In an extraordinary advance for glaciology and climate science, a recent study has unveiled the complex dynamics occurring on Greenland’s ice sheets, revealing an unprecedented co-occurrence of superimposed ice formation and meltwater runoff. This dual process fundamentally alters our understanding of ice sheet hydrology and surface evolution amidst a warming climate, and its consequences ripple far beyond the Arctic landscape, influencing global sea-level projections and polar ecosystem stability.
For decades, scientists have examined meltwater production and ice accumulation on the Greenland Ice Sheet independently. The prevailing view suggested meltwater generated during summer months would either percolate through the porous firn or flow immediately into subglacial channels, while ice formation was typically relegated to winter months when temperatures plunged. However, this groundbreaking research led by Tedstone, Machguth, Clerx, and colleagues, published in Nature Communications, challenges these simplistic assumptions by demonstrating that superimposed ice — a dense form of ice formed from refrozen meltwater — can develop simultaneously with ongoing meltwater runoff. This concurrent process reshapes Greenland’s surface structure and hydrological behavior in ways previously undocumented.
The phenomenon of superimposed ice formation involves meltwater percolating downward through firn, the compacted snow layer atop the ice sheet, and refreezing at deeper, colder layers. This refreezing mechanism densifies the firn, reducing its porosity and thereby impeding future infiltration of meltwater. Until now, it was believed that superimposed ice formation peaked during colder months when refreezing conditions were optimal and when meltwater input was minimal. However, Tedstone and colleagues’ observations from multiple field sites and satellite data reveal that under current warming trends, these two processes are not temporally segregated but vividly overlap.
Crucially, the concurrent formation of superimposed ice alongside active surface runoff suggests that meltwater pathways are dynamically evolving and that some meltwater is trapped within the firn and refrozen, while other portions continue to flow on the surface or within the ice. This dichotomy alters the energy balance and albedo characteristics of the ice sheet, with denser superimposed ice lowering the surface’s reflectivity. As a result, solar radiation absorption increases, amplifying melt rates and creating a feedback loop that accelerates ice sheet degradation.
The methodology underpinning this discovery synthesized high-resolution remote sensing with ground-based firn monitoring. The team deployed instrument arrays measuring temperature, density, and meltwater flux at multiple elevations across southwestern Greenland, alongside state-of-the-art radar and optical satellite imagery. These data streams were integrated into a sophisticated firn hydrology model, simulating the competing processes of meltwater infiltration, refreezing, and runoff at unprecedented temporal and spatial resolutions.
One pivotal insight emerged from detailed analyses of slab-like ice formations, known as “ice slabs,” which exhibit low permeability and extended meltwater retention times. These slabs, formed through episodic refreezing of meltwater, create physical barriers that redirect runoff laterally and influence local drainage patterns. The study revealed that superimposed ice develops prominently within these slabs during active melt seasons, obstructing meltwater infiltration and forcing excess meltwater to runoff more efficiently than previously understood.
The implications of this discovery extend to broader cryospheric and climatological contexts. Models forecasting Greenland’s contribution to global sea level rise have historically underestimated the volume and timing of meltwater runoff because they relied on oversimplified representations of firn freezing and melting cycles. Integrating the concurrent formation of superimposed ice with runoff into predictive models enhances their accuracy, informing coastal resilience strategies worldwide.
Furthermore, the enhanced meltwater runoff, facilitated by impermeable ice slabs, can lead to more substantial pulses of freshwater entering the North Atlantic Ocean. These pulses affect ocean circulation patterns including the Atlantic Meridional Overturning Circulation (AMOC), with possible knock-on effects on weather patterns across Europe, North America, and beyond. Thus, dynamics first observed on isolated ice slabs connect intrinsically to planetary-scale climatic processes.
The presence of superimposed ice also impacts the ecology within and atop the ice sheet. Water trapped and frozen within the firn layers influences microbial habitats, nutrient cycling, and the survival of extremophile organisms uniquely adapted to this environment. Changes in the balance between freezing and runoff may therefore shift these fragile ecosystems, highlighting the interconnectedness between physical ice properties and biological communities.
Moreover, the team identified seasonal variability in these processes’ intensity, with peak superimposed ice formation occurring during late spring and early summer, coinciding with periods of sustained but moderate meltwater production. This temporal window marks a critical phase where surface energy inputs and firn thermal structures align to optimize both meltwater penetration and refreezing efficacy. Understanding this temporal nuance is vital for accurate seasonal forecasting and resource management in Arctic regions.
Technological advancements played a central role in revealing these intricate processes. Using satellite missions equipped with laser altimeters and radars, researchers could map changes in ice surface elevation and density with centimeter-scale precision. Such measurements were critical for detecting vertical growth of superimposed ice layers and quantifying runoff volumes. Coupled with in situ sensors, this comprehensive approach sets a new standard for high-fidelity data acquisition in polar research.
The implications for future Greenland ice sheet stability are profound. As rising atmospheric and oceanic temperatures intensify, the interplay between meltwater runoff and superimposed ice formation could become more pronounced, potentially accelerating ice mass loss. This paradigm shift challenges existing mitigation frameworks and underscores the urgency for integrated climate modeling that accounts for these coupled hydrological and cryospheric feedbacks.
Importantly, the study’s authors emphasize the need for continued interdisciplinary collaboration. Glaciologists, climatologists, hydrologists, microbiologists, and remote sensing specialists must converge to further unravel these complex interactions. Only through such synergy can we refine projections of ice sheet behavior and implement effective adaptation strategies for vulnerable human and ecological systems.
This pioneering work also opens exciting avenues for applying similar methodologies to other glaciated regions, such as Antarctica and alpine glaciers, where layered ice processes and meltwater dynamics play crucial roles. Lessons learned from Greenland’s ice slabs could inform global models of ice mass balance, advancing our broader understanding of cryosphere responses to climate change.
In summary, the revelation that Greenland’s ice slabs simultaneously experience superimposed ice formation and meltwater runoff departs sharply from previous assumptions and signals a major leap forward in polar science. It accentuates the nuanced complexity of ice sheet surface processes and their profound cascading effects, from local hydrology to global climate systems. As we refine our grasp of these mechanisms, we enhance humanity’s ability to anticipate, adapt, and respond to the multifaceted challenges posed by a warming Earth.
Subject of Research: Concurrent superimposed ice formation and meltwater runoff processes on Greenland’s ice slabs and their impact on ice sheet hydrology and climate feedbacks.
Article Title: Concurrent superimposed ice formation and meltwater runoff on Greenland’s ice slabs.
Article References:
Tedstone, A., Machguth, H., Clerx, N. et al. Concurrent superimposed ice formation and meltwater runoff on Greenland’s ice slabs. Nat Commun 16, 4494 (2025). https://doi.org/10.1038/s41467-025-59237-9
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