| 英文摘要: | Supraglacial lakes (SGLs) form annually on the Greenland ice sheet1, 2 and, when they drain, their discharge enhances ice-sheet flow3 by lubricating the base4 and potentially by warming the ice5. Today, SGLs tend to form within the ablation zone, where enhanced lubrication is offset by efficient subglacial drainage6, 7. However, it is not clear what impact a warming climate will have on this arrangement. Here, we use an SGL initiation and growth8 model to show that lakes form at higher altitudes as temperatures rise, consistent with satellite observations9. Our simulations show that in southwest Greenland, SGLs spread 103 and 110 km further inland by the year 2060 under moderate (RCP 4.5) and extreme (RCP 8.5) climate change scenarios, respectively, leading to an estimated 48–53% increase in the area over which they are distributed across the ice sheet as a whole. Up to half of these new lakes may be large enough to drain, potentially delivering water and heat to the ice-sheet base in regions where subglacial drainage is inefficient. In such places, ice flow responds positively to increases in surface water delivered to the bed through enhanced basal lubrication4, 10, 11 and warming of the ice5, and so the inland advance of SGLs should be considered in projections of ice-sheet change. The volume of water stored in SGLs on the surface of the Greenland ice sheet is determined by the presence of depressions in the local terrain2, by the amount of runoff8 (melt water plus rain minus refreezing in the snowpack) and by lake drainage3. It is estimated that 13% of Greenland’s SGLs drain on timescales of the order of a few hours12, often by the creation of moulins as water-filled fractures propagate through the full thickness of the ice sheet (termed hydro-fracture)13. SGLs act as a source of en- and subglacial water when they drain and afterwards, the moulin acts as a conduit allowing runoff to pass between the ice-sheet surface and base1, 3. Satellite and ground-based observations show a correlation between the degree of runoff and the rate of ice motion4, 6, 7; however, there are known spatial and temporal variations in the magnitude and sign of this relationship. For example, near the ice-sheet margin, lower annual ice speeds have been recorded in years of high melting6, 7 but further inland—at higher elevations—the reverse seems to be the case4, 11. This dichotomy can be attributed to an abundance of melt water at the margin, enabling the evolution of efficient subglacial drainage early in the melt season6, 10, and thicker ice and less water farther inland hindering the development of an efficient evacuation system14, 15. In addition to their impact on basal sliding, draining SGLs, and moulins that persist post-drainage, can exert a local warming as relatively warm water passes through the colder ice (termed cryo-hydrologic warming)5. This—by rendering the ice sheet more fluid—can potentially enable faster ice-sheet flow due to internal deformation5. Ultimately, faster flow may result in mass loss as ice-sheet thinning promotes an inland expansion of the melt zone. In southwest Greenland, the maximum elevation at which SGLs occur has migrated 53 km inland over the past 40 years, following an upwards shift in the ice-sheet equilibrium line9, which, historically, has fallen close to (within 10 km on average) the maximum elevation of SGLs (Supplementary Table 1). This migration has accelerated over the past two decades, in response to rapid changes in regional temperature16 associated with global warming and an increase in frequency of negative North Atlantic Oscillation indices during boreal summer (favouring warmer and drier atmospheric conditions than normal)17. To study the long-term response of SGLs to this and future climate change, we simulate their initiation and growth over the period 1971–2060 in the vicinity of the Russell and Leverett glaciers (Fig. 1). Our simulations are performed using the SGL Initiation and Growth (SLInG) model8, a hydrologic model that routes runoff over a model of the ice-sheet surface, allowing water to form lakes in topographic depressions (Methods). Here we focus on a 19,441 km2 section of the ice sheet situated at elevations more than 1,100 m above sea level (a.s.l.), where subglacial drainage is expected to be inefficient10, 15 and the impact of SGLs on ice-sheet hydrology is potentially large. The SLInG model is forced with estimates of runoff derived from high-resolution (25 km) regional climate model18 reanalyses (1971–2010) and future projections (2006–2100). Future simulations are performed under both moderate and extreme climate projections characterized by Intergovernmental Panel on Climate Change Representative Concentration Pathways (RCPs) 4.5 and 8.5 (ref. 19), respectively.
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) beneath SGLs is twice that of the surrounding ice and using equation (2). Total lake area (Alakes) is estimated for the entire ice sheet by multiplying lake density modelled in the study region by the total lake-covered area (including that which lies below 1,100 m a.s.l.) observed in the present and simulated in the future.
