For millions of people around the world, rising sea levels are no longer a distant theoretical projection—they are already reshaping economies, livelihoods, cultures, and daily geographies. Driven primarily by human activity since at least 1970, the world’s oceans are currently rising at a faster rate than at any time in at least the past three millennia.
Against this backdrop at the United Nations General Assembly in New York, world leaders are adopting a high-level declaration addressing the existential threats posed by sea level rise. The declaration underscores that rising oceans are not a future scenario, but a lived reality for vulnerable populations across the globe.
On average, global sea levels rose by 20 centimeters between 1901 and 2018. This historical increase is driven by a combination of factors, including the melting of glaciers and ice sheets, the thermal expansion of seawater as it absorbs heat, and shifts in land-water storage. The rate of this rise has dramatically accelerated in recent decades, with ocean levels climbing an additional 10.6 centimeters since 1993 alone. Local sea levels can vary significantly from this global average due to seismic and volcanic activity, groundwater extraction, and localized changes to the Earth’s surface resulting from ice melt and gravitational shifts. Under a moderate-emissions scenario, scientists project that global average sea levels will rise an additional 56 centimeters by 2100, relative to a 1995-2014 baseline.
Cities and Coastal Communities
Around 770 million people—representing roughly 10% of the world’s population—are currently at acute risk of negative impacts from sea level rise, according to a report released by the UN secretary general. The report defines locations at acute risk as those situated less than five meters above the high-tide line.
This vulnerable population includes residents of several of the world’s largest and most economically vital urban centers, including Mumbai and Kolkata in India, as well as Shenzhen and Guangzhou in China. It also encompasses the entire populations of numerous low-lying island nations.
Understanding the mechanics of rising water requires distinguishing between global-average sea level rise, which measures the upward movement of the ocean surface relative to a global baseline, and relative sea level rise. Relative sea level rise accounts for how much the ocean has risen in a specific locality, factoring in local land motion, ocean circulation, and changes to the Earth’s surface and rotation caused by melting ice sheets.
Higher sea levels unleash a cascade of dangers for coastal communities, including persistent flooding and inundation, stronger and more destructive storm surges, and accelerated erosion of beaches and cliffs. Furthermore, rising seas cause the water table of coastal land to rise, significantly compounding flood risks.
The frequency of 100-year extreme sea level events—where high tides, storm surges, and relative sea level rise combine to produce exceptionally high waters—has increased twelvefold since 1900. Under a moderate-emissions scenario, these severe events are projected to occur at least annually, and potentially even more frequently, in many coastal regions by the end of the century.
Beyond direct damage to residential and commercial buildings, critical municipal infrastructure such as water supply systems and wastewater management facilities faces mounting vulnerability. Flooding frequently cuts communities off from essential services like hospitals and commercial markets, with low-income and marginalized populations bearing a disproportionate share of the burden. Inland encroachment of seawater also drives saltwater intrusion, threatening agricultural productivity in low-lying coastal breadbaskets.
Vertical land motion can drastically amplify these risks. Phenomena such as seismic activity, volcanic shifts, and land subsidence—the sinking of the ground—can cause local sea level changes that equal or surpass the contributions of climate-driven ocean rise. Many coastal cities experiencing the most extreme relative sea level changes are located in east and south-east Asia. Jakarta, Indonesia, has famously sunk by up to 15 centimeters per year over the past decade, largely driven by the overextraction of groundwater that causes underground aquifers to collapse.
Even as these risks escalate, human populations in coastal zones continue to outstrip growth in inland areas. Between 2000 and 2018, the global population grew by slightly more than 23%, while the population living within five kilometers of a coast surged by 28%. Consequently, even without factoring in future sea level rise, global economic losses from flooding across the world’s 136 largest coastal cities could reach up to $52 billion per year by 2050, up sharply from $6 billion in 2005.
Communities worldwide have turned to varied adaptive measures to counter these threats. In 2003, Italy initiated a massive engineering project in Venice involving three massive mobile floodgates designed to rise during high tides and protect the city’s lagoon from the Adriatic Sea. The system was first deployed in October 2020 and has since been engaged dozens of times. Other regions have implemented less technologically intensive defenses, including the construction of seawalls, the restoration of mangrove forests and marshes, restrictions on development in high-risk zones, and the relocation of vulnerable infrastructure to higher ground. However, existing adaptations may prove insufficient; under a low-emissions scenario, current protective measures could be breached ten times as frequently over the coming three decades.
Biodiversity and Coastal Ecosystems
Coastal ecosystems worldwide are undergoing rapid degradation driven by the dual pressures of coastal development and sea level rise. This phenomenon, known as coastal squeeze, occurs when ecosystems that would naturally migrate inland as sea levels rise find their paths blocked by human-made infrastructure. Coastal squeeze has been a primary driver behind the widespread loss of global wetlands.
Globally, nearly 28 million hectares of coastal wetlands—encompassing estuaries, tidal flats, mangroves, and seagrass meadows—have vanished since 1970, an area roughly equivalent to the size of Ecuador. While agricultural conversion remains the largest historical driver of wetland loss, climate change increasingly intensifies these pressures through altered weather extremes, fires, floods, droughts, and rising seas. In the continental United States, only 16% of coastal wetlands are migrating inland at a pace exceeding local sea level rise, while nearly three-quarters are moving too slowly to keep pace, and 11% are actively submerging.
Low-lying islands face acute threats due to their high ratio of coastline to total land area, while simultaneously serving as global biodiversity hotspots that harbor endemic species found nowhere else on Earth. More than 20% of all known plant species exist exclusively on islands. Modeling of island biodiversity hotspots indicates that one meter of sea level rise would completely submerge roughly 6% of island habitat areas, while reducing the land area of more than 11% of hotspot islands by at least half, placing numerous species at risk of extinction.
In 2024, researchers documented the first known local extinction of a plant species in the United States directly attributed to sea level rise. Hurricanes and storm surges, amplified by higher baseline waters, decimated the only US population of the Key Largo cactus during the 2010s. Soil erosion and saltwater intrusion severely damaged the remaining plants, forcing conservationists to remove the final specimens in 2021 for greenhouse cultivation.
As native flora and fauna decline, coastal ecosystems become increasingly susceptible to colonization by invasive alien species, compounding biodiversity loss. Furthermore, as human coastal communities are forced to relocate inland, secondary development pressures create knock-on ecological impacts in new regions, a trend expected to be particularly pronounced in densely populated low-lying areas of south-east Asia.
Small Island Developing States
Sea level rise poses an acute and disproportionate threat to small island developing states. The UN report warns that even under moderate emissions scenarios, rising seas will render many low-lying coastal zones and island nations increasingly uninhabitable without extraordinary adaptation efforts. Climate change is already inflicting tangible loss and damage on these nations, whose vulnerability is rooted in their unique geography. Several Pacific island states consist of atolls—ring-shaped coral or sandy structures encircling lagoons—with average elevations of just one to two meters above sea level and maximum heights of three to five meters.
While some scientific modeling indicates that reef islands can grow vertically as waves wash ocean sediment onto their surfaces, the high energy of those waves often renders the land unsuitable for human habitation. Furthermore, most small island nations in the Pacific are located in a region where relative sea level rise resulting from the melting of the Antarctic ice sheet is projected to run 11% to 33% higher than the global average by 2100, regardless of future emissions trajectories.
The consequences of this amplified rise are already visible. In 1999, Kiribati lost two small, uninhabited islands to the sea. Several uninhabited islands in the Solomon Islands vanished by 2014, and six others suffered severe erosion, forcing community relocations. Additionally, small island developing states frequently lie in paths prone to tropical cyclones, where elevated baseline seas intensify destructive storm surges.
Despite facing these disproportionate impacts, small island developing states often lack the domestic financial resources to mount comprehensive defenses. According to UN estimates, these nations will require up to $6 billion annually by 2035 to adapt effectively to climate change, yet they received just $1.2 billion in public adaptation finance in 2022-23. Current expected annual climate damages from coastal flooding stand at $1.64 billion for these nations, equivalent to 0.13% of their cumulative GDP, but these costs are projected to soar to $24 billion even if global warming is limited to 1.5 degrees Celsius above pre-industrial levels.
International policy discussions have also grappled with the legal status of maritime boundaries as island territories are gradually enveloped by the sea, since exclusive economic zones are tied to land borders. A 2025 report by the UN International Law Commission concluded that international law permits existing national borders to remain static despite climate-driven coastline changes, while emphasizing the urgent need for legal and practical protections for displaced populations.
Coral Reefs
Coral reefs rank among the ecosystems most vulnerable to climate change, suffering widespread degradation driven primarily by ocean warming. While reefs are permanently submerged, sea level rise exerts profound secondary effects on their survival.
As oceans rise, the water column above shallow coral ecosystems deepens. This increased depth offers a dual-edged effect. Deeper water can reduce peak temperatures experienced by reefs, acting as a temporary buffer against marine heatwaves and broader ocean warming. Conversely, the greater water depth reduces the amount of sunlight penetrating to the coral communities, which can inhibit their growth and survival.
Furthermore, sea level rise-driven coastal erosion adds large volumes of sediment to near-shore waters. These particles settle on coral structures, impairing their ability to feed, reproduce, and conduct photosynthesis via their symbiotic zooxanthellae algae, ultimately causing heightened physiological stress and slower growth rates. While some regional reefs have historically managed to keep pace with sea level rise by accelerating vertical growth to maintain adequate light levels, modeling indicates that very few reefs possess the capacity to maintain their proximity to the surface under moderate-emissions scenarios.
As coral reefs degrade, the underlying seafloor erodes, accelerating apparent sea level rise in locations like the Caribbean, Florida, and Hawaii. Conversely, sea level rise has occasionally spurred reef growth in shallow environments previously deemed uninhabitable. In Sanya Bay within the northern South China Sea, rising seas since the mid-1980s have facilitated the recolonization of a reef dormant for over five millennia. However, these localized recoveries are vastly outweighed by global losses; the world has lost nearly 10% of its coral cover since 1980 due to ocean warming, leading the UN declaration to emphasize that every fraction of a degree of global warming increases risks to coral reefs worldwide.
Heritage Sites
Throughout history, human societies naturally concentrated along rivers and coastlines to take advantage of food security and transportation access. Today, this historical proximity to water places countless cultural and natural heritage sites at severe risk of damage or destruction from sea level rise. Cultural heritage encompasses physical structures, living traditions, traditional lands, burial grounds, underwater assets, and archaeological landscapes.
Numerous assessments have mapped the cultural and natural landmarks facing the greatest exposure to coastal flooding and erosion. Along the Mediterranean Sea, 49 UNESCO World Heritage sites sit at low elevations, with nearly every one facing immediate threats from erosion or severe flooding. Locations at risk include the ancient Carthaginian archaeological sites in Tunisia, Ephesus in Turkey, the ruins of Pompeii and Herculaneum in Italy, and the medieval Cathedral of St James in Šibenik, Croatia.
A temperature increase of three degrees Celsius above pre-industrial levels would threaten nearly one-fifth of all UNESCO cultural world heritage sites, including Japan’s Hiroshima Peace Memorial, South Africa’s Robben Island, Chile’s Rapa Nui, and the Sydney Opera House—many of which become vulnerable at much lower warming thresholds. In Africa, 56 out of 284 cultural and natural heritage sites already face risks from flooding or erosion, a figure projected to nearly triple to 191 sites by 2050 under a moderate-emissions scenario. Globally, 386 coastal UNESCO heritage sites sit at elevations of no more than 20 meters, placing them within reach of ongoing coastal hazards. The UN declaration underscores that the protection, preservation, and documentation of this irreplaceable cultural heritage remains an urgent global priority.
