For millions of people across the globe, rising sea levels are no longer a distant theoretical projection—they are a daily reality actively reshaping economies, traditional livelihoods, and cultural landscapes. 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 set to formally adopt a high-level declaration addressing the existential threats posed by accelerating sea level rises. The declaration explicitly notes that the phenomenon is not a distant scenario, but a real and lived experience for communities worldwide.
On average, global sea levels rose by 20 centimetres between 1901 and 2018. This historical shift is primarily due to a combination of melting glaciers and polar ice sheets, the thermal expansion of seawater as global temperatures warm, and shifting patterns of land-water storage. However, the velocity of this rise has dramatically accelerated in recent decades. Ocean levels have climbed by 10.6 centimetres since 1993 alone. Under a moderate-emissions scenario, scientists project that the global average sea level will rise an additional 56 centimetres by the year 2100, relative to a baseline established between 1995 and 2014.
While global averages provide a baseline, local impacts vary widely due to factors such as seismic and volcanic activity, groundwater extraction, and localized changes in the Earth’s surface resulting from ice melt. To understand the full scope of this planetary crisis, experts continue to examine how rising waters threaten human societies and natural ecosystems alike.
Cities and Coastal Communities
Around 770 million people—representing roughly 10% of the global population—currently face acute risks from the negative impacts of sea level rise, according to a recent report released by the UN secretary general. The report defines locations at acute risk as areas situated less than five metres above the high-tide line.
This vulnerable population includes residents living in some of the world’s largest urban centers, such as Mumbai and Kolkata in India, and 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—the upward movement of the ocean surface relative to a baseline—and relative sea level rise. Relative sea level rise accounts for how much the water has risen in a specific geographic location, influenced by local land motion, ocean circulation, and shifts in the Earth’s rotation and gravitational pull caused by melting ice sheets.
Higher sea levels unleash a cascade of dangers for coastal populations, ranging from persistent flooding and severe coastal inundation to the dangerous reinforcement of storm surges and the accelerated erosion of beaches and sea cliffs. Furthermore, rising sea levels cause coastal water tables to rise, which significantly exacerbates overall flood risks.
The frequency of 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 hundred-year extreme events are projected to occur at least annually, and potentially even more frequently, in many coastal regions by the end of the century.
Beyond residential damage, critical public infrastructure, including water distribution systems and wastewater management projects, faces mounting vulnerability to flooding. When floodwaters submerge vital transport and utility networks, communities can be cut off from essential services such as hospitals and food markets, with low-income and marginalized populations bearing the brunt of the impact. Additionally, the inland encroachment of seawater leads to severe saltwater intrusion, threatening agricultural productivity in low-lying coastal plains.
Vertical land motion can drastically amplify these risks. Phenomena such as tectonic shifts, volcanic activity, and land subsidence—the sinking of the ground—can cause local sea level changes that equal or surpass the contributions of climate-driven ocean expansion. Many coastal cities experiencing the most dramatic shifts in relative sea level are located in east and south-east Asia. Jakarta, Indonesia, for instance, has sunk by up to 15 centimetres per year over the past decade, driven largely by the overextraction of groundwater that causes underground aquifers to collapse.
Even as these environmental hazards intensify, human populations continue to flock to the coastlines at rates that outstrip inland growth. Between 2000 and 2018, while the global population grew by slightly more than 23%, the population living within five kilometres of a coast surged by 28%. Consequently, development pressures mean that even without accounting for future sea level rise, global losses from flooding across the world’s 136 largest coastal cities could reach up to $52 billion per year by 2050, a steep increase from $6 billion in 2005.
In response, communities worldwide have turned to various adaptation strategies. In 2003, the Italian city of Venice initiated a massive engineering project to construct three movable floodgates designed to rise during high tides and protect the city’s delicate 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, ranging from the construction of seawalls and the restoration of natural protective barriers like mangrove forests and marshes, to disincentivizing new development in high-risk zones and relocating vulnerable infrastructure to higher ground. However, experts warn that existing adaptations may prove insufficient; under a low-emissions scenario, current protective measures could be breached ten times as frequently over the coming decades.
Biodiversity and Coastal Ecosystems
Coastal ecosystems globally are facing rapid degradation from a combination of human development and rising seas, a dual pressure known to scientists as "coastal squeeze." This phenomenon occurs when natural habitats that would typically migrate inland in response to rising waters find their pathways blocked by human-made infrastructure, leading to the widespread loss of vital wetlands.
According to the global wetland outlook report, nearly 28 million hectares of coastal wetlands—including 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 driver of historical wetland loss, climate change and sea level rise are increasingly compounding these pressures by altering the frequency and intensity of extreme weather events, fires, floods, and droughts.
In the continental United States, only 16% of coastal wetlands are currently migrating inland at a pace that outstrips local sea level rise. Nearly three-quarters of sites are migrating too slowly to keep pace, while 11% are actively submerging.
Low-lying islands are particularly vulnerable due to their high ratio of coastline to land area. At the same time, these islands serve as critical biodiversity hotspots, home to thousands of plant and animal species found nowhere else on Earth. More than 20% of the world’s known plant species exist exclusively on islands.
Modelling studies examining the potential impact of one metre of sea level rise across thousands of island biodiversity hotspots indicate that roughly 6% of island habitat areas would be entirely submerged, while more than 11% of the islands would see their land areas reduced by at least half, placing numerous endemic species at severe risk of extinction.
The threat is already materializing. In 2024, researchers documented the first known local extinction of a plant species in the United States directly attributed to sea level rise. Amplified by rising seas, hurricanes and storm surges devastated the sole domestic population of the Key Largo cactus during the 2010s. Severe soil erosion and saltwater intrusion compromised the remaining plants, prompting conservationists to rescue the final specimens for greenhouse cultivation in 2021, though surviving populations persist in other Caribbean locations like Cuba.
As native flora and fauna decline, coastal ecosystems become increasingly susceptible to colonization by invasive alien species, compounding the loss of biodiversity. Furthermore, as human coastal communities are forced to relocate inland, secondary pressures on biodiversity emerge in newly developed regions, a trend expected to be particularly acute in south-east Asia given its massive low-lying rural and urban populations.
Small Island Developing States
Small island developing states face acute and disproportionate threats from rising seas. Even under moderate emissions scenarios, rising waters threaten to render many low-lying coastal zones and small island nations largely uninhabitable without extraordinary adaptation efforts.
The physical geography of these nations heightens their exposure. Many Pacific island states are built upon atolls—ring-shaped coral or sandy formations enclosing shallow lagoons—which typically feature average elevations of just one to two metres above sea level. While some scientific models suggest certain reef islands can grow vertically through wave-driven sediment accumulation, the force of those high-energy waves generally renders the resulting land unsuitable for human habitation or infrastructure.
Additionally, most Pacific island nations are situated in a regional band where relative sea level rise driven by the melting of the Antarctic ice sheet is projected to exceed the global average by 11% to 33% by the year 2100, regardless of future emissions pathways.
The consequences are already visible. Kiribati lost two small, uninhabited islands to rising seas in 1999, and several uninhabited islands in the Solomon Islands had completely vanished by 2014, with six others heavily eroded, forcing local community relocations. Furthermore, because small island states frequently lie in the paths of tropical cyclones, higher sea levels amplify destructive storm surges.
Despite their extreme vulnerability, small island developing states often lack the financial resources to mount adequate defenses independently. According to UN estimates, these nations will require up to $6 billion annually by 2035 for climate adaptation, yet they received only $1.2 billion in public adaptation finance between 2022 and 2023. Current expected annual climate damages from coastal flooding stand at $1.64 billion—equivalent to 0.13% of their cumulative gross domestic product—and are projected to surge to $24 billion even if global warming is successfully limited to 1.5 degrees Celsius above pre-industrial levels.
The crisis has also sparked international legal debates regarding the status of maritime boundaries, such as exclusive economic zones, which are conventionally tied to a nation’s land borders. Addressing this vacuum, a report by the UN International Law Commission concluded that international law permits existing maritime borders to remain intact despite coastline changes caused by climate-driven sea level rise, 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. While ocean warming remains the primary driver of their current widespread degradation, sea level rise also exerts a complex physical influence.
As the ocean rises, the depth of water overlying shallow coral ecosystems increases. This deepening can have dual effects; deeper water can reduce ambient temperatures and act as a buffer against marine heatwaves and global ocean warming, but it simultaneously reduces the amount of sunlight penetrating down to the coral communities, impairing their growth and survival.
Furthermore, sea level rise-driven coastal erosion introduces increased sediment loads into near-shore waters. These suspended particles can settle directly onto corals, impeding their ability to feed, reproduce, and carry out photosynthesis alongside their symbiotic zooxanthellae algae, ultimately causing elevated physiological stress and stunted growth.
While certain reef systems in parts of the world have historically managed to grow vertically at accelerated rates to keep pace with rising waters, modelling indicates that very few reefs possess the biological capacity to maintain their proximity to the surface under moderate-emissions scenarios. As reefs degrade, the underlying seafloor erodes, exacerbating apparent relative sea level rise in regions like the Caribbean, Florida, and Hawaii. Although rising waters have occasionally enabled the rare recolonization of long-dormant reefs in shallow environments—such as Sanya Bay in the northern South China Sea—these localized revivals are vastly outweighed by global coral losses. Since 1980, the world has lost nearly 10% of its coral cover due to ocean warming.
Heritage Sites
Human history has long been anchored to rivers and coasts due to the abundance of food and logistical ease of transportation. Consequently, countless cultural and natural heritage sites now face severe threats of damage or destruction from rising seas.
The UN sea level rise report defines cultural heritage broadly to encompass physical sites, living traditions, ancestral lands, burial grounds, underwater archaeological finds, sacred locations, and culturally significant coastal landscapes. In the Mediterranean Sea alone, 49 UNESCO world heritage sites sit at low elevations along the coast, with nearly all facing immediate risks from erosion or severe flooding events. Among the prominent locations at risk are the ancient archaeological ruins of Carthage in Tunisia, Ephesus in Turkey, Pompeii and Herculaneum in Italy, and the medieval Cathedral of St James in Šibenik, Croatia.
Projections indicate that a global temperature rise of three degrees Celsius above pre-industrial levels would imperil nearly one-fifth of all UNESCO cultural world heritage sites. Vulnerable locations at this threshold include Japan’s Hiroshima Peace Memorial, South Africa’s Robben Island, Chile’s Rapa Nui, and the Sydney Opera House, with many of these sites becoming vulnerable at even lower warming levels.
In Africa, 56 out of 284 cultural and natural heritage sites currently face risks from coastal flooding and erosion, a figure projected to nearly triple to 191 threatened sites by 2050 under a moderate-emissions scenario. Globally, 386 coastal UNESCO heritage sites lie within 20 metres of sea level, leaving them exposed to persistent flooding and erosion hazards. Responding to these mounting risks, the UN declaration underscores that the active protection, preservation, and documentation of cultural heritage must remain a critical global priority in the years ahead.
