contemporary satellite data to track ongoing changes in more than 10,000 large lakes in rapidly warming Siberia. A widespread decline in lake abundance and area has occurred since 1973, despite slight precipitation increases to the region. The spatial pattern of lake disappearance suggests (i) that thaw and "breaching" of permafrost is driving the observed losses, by enabling rapid lake draining into the subsurface; and (ii) a conceptual model in which high-latitude warming of permafrost triggers an initial but transitory phase of lake and wetland expansion, followed by their widespread disappearance.

Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km 2 of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54-2.81 cm·d) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41-98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056-0.112 km ), and when integrated over the melt season, totaled just 37-75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.Greenland ice sheet | supraglacial hydrology | meltwater runoff | mass balance | remote sensing M eltwater runoff from the Greenland ice sheet (GrIS) accounts for half or more of its total mass loss to the global ocean (1, 2) but remains one of the least-studied hydrologic processes on Earth. Each summer, a complex system of supraglacial meltwater ponds, lakes, streams, rivers, and moulins develops across large areas of the southwestern GrIS surface, especially below ∼1,300 m elevation (3-7), with supraglacial erosion driven by thermal and radiative processes (5). Digital elevation models (DEMs) suggest a poorly drained surface resulting from abundant topographic depressions, which computational flow routing models must artificially "fill" to allow hydrological flow paths extending from the ice sheet interior to its edge (8-11). The realism of such modeled flow paths remains largely untested by real-world observations. To date, most observational studies of GrIS supraglacial hydrology have focused on large lakes (∼1 km 2

An analysis of 1516 radiocarbon dates demonstrates that the development of the current circumarctic peatlands began approximately 16.5 thousand years ago (ka) and expanded explosively between 12 and 8 ka in concert with high summer insolation and increasing temperatures. Their rapid development contributed to the sustained peak in CH4 and modest decline of CO2 during the early Holocene and likely contributed to CH4 and CO2 fluctuations during earlier interglacial and interstadial transitions. Given the decreased tempo of peatland initiation in the late Holocene and the transition of many from fens (which generated high levels of CH4) to ombrotrophic bogs, a neoglacial expansion of northern peatlands cannot explain the increase in atmospheric CH4 that occurred after 6 ka.

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Large uncertainties in the budget of atmospheric methane (CH 4 ) limit the accuracy of climate change projections. Here we describe and quantify an important source of CH 4 -point-source ebullition (bubbling) from northern lakes-that has not been incorporated in previous regional or global methane budgets. Employing a method recently introduced to measure ebullition more accurately by taking into account its spatial patchiness in lakes, we estimate point-source ebullition for 16 lakes in Alaska and Siberia that represent several common northern lake types: glacial, alluvial floodplain, peatland and thermokarst (thaw) lakes. Extrapolation of measured fluxes from these 16 sites to all lakes north of 458 N using circumpolar databases of lake and permafrost distributions suggests that northern lakes are a globally significant source of atmospheric CH 4 , emitting approximately 24.2G10.5 Tg CH 4 yr K1. Thermokarst lakes have particularly high emissions because they release CH 4 produced from organic matter previously sequestered in permafrost. A carbon mass balance calculation of CH 4 release from thermokarst lakes on the Siberian yedoma ice complex suggests that these lakes alone would emit as much as approximately 49 000 Tg CH 4 if this ice complex was to thaw completely. Using a space-for-time substitution based on the current lake distributions in permafrost-dominated and permafrost-free terrains, we estimate that lake emissions would be reduced by approximately 12% in a more probable transitional permafrost scenario and by approximately 53% in a 'permafrost-free' Northern Hemisphere. Long-term decline in CH 4 ebullition from lakes due to lake area loss and permafrost thaw would occur only after the large release of CH 4 associated thermokarst lake development in the zone of continuous permafrost.

Recent historic observed lows in Arctic sea ice extent, together with climate model projections of additional ice reductions in the future, have fueled speculations of potential new trans-Arctic shipping routes linking the Atlantic and Pacific Oceans. However, numerical studies of how projected geophysical changes in sea ice will realistically impact ship navigation are lacking. To address this deficiency, we analyze seven climate model projections of sea ice properties, assuming two different climate change scenarios [representative concentration pathways (RCPs) 4.5 and 8.5] and two vessel classes, to assess future changes in peak season (September) Arctic shipping potential. By midcentury, changing sea ice conditions enable expanded September navigability for common open-water ships crossing the Arctic along the Northern Sea Route over the Russian Federation, robust new routes for moderately ice-strengthened (Polar Class 6) ships over the North Pole, and new routes through the Northwest Passage for both vessel classes. Although numerous other nonclimatic factors also limit Arctic shipping potential, these findings have important economic, strategic, environmental, and governance implications for the region.climate change | human-environment | marine transportation modeling | Arctic maritime development | global change

Rivers provide critical water supply for many human societies and ecosystems, yet global knowledge of their flow rates is poor. We show that useful estimates of absolute river discharge (in cubic meters per second) may be derived solely from satellite images, with no ground-based or a priori information whatsoever. The approach works owing to discovery of a characteristic scaling law uniquely fundamental to natural rivers, here termed a river's at-many-stations hydraulic geometry. A first demonstration using Landsat Thematic Mapper images over three rivers in the United States, Canada, and China yields absolute discharges agreeing to within 20-30% of traditional in situ gauging station measurements and good tracking of flow changes over time. Within such accuracies, the door appears open for quantifying river resources globally with repeat imaging, both retroactively and henceforth into the future, with strong implications for water resource management, food security, ecosystem studies, flood forecasting, and geopolitics. S ome 80% of the world's population and 65% of its river ecosystems are threatened by insecure water supply, yet global knowledge of the river discharges upon which these depend is surprisingly poor (1, 2). For much of the world, river gauge measurements are rare, nonexistent, or proprietary. Even wellmonitored countries have sparsely distributed networks, thus limiting current understanding of water losses along river courses, habitat changes, and flood risk (3, 4). Satellites, in contrast, provide spatially dense coverage globally, attracting calls for a global river discharge mapping capacity from space (5-10). However, previous efforts to estimate river discharge from remotely sensed observations have all required inclusion of some form of ancillary ground-based information, such as gauge measurements, bathymetric surveys, and/or calibrated hydrology models that are simply unavailable for most of the planet (11-18). To remove this dependence on ground-based information, we show that useful estimates of absolute river discharge (i.e., in units of cubic meters per second) may be derived solely from multiple satellite images of a river, with no ground-based or a priori information whatsoever, through use of a characteristic scaling law, here termed a river's atmany-stations hydraulic geometry (AMHG). As will be shown in this paper, AMHG effectively halves the number of parameters required by traditional hydraulic geometry, thus paving the way for remote estimation of a single remaining parameter-and thus river discharge-through repeated satellite image observations along a river course. The presence of AMHG is verified in 12 of 12 rivers examined, using 88 in situ gauging stations, three fieldcalibrated hydrodynamic models incorporating 772 field-surveyed bathymetric cross-sections, and 42 Landsat Thematic Mapper (TM) satellite images (SI Text, section S1, Materials and Methods and Tables S1 and S2). Following a description of width AMHG, an innovative satellite discharge estimation approach i...

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.