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Significance Meltwater runoff from the Greenland ice sheet is a key contributor to global sea level rise and is expected to increase in the future, but it has received little observational study. We used satellite and in situ technologies to assess surface drainage conditions on the southwestern ablation surface after an extreme 2012 melting event. We conclude that the ice sheet surface is efficiently drained under optimal conditions, that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater release from the ice sheet. 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 ⁻¹) 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 Atmosphèérique Rèéégional (MAR) regional climate model (0.056–0.112 km ³⋅d ⁻¹ vs. ∼0.103 km ³⋅d ⁻¹), 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.

The additional water from the Antarctic ice sheet and ice shelves due to climate-induced melt can impact ocean circulation and global climate. However, the major processes driving melt are not adequately represented in Coupled Model Intercomparison Project phase 6 (CMIP6) models. Here, we analyze a novel multi-model ensemble of CMIP6 models with consistent meltwater addition to examine the robustness of the modeled response to meltwater, which has not been possible in previous single-model studies. Antarctic meltwater addition induces a substantial weakening of open-ocean deep convection. Additionally, Antarctic Bottom Water warms, its volume contracts, and the sea surface cools. However, the magnitude of the reduction varies greatly across models, with differing anomalies correlated with their respective mean-state climatology, indicating the state-dependency of the climate response to meltwater. A better representation of the Southern Ocean mean state is necessary for narrowing the inter-model spread of response to Antarctic meltwater.

The stability of widespread methane hydrates in shallow subsurface sediments of the marine continental margins is sensitive to temperature increases experienced by upper intermediate waters. Destabilization of methane hydrates and ensuing release of methane would produce climatic feedbacks amplifying and accelerating global warming. Hence, improved assessment of ongoing intermediate water warming is crucially important, especially that resulting from a weakening of Atlantic meridional overturning circulation (AMOC). Our study provides an independent paleoclimatic perspective by reconstructing the thermal structure and imprint of methane oxidation throughout a water column of 1,300 m. We studied a sediment sequence from the eastern equatorial Atlantic (Gulf of Guinea), a region containing abundant shallow subsurface methane hydrates. We focused on the early part of the penultimate interglacial and present a hitherto undocumented and remarkably large intermediate water warming of 6.8 °C in response to a brief episode of meltwater-induced, modest AMOC weakening centered at 126,000 to 125,000 y ago. The warming of intermediate waters to 14 °C significantly exceeds the stability field of methane hydrates. In conjunction with this warming, our study reveals an anomalously low δ13C spike throughout the entire water column, recorded as primary signatures in single and pooled shells of multitaxa foraminifers. This extremely negative δ13C excursion was almost certainly the result of massive destabilization of methane hydrates. This study documents and connects a sequence of climatic events and climatic feedback processes associated with and triggered by the penultimate climate warming that can serve as a paleoanalog for modern ongoing warming.

Meltwater runoff from the Greenland ice sheet surface influences surface mass balance (SMB), ice dynamics, and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-h trial for a 63.1-km2 moulin-terminating internally drained catchment (IDC) on Greenland’s midelevation (1,207–1,381 m above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6, RACMO2.3, MERRA-2, and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins but are improved using synthetic unit hydrograph (SUH) theory. Retroactive SUH applications to two older field studies reproduce their findings, signifying that remotely sensed IDC area, shape, and supraglacial river length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6, and RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment, models overestimated runoff by +21 to +58%, linked to overestimated surface ablation and possible meltwater retention in bare, porous, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of SMB with ice dynamics and subglacial systems.

Continued melting of Antarctic ice sheets and shelves adds freshwater to the Southern Ocean (SO), enhancing stratification and inducing surface cooling. This cooling influences tropical climate through coupled atmosphere–ocean interactions, though model responses vary. Using coordinated coupled model experiments with idealized Antarctic meltwater forcing, we assess the remote impacts of SO surface cooling. All 11 models simulate equatorial surface cooling and a northward Intertropical Convergence Zone shift, but show discrepant responses in the equatorial Pacific zonal temperature gradient and Atlantic meridional dipole. When normalized by SO cooling amplitude, these tropical metrics are positively correlated with shortwave cloud feedback strength. Surface energy budget analysis indicates that the previously proposed teleconnection mechanisms in the eastern Pacific are not robust across models. The timescale of tropical cooling and the relative roles of wind‐driven latent heat and shortwave fluxes differ across models and basins, highlighting the uncertainty in SO–tropics teleconnections.

The Source Region of the Yangtze River (SRYR), located on the hinterland of the Tibetan Plateau (TP), is significantly affected by cryospheric components such as snow, permafrost and glaciers. Nevertheless, there remains a lack of consensus regarding the contribution of cryospheric meltwater to river runoff and its potential changes in a changing climate. This study quantifies the contributions of different types of cryospheric meltwater in the SRYR during 1961–2100 using a physically based cryospheric‐hydrological model. Results show that snowmelt, ground ice melt from thawing permafrost and glacier melt contributed 23%, 3%, and 6% to river runoff in the historical period, respectively. Due to cryospheric degradation as indicated by diminishing snow cover, thickening permafrost active layer and declining ice storage, the runoff contributions of all three types of cryospheric meltwater are projected to decline in a warming climate, with the tipping point of snowmelt runoff already reached in the historical period. The tipping point of glacier runoff is expected to be reached soon, near the 2030s, followed by the contributions of thawing permafrost later in this century. The total contribution of cryospheric meltwater to river runoff is expected to decline in the future, with meltwater from thawing permafrost replacing snowmelt as the dominant cryospheric meltwater component. Consequently, the drought mitigation capacity of cryospheric meltwater will diminish in the future, despite an expected increase in total river runoff. This study highlights the widespread risks of declining cryospheric meltwater supply both in the TP and in other cold region catchments in a warming climate.

Despite clearly documented in instrumental temperature records, the warming amplitude over the current warm period (CWP) inferred from lacustrine archive is very limited. The potential role of meltwater pulses in lake hydrology and their influence on lake water temperature under anthropogenic warming remain poorly investigated. Here we present and summarize lake water temperature and hydrology records in northern China to address potential meltwater effects over the last millennium. Our results show abrupt freshening and muted warming of lake water over the CWP, which appears to have also occurred along with climatic warming previously. Hence, substantial meltwater pulses to lakes, a transient response to climatic warming, could partly account for the muted warming over the CWP as indicated by lake water temperature records. Plain Language Summary Compared to the instrumental temperature data, synthesized North Hemisphere temperature records and temperature records inferred from lacustrine archives show muted warming amplitudes since ∼1860 AD. The potential effects of meltwater pulses on lake conditions under anthropogenic warming remain poorly investigated. Our lake water temperature and hydrology records in northern China, together with previously reported records from this region, collectively show abrupt freshening and muted warming of lake water over the current warm period. This indicates that meltwater pulses appear to disrupt the warming signal in lake water temperature records, as occurred in previous climatic warming periods. Our results suggest that meltwater pulses, associated with climatic warming, could mute the amplitude of warming in the climatic transition period and has important implications for hydroclimatic reconstructions and projections in northern China and perhaps over the globe. Key Points Meltwater pulses over the last millennium inferred from lake water temperature and hydrology records in northern China Abrupt freshening and muted warming of lake water over current warm period linked to meltwater pulses under anthropogenic warming Meltwater effect on lake conditions in climatic warming periods would mute the amplitude of warming in climatic transition period

Glaciers in High Mountain Asia generate meltwater that supports the water needs of 250 million people, but current knowledge of annual accumulation and ablation is limited to sparse field measurements biased in location and glacier size. Here, we present altitudinally-resolved specific mass balances (surface, internal, and basal combined) for 5527 glaciers in High Mountain Asia for 2000–2016, derived by correcting observed glacier thinning patterns for mass redistribution due to ice flow. We find that 41% of glaciers accumulated mass over less than 20% of their area, and only 60% ± 10% of regional annual ablation was compensated by accumulation. Even without 21 st century warming, 21% ± 1% of ice volume will be lost by 2100 due to current climatic-geometric imbalance, representing a reduction in glacier ablation into rivers of 28% ± 1%. The ablation of glaciers in the Himalayas and Tien Shan was mostly unsustainable and ice volume in these regions will reduce by at least 30% by 2100. The most important and vulnerable glacier-fed river basins (Amu Darya, Indus, Syr Darya, Tarim Interior) were supplied with >50% sustainable glacier ablation but will see long-term reductions in ice mass and glacier meltwater supply regardless of the Karakoram Anomaly. Glaciers in High Mountain Asia are a key water resource. The authors use remote sensing data and a regional implementation of the continuity equation to quantify glacier ablation and accumulation rates for 2000–2016, and establish current climatic-geometric imbalances that imply strong reductions in ice volume by 2100.