Coastal cities from Miami to Mumbai sit on land that would vanish beneath the ocean if Earth’s two largest ice sheets, Greenland and Antarctica, surrendered all their frozen mass. The combined result, an estimated 67 meters of global sea-level rise, or about 223 feet, would redraw every coastline on the planet. That figure is not a single headline number plucked from thin air; it is assembled from separate volume measurements of each ice sheet, and the precise totals depend on which datasets and which land‑ice categories researchers include.
Why the 67-Meter Estimate Demands Sharper Scrutiny in 2026
The tension behind this number is straightforward: two major U.S. government agencies frame the total differently, and the gap between their figures reveals how much depends on what counts as “ice.” According to the NASA portal, if all glaciers and ice sheets melted, global sea level would rise by more than 195 feet, or about 60 meters. The U.S. Geological Survey, by contrast, puts the figure higher: approximately 70 meters, or roughly 230 feet, if all glaciers and ice caps melted. The difference, on the order of 10 meters, hinges on whether smaller mountain glaciers and polar ice caps are folded into the total or treated separately from the two great ice sheets.
For the Greenland component specifically, the most detailed mapping effort to date produced a tightly constrained number. The BedMachine v3 dataset, published in the Proceedings of the National Academy of Sciences, used multibeam echo sounding combined with mass conservation to calculate Greenland’s sea‑level potential at 7.42 plus or minus 0.05 meters. That narrow uncertainty band, just 5 centimeters on either side, reflects the precision of bed‑topography mapping beneath the ice. With Greenland contributing roughly 7.4 meters of the total and Antarctica holding the vast majority of the remainder, the 67‑meter headline figure emerges when both ice sheets are combined and smaller glaciers are excluded.
A central question for the coming decades is not whether all of this ice will melt, a scenario that would take centuries or millennia, but how fast specific glaciers begin losing mass in a nonlinear fashion. BedMachine’s bed‑elevation data show that many of Greenland’s marine‑terminating glaciers sit in fjords that deepen inland. Once a glacier’s grounding line retreats past a shallow ridge and enters a deeper basin, the ice face exposed to warm ocean water grows taller, accelerating calving and thinning. Current linear projection models may not capture this threshold behavior. If bed topography triggers faster retreat into deeper basins at multiple glaciers simultaneously, Greenland’s contribution to sea‑level rise could jump detectably within the next two decades, well ahead of what smooth trend lines predict.
BedMachine Mapping and the Numbers Behind the Headline
The BedMachine v3 paper, led by researchers who combined multibeam sonar surveys of coastal fjords with mass‑conservation physics applied to radar‑derived ice thickness, produced the most granular picture of Greenland’s bedrock to date. The 7.42‑meter sea‑level equivalent it calculated refined earlier estimates by filling data gaps in narrow fjords where airborne radar struggled to resolve bed geometry. That improvement matters because the shape of the bed directly controls how glaciers respond to warming ocean water. A fjord that widens and deepens inland creates conditions for runaway retreat; a fjord that narrows acts as a brake.
Antarctica’s ice‑sheet volume is far larger than Greenland’s, holding an estimated 58 to 60 meters of sea‑level equivalent depending on the dataset and on how floating ice shelves are treated. No single peer‑reviewed study in the available evidence provides an Antarctic figure with the same multibeam and mass‑conservation precision that BedMachine v3 delivered for Greenland. That asymmetry in data quality is itself a risk factor: the ice sheet holding the most water is the one whose bed topography is least well mapped in many sectors, particularly in East Antarctica, where thick ice and remote terrain limit direct observations.
The NASA and USGS figures both serve as institutional benchmarks but address slightly different questions. NASA’s roughly 60‑meter figure covers all glaciers and ice sheets globally, using broad categories that group Greenland, Antarctica, and smaller ice bodies into a single global sum. The USGS estimate of about 70 meters includes glaciers and ice caps beyond the two main ice sheets and expresses the result in rounded terms suitable for public communication. Neither agency’s public explainer isolates a Greenland‑plus‑Antarctica‑only total, which is why the 67‑meter headline number must be reconstructed from component estimates rather than cited from a single source.
Gaps in Antarctic Bed Data and the Nonlinear Retreat Question
Several questions remain open. First, the Antarctic ice sheet lacks a BedMachine‑equivalent dataset with dense multibeam coverage of its marine‑terminating glacier troughs and grounding zones. Airborne radar and satellite gravity data provide broad‑scale maps of bed elevation, but narrow channels that can control glacier flow are often unresolved. Until that mapping is improved, the Antarctic component of the 67‑meter figure carries wider uncertainty than Greenland’s tightly bounded estimate, especially in regions where deep basins may lie hidden beneath the ice.
Second, the physics of marine ice‑sheet instability depend sensitively on bed shape. Where the bed slopes downward inland, retreating grounding lines can become unstable, leading to self‑sustaining loss of ice. Without high‑resolution maps of these slopes around Antarctica’s perimeter, models may underestimate the potential for rapid, localized collapse of outlet glaciers that currently buttress interior ice. That underestimation would not change the ultimate ceiling of roughly 67 meters from the two big ice sheets, but it could significantly alter the timeline over which a portion of that water is added to the oceans.
Third, the interaction between thinning ice shelves and grounded ice remains difficult to quantify. Ice shelves themselves are already floating and therefore do not directly raise sea level when they melt, but they act as brakes on the glaciers behind them. Warmer ocean water eroding an ice shelf from below can remove that buttressing, allowing grounded ice to accelerate toward the sea. In sectors of West Antarctica where shelves are thinning rapidly, incomplete knowledge of the bed beneath upstream glaciers makes it hard to say how far and how fast that acceleration could propagate inland.
These gaps do not render the 67‑meter estimate meaningless; rather, they define its limits. Greenland’s contribution is now constrained to within a few centimeters of sea‑level equivalent by detailed topographic mapping. Antarctica’s contribution remains a broader range whose central value is reasonably well known but whose internal pathways to retreat are not. The contrast underscores a basic point for policymakers and coastal planners: uncertainty today is less about whether the water exists and more about how quickly different portions of it can reach the ocean.
Improving that picture will require sustained investment in polar observations. Multibeam sonar campaigns in Antarctic fjords, expanded airborne radar surveys, and new satellite missions that can resolve ice thickness and bed elevation at finer scales all have a role to play. So do advances in ice‑sheet modeling that explicitly incorporate threshold behaviors tied to bed geometry, rather than relying on smooth extrapolations of past trends. As these efforts converge, the global community will gain a clearer view not just of the ultimate 67‑meter ceiling, but of the nearer‑term steps on the way there-steps that determine how quickly coastal defenses must rise, how zoning laws should evolve, and how much room remains to avoid the most extreme outcomes.
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*This article was researched with the help of AI, with human editors creating the final content.
