The warning comes as new research synthesizes the latest data on ice sheet behaviour and long-term sea level changes, concluding that 1.5°C should not be regarded as a "safe" level for coastal communities around the world.
While nearly 200 countries have pledged under the Paris Agreement to pursue efforts to keep warming below 1.5°C, scientists say this target—though ambitious—is still likely to trigger major ice melt in Greenland and Antarctica that would eventually submerge low-lying areas.
"Limiting warming to 1.5°C would be a landmark achievement. But it won't halt sea-level rise or save ice sheets from retreat," said lead author Professor Chris Stokes, a glaciologist at Durham University. "The message is clear: even the best-case scenario leaves a legacy of rising seas."
The world is currently on track for around 3°C of warming by 2100 based on existing government policies, far beyond the threshold scientists say is needed to avoid catastrophic climate impacts. Yet even 1.5°C of warming—if achieved and maintained—would continue to drive glacial loss due to the long response times of Earth's massive ice sheets.
The researchers, writing in Communications Earth & Environment, drew on three key types of evidence: geological records of ancient warm periods, direct observations of current ice sheet changes, and projections from climate models.
Lessons from the Past
Looking back tens of thousands to millions of years, scientists note that when Earth's temperatures were similar to today—such as 125,000 years ago—sea levels were several metres higher. Even more dramatically, around three million years ago, when atmospheric CO₂ levels last matched today's concentrations, global sea levels stood 10–20 metres above current levels.
Signals in the Present
Glaciologists are already seeing alarming shifts in both Greenland and West Antarctica. "We're witnessing disturbing changes—rapid ice loss in key regions that suggest we may be crossing tipping points," said co-author Professor Jonathan Bamber, director of the Bristol Glaciology Centre. East Antarctica remains relatively stable, but for how long is uncertain.
Models of the Future
Computer simulations further support the concern. "The vast majority of models show sea-level rise not only continuing, but accelerating, even if temperatures stabilise at 1.5°C," explained Professor Stokes. The models suggest there's little chance of reversing sea-level rise on any timescale relevant to human civilisation.
The study's strength lies in combining data from multiple disciplines—paleoclimate, present-day satellite measurements, and predictive modelling—to paint a consistent and deeply concerning picture of long-term sea-level rise.
A Long-Term Threat to Coastal Populations
With an estimated 230 million people living within one metre of the current high tide line, rising seas pose an immense risk to lives, livelihoods, and entire cities. "We're not just talking about low-lying islands. This will reshape coastlines globally," said Professor Andy Shepherd, a glaciologist at Northumbria University, who was not involved in the study.
As warming continues, so too does the risk of ice sheet collapse beyond critical thresholds—so-called tipping points—though exactly when and how this would occur remains uncertain.
By the end of the century, a sea-level rise of more than a centimetre per year—driven by melting ice and thermal expansion of the oceans—could overwhelm even the wealthiest nations' capacity to adapt, the authors warn. "At that point, relocation and retreat become unavoidable," said Professor Bamber. "We're talking about migration on an unprecedented scale."
But Every Tenth of a Degree Counts
Despite the sobering conclusions, the researchers say the findings are not a reason for despair. On the contrary, they underscore the urgency of action. "The faster we cut emissions, the more damage we prevent," said Professor Stokes. "Every tenth of a degree avoided now matters for the future of our coastlines."
Sea-level rise may be inevitable, but the rate and magnitude of that rise remain within our influence—at least for now.