AI Insight
Global sea levels have risen 20 centimeters between 1901 and 2018, with the rate accelerating in recent decades (10.6 cm since 1993 alone), primarily due to glacier and ice sheet melting and thermal expansion of warming seawater. Human influence has been the dominant cause of sea level rise since at least 1970, with current rates exceeding any seen in the past three millennia. Under moderate emissions scenarios, scientists project an additional 56 cm rise by 2100 relative to 1995-2014 levels, though local variations occur due to factors like land subsidence, groundwater extraction, and gravitational changes from ice melt.
Why it matters
Approximately 770 million people (10% of global population) living in areas less than five meters above high-tide lines face acute risks from sea level rise, including residents of major cities like Mumbai, Kolkata, Shenzhen, and Guangzhou, as well as entire island nation populations. Rising seas threaten coastal communities through increased flooding, storm surge amplification, ecosystem degradation including coral reefs, and damage to cultural heritage sites.
Understand the Science
For millions of people around the world, rising sea levels are already reshaping economies, livelihoods and cultures.
The world’s oceans are currently rising at a faster rate than at any time in at least the past three millennia, with human influence the “dominant cause” of sea level rise since at least 1970.
At the UN general assembly in New York this week, world leaders are set to adopt a high-level declaration on the “existential threats” posed by sea level rise.
The declaration notes: “Sea level rise is not a distant scenario, but a real and lived experience for many.”
On average, global sea levels rose by 20 centimetres (cm) between 1901 and 2018.
This is due to both the melting of glaciers and ice sheets and the expansion of seawater as it warms, as well as changes in land-water storage.
The rate of the rise has accelerated in recent years, with ocean levels rising 10.6cm since 1993.
Sea level rise can vary locally due to seismic and volcanic activity, groundwater extraction and changes to the Earth’s surface resulting from ice melt.
Under a moderate-emissions scenario, scientists predict that global average sea level will rise an additional 56cm by 2100, relative to a 1995-2014 baseline.
Here, Carbon Brief unpacks some of the key ways that sea level rise threatens both societies and ecosystems.
Cities and coastal communities
Around 770 million people – 10% of the world’s population – are at “acute risk” of negative impacts from sea level rise, according to a report from the UN secretary general released last month.
(The report defines locations at acute risk as those that are less than five metres above the high-tide line.)
The people at risk include the residents of several of the world’s largest cities, including Mumbai and Kolkata in India and Shenzhen and Guangzhou in China. It also encompasses the entire populations of many island nations. (See: Small island developing states.)
There are two ways to consider sea level rise.
Global-average sea level rise is the amount the ocean surface has moved upwards, on average, relative to a baseline.
Relative, or local, sea level rise, is how much the ocean has risen in a given place. This can vary from the global average due to a number of factors, including land motion and ocean circulation, as well as changes to the Earth’s surface, rotation and gravitational pull due to the melting of the ice sheets.
Higher sea levels bring with them myriad dangers for coastal communities: they can increase persistent flooding and inundation, strengthen dangerous storm surges and erode beaches and cliffs. Sea level rise also causes the water table of coastal land to rise, which exacerbates flood risk.
The frequency of 100-year “extreme sea level events” has already increased 12-fold since 1900. These are events where high tides, storm surges and relative sea level rise combine to produce exceptionally high sea levels. Under a moderate-emissions scenario, these events are likely to occur at least annually – and, potentially, even more frequently – in many places by the end of the century.
In addition to damage to homes and other buildings, critical infrastructure – such as water systems and wastewater management projects – is increasingly vulnerable to flooding as a result of sea level rise. Flooding can cut communities off from essential services, such as hospitals and markets, with low-income and other marginalised groups disproportionately affected.
Inland encroachment of seawater leads to the saltwater intrusion and threatens agriculture in low-lying coastal areas.
Vertical land motion can also amplify the risk of rising seas. This includes a shifting of the land in response to seismic or volcanic activity or to land sinking, known as subsidence. The changes in local sea level due to these types of vertical motion can equal or surpass the contributions of climate-driven sea level rise.
Many of the coastal cities experiencing the largest changes in their relative sea level are located in east and south-east Asia. One notable example is Jakarta, Indonesia, which has been sinking by up to 15cm per year over the past decade, due largely to the overextraction of groundwater, which leads to the collapse of underground aquifers.
But even as the risks from sea level rise increase, population growth in coastal areas continues to outstrip that of inland areas. Between 2000 and 2018, the global population grew by slightly more than 23%. The population living within 5km of a coast increased by 28% over that same time.
The associated development of coastal areas means that, even without future sea level rise, global losses from flooding in the world’s largest 136 coastal cities could reach up to $52bn per year by 2050 – up from $6bn in 2005.
In response to the growing threats posed by sea level rise, communities around the world have implemented a number of adaptive actions.
In 2003, the Italian city of Venice began a years-long project to construct three floodgates that could be raised during high tide events to protect the city’s lagoon from the encroachment of the Adriatic Sea. The system was engaged for the first time in October 2020 and then another 48 times in the following two years.
Other communities have opted for less technologically intensive adaptations, including constructing seawalls, restoring mangrove forests and marshes, disincentivising development in high-risk areas and relocating residents, buildings and infrastructure to higher ground or inland areas.
However, existing adaptations may not be sufficient to protect communities. Under a low-emissions scenario, these protections may be breached 10 times as frequently over the next 30 years as they currently are.
Biodiversity and coastal ecosystems
The world’s coastal ecosystems are rapidly being destroyed due to both development and sea level rise.
This combination of pressures is called “coastal squeeze”, where ecosystems that may have otherwise shifted inland in response to sea level rise find their paths blocked by human-made structures.
Coastal squeeze has contributed to the widespread loss of the world’s wetlands.
Globally, nearly 28m hectares of coastal wetlands – including estuaries, tidal flats, mangroves and seagrass – have disappeared since 1970, according to the 2025 “global wetland outlook” report. This is an area equivalent to roughly the size of Ecuador.
Although the report names conversion to agriculture as the largest driver of wetland loss, it notes:
“Climate change is increasingly exacerbating the impact of other drivers on wetlands and human wellbeing through changes in the frequency and intensity of extreme weather events, associated fires, floods and droughts and through sea level rise.”
In the continental US, just 16% of coastal wetlands are migrating inland at rates that exceed local sea level rise. Nearly three-quarters of sites are moving at rates that do not outpace sea level rise, while 11% are submerging.
Low-lying islands are particularly threatened by sea level rise, due to their large amounts of coastline relative to their land areas. At the same time, islands are often “hotspots” of biodiversity, with many home to species found nowhere else in the world. More than 20% of the Earth’s known plant species are found only on islands.
A 2013 study modelled the impact of different amounts of sea level rise on 10 island biodiversity hotspots, comprising nearly 4,450 individual islands. It found that in a future with one metre of sea level rise, around 6% of the island habitat area would be completely submerged, while more than 11% of the hotspot islands would see their land area reduced by at least half. This could put dozens of species at risk of extinction, the study said.
In 2024, researchers documented the first known extirpation, or local extinction, of a plant species in the US due to sea level rise. Hurricanes and storm surges – amplified by sea level rise – began to kill off the only US population of the Key Largo cactus in the 2010s.
The remaining cacti suffered from soil erosion and saltwater intrusion and the final remaining specimens were removed in 2021 in an effort to cultivate them in greenhouses. (Other Caribbean islands, including Cuba, do still have surviving populations of the cactus.)
As native flora and fauna are diminished or even eliminated by rising sea levels, coastal ecosystems may become vulnerable to colonisation by invasive alien species, further harming biodiversity.
And as coastal communities are forced to relocate due to sea level rise, there are knock-on effects for biodiversity as they develop on new lands. These secondary biodiversity impacts are likely to be particularly prevalent in south-east Asia, due to the large number of people living in low-lying areas who may be forced to migrate due to sea level rise.
Small island developing states
Sea level rise poses an “acute and disproportionate” threat to small island developing states, says the recent UN report. It adds:
“Even under moderate scenarios, rising seas will render many low-lying coastal zones and small island developing states increasingly uninhabitable without extraordinary adaptation.”
Climate change is already resulting in loss and damage to small island nations, which are particularly vulnerable to both climate change in general and sea level rise specifically.
This vulnerability is in large part due to the geography of these countries. Several small island Pacific states are made up of atolls – ring-shaped coral or sandy islands that encircle lagoons. These often have average elevations of 1-2 metres above sea level and maximum elevations of 3-5 metres above sea level.
Some modelling evidence has shown that reef islands can grow vertically in response to sea level rise, as waves washing over the islands transport sediment from the ocean onto the surface. However, the strength of such waves would likely make these islands unsuitable for building on.
Other research has shown that Pacific islands respond in many different ways to rising sea levels, with “complex” outcomes, both positive and negative.
In addition, most of the small-island nations in the Pacific Ocean are located in a region where relative sea level rise from the melting of the Antarctic ice sheet is projected to be 11-33% higher than the global average rise in 2100 – regardless of emissions scenario.
The effects of this higher-than-average sea level rise is already evident.
In 1999, Kiribati lost two small, uninhabited islands to the rising seas. Several uninhabited islands in the Solomon Islands had vanished by 2014, while a further six islands had been severely eroded by the ocean, necessitating the relocation of some communities.
In addition, most small island developing states are located in parts of the ocean that are often hit by tropical cyclones. Sea level rise can enhance storm surge, leading to greater destruction during such storms.
However, small island developing states are also vulnerable “because they lack the means to address the impacts on their own”, reads the UN report.
According to the UN, these countries will require up to $6bn annually by 2035 in order to adapt to climate change. However, they received just $1.2bn in public adaptation finance in 2022-23.
Currently, small island developing states experience “expected” annual climate damages of $1.64bn due to coastal flooding, equivalent to 0.13% of their cumulative GDP. But, even if warming were limited to 1.5C above pre-industrial temperatures, these annual damages are projected to grow to $24bn.
In the international policy arena, questions have arisen over what should happen to island nations’ maritime boundaries as their land is enveloped by the sea. This is because maritime holdings, such as exclusive economic zones, are determined based on a country’s land borders.
However, a 2025 report by the UN International Law Commission considered the legal implications of sea level rise. It concluded that international law allows for countries’ borders to stay the same, “notwithstanding changes to the coastline as a result of climate change-related sea level rise”. It also noted:
“There is a need to develop legal and practical solutions to better protect persons affected by sea level rise, including those who remain in situ and those who are internally or externally displaced by it.”
Other small islands also face similar issues in their exposure to threats posed by sea level rise.
Coral reefs
Coral reefs are among the ecosystems that are most vulnerable to climate change.
They are also being visibly affected already – almost entirely due to ocean warming. Even though these ecosystems are completely submerged to begin with, they are also impacted by sea level rise.
As the ocean rises, the water over shallow ecosystems deepens.
The effects of this are twofold. Deeper water reduces the temperatures experienced by reefs. This can act as a buffer against marine heatwaves and global ocean warming.
At the same time, the increased depth reduces the amount of light that can reach the coral communities, which can impact their survival.
In addition, sea level rise-assisted erosion will add more sediment to the near-shore waters. These particles can settle on corals, impeding their ability to feed and reproduce, as well as interfering with photosynthesis by the zooxanthellae algae that live symbiotically with corals. Together, this leads to slower coral growth and increased stress on reefs.
So far, reefs in some parts of the world have been able to “keep pace” with sea level rise, growing vertically at accelerated rates and therefore maintaining suitable levels of light availability.
However, modelling has shown that few reefs have the capacity to continue to maintain their distance from the surface under a moderate-emissions scenario.
As coral reefs degrade, the seafloor below them can wear away. This erosion is contributing to greater apparent levels of sea level rise on coral reefs in the Caribbean, as well as the US states of Florida and Hawaii.
Sea level rise may also have the ability to spur reef growth in shallow environments previously thought to be uninhabitable for corals. In Sanya Bay in the northern South China Sea, sea level rise since the mid-1980s has allowed for the recolonisation of a reef that had been dormant for more than five millennia.
But the opportunities for such recolonisation are far outstripped by the loss of coral elsewhere. Since 1980, the world has lost nearly 10% of its coral cover due to climate change-induced ocean warming. The UN declaration reads:
“Every fraction of a degree of global warming increases the risks to coral reefs.”
Heritage sites
Throughout human history, many societies developed along rivers and coastlines, due to the abundance of food and ease of transportation. However, their proximity to the sea means that many of these sites are now at risk of being damaged or destroyed by sea level rise.
Cultural heritage includes “physical sites, living heritage, traditional lands, burial grounds, underwater cultural heritage, archaeological and sacred sites and culturally significant coastal landscapes”, according to the UN sea level rise report.
Several studies have mapped the cultural and natural heritage sites that are most at risk from flooding and erosion due to sea level rise.
There are 49 Unesco world heritage sites located at low elevations along the coast of the Mediterranean Sea. Nearly every one of these is already at risk from erosion or severe flooding events – 42 face issues with erosion, while 37 are at risk from a 100-year flood event. Both of these risks will increase over the remainder of the century as sea levels continue to rise.
The locations at risk include the archaeological sites of the ancient cities of Carthage in present-day Tunisia and Ephesus in Turkey, the ruins of Pompeii and Herculaneum in Italy and the medieval Cathedral of St James in Šibenik, Croatia.
The sea level rise associated with warming of 3C above pre-industrial temperatures would impact nearly one-fifth of all Unesco cultural world heritage sites. The sites at risk include Japan’s Hiroshima Peace Memorial, South Africa’s Robben Island, Chile’s Rapa Nui and the Sydney Opera House. Many of these become vulnerable at lower levels of global warming.
In Africa, 56 out of 284 cultural and natural heritage sites already face threats from flooding or erosion due to sea level rise. This number is expected to nearly triple – to 191 threatened sites – by 2050 under a moderate-emissions scenario. However, mitigating emissions could reduce the number of very-highly exposed sites – those with at least 75% of their area vulnerable – by one-quarter.
Globally, there are 386 Unesco heritage sites along the coast that are, at most, 20 metres above sea level and are therefore potentially affected by coastal erosion and flood hazards.
These threatened sites include 289 cultural heritage sites and 91 natural heritage sites, as well as six “mixed” sites that are recognised for both their cultural and natural significance.
The UN declaration calls for action to mitigate damage to significant sites, saying:
“Protection, preservation and documentation of cultural heritage is a priority.”
The post Explainer: How sea level rise poses an ‘existential threat’ to humans, heritage and nature appeared first on Carbon Brief.
Source: Explainer: How sea level rise poses an ‘existential threat’ to humans, heritage and nature