Sea level rise

• 1. Introduction

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  Sea level rise is already reshaping the coastlines that hundreds of millions of people call home. Since the end of the Last Glacial Maximum roughly 20,000 years ago, seas have climbed more than 125 meters. But something changed sharply in the modern era: between 1901 and 2018, the average global sea level rose by 15 to 25 centimeters, and the rate has been accelerating ever since. By 2013 to 2022, the pace had reached 4.62 millimeters per year. The Thwaites Glacier alone now accounts for 4 percent of global sea level rise. In 2023, United Nations Secretary-General António Guterres warned that rising seas risk causing human migrations on a biblical scale. What is driving this acceleration? How much will waters rise, and how fast? And who will bear the heaviest burden when coastlines that have been settled for millennia begin to disappear?

• 2. The Engines of Rise

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  Melting ice sheets and glaciers accounted for 44 percent of sea level rise between 1993 and 2018, while thermal expansion of ocean water contributed another 42 percent. The oceans store more than 90 percent of the extra heat added to the climate system by Earth's energy imbalance. Trend data show the ocean is 1.2 degrees Fahrenheit (0.7 degrees Celsius) warmer than it was in 1950, which has contributed to more than 6 inches of sea level rise on its own. When ocean water gains heat, it expands; warmer water and water under greater pressure expands more, so the effect is uneven across regions. The Southern Ocean accounts for approximately 40 percent of global ocean heat uptake, making it a dominant factor in the global heat budget. Greenland and Antarctica together store roughly 99.5 percent of all land ice on Earth, with a combined sea-level equivalent of more than 65 meters. Total ice loss from the Greenland ice sheet between 1992 and 2018 alone amounted to 3,902 gigatons, equivalent to 10.8 millimeters of sea level rise. Land water storage adds another wrinkle: water redistribution from irrigation was estimated at 2,150 gigatons between 1993 and 2010, equivalent to 6.24 millimeters of sea level rise, and that shift was large enough to cause Earth's rotational pole to drift by 78.48 centimeters, a prediction confirmed in 2023.

• 3. Ice on the Edge

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  West Antarctica's bedrock lies mostly below sea level, making it especially vulnerable to a process called marine ice sheet instability. When underwater glaciers rest on bedrock that slopes downward inland, retreating ice exposes ever-taller walls of ice to warm ocean water, which accelerates breakdown in a self-reinforcing cycle. The Thwaites and Pine Island glaciers buttress the entire West Antarctic Ice Sheet; their collapse would eventually take the whole sheet with them, contributing roughly 3.3 meters to global sea levels over an estimated 2,000 years. The contribution of these glaciers to global sea levels has already accelerated since the year 2000. A still more dramatic idea is marine ice cliff instability: coastal ice cliffs exceeding roughly 90 meters above ground and 800 meters below could collapse under their own weight once the ice shelves propping them up disappear, triggering a cascade. In a 2020 survey of 106 experts, the 2016 paper proposing 1 meter or more of Antarctic sea level rise by 2100 was considered more important than the 2014 IPCC Fifth Assessment Report. Yet observational evidence remains limited and ambiguous. Only one episode of seabed gouging from the Younger Dryas period appears consistent with the theory, and it lasted an estimated 900 years. Scientists Robert DeConto and David Pollard, who originally proposed the hypothesis, have suggested that precise measurements of sea level during the Last Interglacial could resolve the question: if rise at that time was lower than 4 meters, the instability can effectively be ruled out, while values greater than 6 meters would make it very likely. As of 2023, the most recent analysis places the Last Interglacial sea level rise at no more than 2.7 meters.

• 4. Reading the Past

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  Thomas Lempriere, an amateur meteorologist, began measuring sea levels in 1837 and in 1841 established a benchmark on a small cliff on the Isle of the Dead near the Port Arthur convict settlement in Australia. That benchmark, combined with satellite data from 1992 onward, helped establish that global mean sea level rose 19.5 centimeters between 1870 and 2004. The longest continuously running sea-level measurements in the world, the NAP or Amsterdam Ordnance Datum, date to 1675 in Amsterdam. Paleoclimate data confirms that sea level had not risen as fast as it has since 1900 over at least the past 3,000 years, and a research paper published in October 2025 extended that window, finding that current sea level rise is faster than in any century over at least the last 4,000 years. Since the Last Glacial Maximum, sea level has climbed more than 125 meters, with rates at times exceeding 40 millimeters per year when major ice sheets over Canada and Eurasia collapsed. The Eemian interglacial, roughly 120,000 years ago, offers a sobering benchmark: when Earth was about 2 degrees Celsius warmer than pre-industrial temperatures, seas stood at least 5 meters higher than today. Further back, when atmospheric carbon dioxide levels were around 400 parts per million (similar to recent decades), temperatures eventually rose enough to melt one third of Antarctica's ice sheet, pushing seas 20 meters above pre-industrial levels.

• 5. Where the Numbers Land

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  Since the launch of TOPEX/Poseidon in 1992, an overlapping series of altimetric satellites has continuously recorded sea surface heights, estimating a rise of 3.0 plus or minus 0.4 millimeters per year for 1993 to 2017. The Intergovernmental Panel on Climate Change has tracked 21st-century projections across successive reports. Its 2013-2014 Fifth Assessment Report showed that under the lowest scenario, RCP2.6, sea level would rise roughly 44 centimeters by 2100, with a range of 28 to 61 centimeters. The 2021 Sixth Assessment Report revised those figures upward, incorporating improved ice-sheet modeling and structured expert judgements. By 2020, observed ice-sheet erosion in Greenland and Antarctica had already matched the upper end of earlier AR5 projections, validating the need for larger estimates. Under the high-emission SSP5-8.5 scenario in AR6, ice cliff instability could put sea level rise of up to 1.6 meters by 2100 within the realm of possibility. Sea level rise also lags behind temperature change by decades, meaning that warming already in the system will continue to push seas higher through 2050 regardless of near-term emissions cuts. A 2018 paper estimated that for every five years emissions increase before peaking, sea level rise in 2300 grows by a median of 20 centimeters. Even if temperatures stabilize below 2 degrees Celsius, sea level rise in 2300 would still likely exceed 1.5 meters.

• 6. Unequal Ground

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  About 600 million people live directly on the coast around the world, and two thirds of cities with more than five million residents sit in low-lying coastal areas less than 10 meters above sea level. Of the twenty countries most exposed to sea level rise, twelve are in Asia, including Indonesia, Bangladesh, and the Philippines. Bangladesh alone may see 0.9 to 2.1 million people displaced by 2050. In the United States, sea level rise is likely to be two to three times greater than the global average by the end of the century. In places like New Orleans and the Sacramento-San Joaquin River Delta, human-caused subsidence from groundwater extraction and flood management practices has already sunk the land by more than 3 meters and over 9 meters respectively. Environmental scholar Kyle Powys Whyte has argued that climate change can be understood as part of a continuous process that has disproportionately affected Indigenous and minority communities, shaped by a history of settler colonialism. In Miami, according to personal finance reporter Greg Iacurci, historically Black neighborhoods like Overtown and Little Haiti, which were devalued by discriminatory housing practices, now sit at higher elevations than surrounding areas, making them targets for wealthier residents fleeing lower ground in a process called climate gentrification. Along the southeastern U.S. coast, Gullah Geechee communities whose ancestors were enslaved occupy low-lying land at direct risk. In Louisiana, the coastal Indigenous community of Isle de Jean Charles has already experienced significant land loss to sea level rise and erosion, with a state-supported relocation effort underway after the majority of the land there disappeared over several decades.

• 7. Paths Forward

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  Adaptation against flooding is already measurable in economic terms: effective measures could reduce projected annual flooding costs in the world's 136 largest coastal cities from roughly 1 trillion dollars by 2050 to just over 60 billion dollars annually. A survey of 253 planners from 49 countries found that 98 percent are aware of sea level rise projections, but 26 percent have not yet formally integrated them into policy documents. Only around a third of respondents from Asian and South American countries have done so, compared with over 75 percent in Europe, Australasia, and North America. Adaptation strategies range from hard approaches such as seawalls and levees to soft approaches such as dune rehabilitation, beach nourishment, and cultivating saltwater-tolerant crops. Managed retreat is the most straightforward option but also the most disruptive: displacing newly impoverished populations can burden receiving communities and accelerate social tensions. Natural defenses like mangroves and tidal marshes buffer storm surges and tsunamis, but their own survival depends on how fast seas rise; pneumatophores (breathing roots) will be submerged if migration upland is outpaced by inundation. Research suggests that climate engineering to stabilize the glaciers of West Antarctica might delay collapse by centuries, though the proposal is uncertain and would rank among the most expensive projects ever attempted. Greenland's peripheral glaciers and ice caps crossed an irreversible tipping point around 1997, meaning their contribution to sea level rise is now unavoidable. The Thwaites Ice Shelf could potentially fail as early as the mid-2020s, which would remove the main brake on one of the most consequential glaciers on Earth.