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Sea Level Rise: What is Happening?
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“ Sea level rise might not feel as immediate as a wildfire or as dramatic as a hurricane, but make no mistake: it's one of the most far-reaching and irreversible consequences of climate change. And it's already happening. From saltwater creeping into freshwater farms in Bangladesh to billion-dollar flood defences being built in cities like Miami and Rotterdam, the world is waking up to the quiet force reshaping coastlines and displacing communities. ”
At its core, sea level rise is simple physics - warmer temperatures cause ice to melt and oceans to expand. But the ripple effects are anything but simple. As the water rises, so do the costs: economic, ecological, and human.
Despite decades of warnings, global emissions continue to climb. That means rising seas are no longer a distant threat for future generations; they're a present-day challenge for governments, businesses, and communities alike.
In this article, we’ll explore what’s driving sea level rise, where the impacts are already being felt, and what can still be done to avoid the worst-case scenarios.
What is sea level rise?
“ Sea level rise is the long-term increase in the average height of the world’s oceans, caused primarily by climate change. As global temperatures rise, glaciers and ice sheets melt, and warmer water expands, adding more volume to the sea - a trend confirmed by decades of sea level data that shows coastlines steadily moving inland. ”
Although sea level rise is a global phenomenon, it doesn’t happen uniformly. Regional sea level rise varies due to local factors like vertical land movements, land subsidence, regional ocean currents, and gravitational changes caused by melting ice sheets. That’s why scientists distinguish between global mean sea level rise (the overall average) and local sea level or relative sea level rise (the change experienced at a specific location).
“ To monitor these shifts, researchers combine over a century’s worth of tide gauge data with modern satellite data. The results are clear: global average sea levels have risen by around 21 to 24 centimetres since 1900, and the rate is accelerating. In the early 20th century, the rate of sea level rise was about 1.4 mm per year. Today, it’s more than 3.6 mm annually and continuing to speed up. ”
What makes global sea level rise especially concerning is its permanence. Unlike some other climate impacts, it can’t simply be reversed if we reduce emissions. Every millimetre added is here to stay, reshaping coastlines, ecosystems, and lives.
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What causes sea level rise?
“ Sea level rise is a direct consequence of a warming world. As global temperatures climb, ice melts and oceans respond, both by gaining more water and by physically expanding. These changes are happening faster than at any point in recorded history, and they’re largely driven by human activity. ”
While the mechanics of sea level rise are complex, the main drivers fall into three categories: melting glaciers and ice sheets, the thermal expansion of seawater, and a range of other human impacts that alter how water is stored on land.
Melting glaciers and ice sheets
Ice melting on land is the single largest contributor to long-term sea level rise, and it’s accelerating. This includes both small mountain glaciers and massive continental ice sheets. When this ice melts, the runoff flows into rivers and eventually reaches the ocean, raising sea levels in the process.
“ In fact, since the last ice age, melting glaciers and ice sheets have caused sea levels to rise by more than 120 metres - a reminder of how sensitive ocean levels are to changes in global temperature. ”
Mountain glaciers across the world are retreating, from the Alps and the Andes to the Himalayas and Alaska. In many places, these glaciers are now past the point of recovery.
“ Recent research suggests that if current warming trends continue, up to two-thirds of the world’s glaciers could vanish by 2100. This loss would not only contribute to rising seas but also threaten the freshwater supplies that millions of people depend on. ”
But the biggest concern lies with the major ice sheets in the polar regions:
Greenland: Ice loss has more than quadrupled over the past few decades. The island is now losing around 250 billion tonnes of ice per year, and this trend is expected to continue even under moderate warming scenarios. Much of the melting occurs at the edges, where warmer ocean water is undercutting glaciers and accelerating their retreat.
Antarctica: The West Antarctic Ice Sheet is particularly vulnerable. Some of its glaciers, like Thwaites (nicknamed the “Doomsday Glacier”), are grounded below sea level and held back by relatively fragile ice shelves. If these shelves collapse, as some already have, it could trigger a chain reaction of retreat, leading to several metres of sea level rise over the coming centuries.
The concern isn’t just how much ice is being lost, it’s how quickly. Once these systems cross certain thresholds, they may collapse irreversibly, committing the planet to centuries of rising seas even if we eventually reduce emissions.
Thermal expansion of seawater
“ Oceans absorb more than 90% of the excess heat trapped by greenhouse gases. As seawater warms, it expands, a physical process known as thermal expansion. And while this may sound minor, the scale of the ocean makes it a major driver of sea level rise. In fact, thermal expansion has contributed to nearly half of the observed sea level rise since the early 20th century. ”
This process is especially significant because it responds directly to atmospheric warming. As global temperatures climb, so do ocean temperatures, from the sea surface all the way down to deeper layers over time. The expansion that results may be invisible day-to-day, but when measured across vast volumes of ocean water, the effect is measurable and lasting.
One of the challenges with thermal expansion is that it doesn’t stop the moment emissions fall. Even if all ice melt were halted tomorrow, the oceans would continue to absorb heat and expand for decades, possibly even centuries, due to the momentum already built up in the climate system. That makes thermal expansion a kind of built-in, long-term commitment to sea level rise, even under the most optimistic scenarios.
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Other human impacts
“ While climate change is the dominant driver of sea level rise, other human activities are also contributing, often in ways that receive far less attention. ”
One of the most significant is groundwater extraction. Around the world, we pump massive amounts of water from underground aquifers to support agriculture, industry, and drinking water supplies. Once extracted and used, much of this water eventually makes its way into rivers and oceans, effectively transferring stored groundwater into the sea and raising ocean levels over time.
In some regions, this practice has an additional effect: land subsidence. As underground water reserves are depleted, the ground above can sink, contributing to sea level variability and making localised sea level rise appear even more extreme. This is a growing concern in many densely populated coastal cities, including Jakarta, Bangkok, and parts of California, where sinking land is amplifying the impacts of rising seas.
Other land use changes, like deforestation and the draining of wetlands, also influence how much water the land can retain. In some cases, water that would have been absorbed or stored on land now flows more quickly into the sea.
“ While these drivers may seem minor compared to ice melt or thermal expansion, they compound existing risks, especially in urbanised or heavily managed landscapes. And like other sources of sea level rise, their effects are long-lasting and difficult to reverse. ”
Consequences of sea level rise
The impacts of sea level rise aren't limited to distant islands or future generations. From chronic coastal flooding to saltwater intrusion and ecosystem collapse, the effects are wide-ranging, unevenly distributed, and deeply disruptive. As the seas continue to rise, so too does the pressure on communities, infrastructure, economies, and natural systems.
🌊 Coastal flooding and erosion
Even small increases in sea level can significantly raise the risk of coastal flooding impacts, particularly during high tides and storm surges. Rising seas threaten infrastructure and homes, while also removing natural buffers that once protected communities from storms and rising waters.
Low-lying cities like Miami, New York, Jakarta, and Bangkok already experience regular high tide flooding — sometimes called “sunny day” floods — when high tides spill into streets without any storm at all.
Higher seas also accelerate coastal erosion, washing away beaches, wetlands, and protective landforms like dunes. This not only threatens homes and infrastructure, but also removes natural barriers that reduce storm damage.
⛈️ Storm surges and extreme weather
Rising sea levels make storm surges more dangerous. When hurricanes or cyclones make landfall, higher ocean baselines mean more water is pushed inland.
This compounds the destruction from high winds and heavy rain, as seen in disasters like Hurricane Sandy and Typhoon Haiyan, both of which were made more severe by elevated sea levels.
As the climate continues to change, extreme weather events are expected to become more frequent and intense — creating a dangerous feedback loop where rising seas and stronger storms reinforce each other.
💧 Saltwater intrusion and freshwater loss
As seawater pushes further inland, it can infiltrate aquifers, rivers, and farmlands — contaminating vital freshwater supplies. This process, known as saltwater intrusion, is already affecting agricultural regions in countries like Bangladesh, Vietnam, and parts of the southeastern United States.
Once soil and groundwater are salinised, they become far less productive, threatening food security and water access for millions of people.
💸 Economic disruption
Sea level rise carries a high economic price tag. Coastal cities face rising costs for infrastructure repair, flood defences, and emergency response. Insurance premiums are increasing — or disappearing altogether — in high-risk areas.
In some cases, governments are already buying out homeowners in flood-prone zones.
The World Bank estimates that without adaptation, climate-related flooding in coastal cities could cost $1 trillion per year by 2050. And that doesn’t even account for losses in tourism, real estate, agriculture, or fisheries.
🐚 Ecosystem loss and biodiversity decline
Rising seas also threaten coastal ecosystems — particularly wetlands, mangroves, coral reefs, and estuaries. These habitats support rich biodiversity and play a vital role in carbon storage, storm protection, and water purification.
As seawater rises, many of these systems are being drowned or squeezed out. In some cases, inland migration is blocked by human development — leaving ecosystems with nowhere to go. Species that rely on these habitats for breeding or food face a heightened risk of collapse.
🏚️ Displacement and climate migration
In the most vulnerable regions, rising seas are already forcing people to leave their homes. Entire communities in the Pacific Islands, the Bay of Bengal, and parts of the U.S. Gulf Coast have faced relocation due to repeated flooding, land loss, or freshwater scarcity.
According to the Internal Displacement Monitoring Centre, climate-related disasters displaced over 9.8 million people in 2024 — many from low-lying coastal areas. As sea levels continue to rise, this figure is likely to grow, raising complex questions about equity, global responsibility, and the rights of climate migrants.
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How much will sea levels rise?
The extent of sea level change will depend largely on how quickly we cut greenhouse gas emissions.
“ According to sea level projection data from the Intergovernmental Panel on Climate Change (IPCC), sea levels are likely to rise by 28 to 55 centimetres by 2100 under a low-emissions scenario, and up to 1 metre or more if emissions continue at their current pace. ”
But that’s just the beginning. Because oceans respond slowly to warming, sea level trends suggest that oceans will keep rising for hundreds of years, even if the global mean temperature stabilises. The more we emit now, the more we “lock in” future sea level rise that will unfold long after this century ends.
Tipping points and long-term risks
One of the biggest concerns is the potential for ice sheet tipping points, thresholds beyond which ice loss becomes irreversible. If parts of the West Antarctic or Greenland Ice Sheets collapse, it could trigger several metres of sea level rise over the next few centuries. These shifts would reshape global coastlines, submerge entire island nations, and render many major cities uninhabitable without extensive defences or retreat.
While scientists are still working to understand exactly when, or if, these tipping points might be crossed, the risk increases the longer global temperatures remain high.
The bottom line
“ Even under the most optimistic scenarios, some sea level rise is now unavoidable. What we can still influence is how much, how fast, and how well we prepare. ”
Whether it’s half a metre or two metres by the end of the century, the implications are enormous. Infrastructure will need to be rethought. Coastal populations will need protection or relocation. And governments will need to plan not just for the next decade, but for the next century and beyond.
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How are we responding?
Sea level rise may be inevitable, but its impacts don’t have to be catastrophic. Around the world, governments, cities, and communities are beginning to respond. Some are investing in hard infrastructure to hold back the sea. Others are rethinking land use, restoring natural defences, or preparing for managed retreat. These responses generally fall into two categories: adaptation and mitigation.
Adapting to higher seas
Adaptation is about learning to live with rising water, reducing the damage it can cause, and protecting people and places at risk.
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Coastal defences
Countries are building or upgrading sea walls, flood gates, and storm surge barriers. The Netherlands is a global leader in this space, with decades of engineering experience keeping the sea at bay.
🌿
Nature-based solutions
Regions are restoring mangroves, salt marshes, and wetlands to act as natural buffers. These ecosystems absorb wave energy, reduce erosion, and support biodiversity.
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Urban planning
Cities like New York and Rotterdam are raising infrastructure, improving upstream flood control, and redesigning waterfronts with climate resilience in mind.
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Managed retreat
In some areas, relocation is already happening. Communities in the Pacific Islands are moving away from coastlines as the land disappears.
Reducing emissions to limit future rise
Adaptation can reduce short-term damage, but the only way to prevent runaway sea level rise is to cut emissions at the source.
🌡️
Limiting global warming to 1.5°C, as set out in the Paris Agreement, could avoid several metres of sea level rise over the long term.
🔋
Transitioning to low-carbon energy, protecting carbon sinks, and transforming food and transport systems are key to slowing the pace of sea level rise.
🛑
Long-term, deep decarbonisation is the only way to prevent sea level rise from spiralling beyond our ability to manage it.
Who can afford to act?
One of the most pressing challenges is that adaptation isn’t evenly distributed. Wealthier countries and cities may have the means to build expensive defences, but many vulnerable nations and communities do not.
“ Island states, low-income coastal populations, and informal settlements often lack access to funding, planning resources, or political support. For them, international climate finance and capacity-building are critical to ensure that adaptation is not just possible, but fair. ”
Rising seas are a global issue, but our ability to respond is shaped by politics, geography, and inequality. Closing that gap, while pushing for stronger global climate action, will be key to managing what comes next.
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How can Greenly help?
At Greenly, we help companies take climate action into their own hands. Our suite of carbon management services is designed to help businesses track, analyse, and minimise their emissions, making sustainability both actionable and achievable.
Here’s how we support organisations on their climate journey:
- Carbon accounting: We calculate your full Scope 1, 2, and 3 emissions using a robust, standards-aligned methodology.
- Customised action plans: We identify the most effective ways for your business to reduce its carbon footprint with tailored solutions for your sector.
- Supplier engagement: We help you make your supply chain more sustainable, including access to vetted low-carbon suppliers.
- Compliance support: We guide you through reporting frameworks like the CSRD, SECR, and more, ensuring your disclosures are accurate and up to date.
- Science-Based Targets: We assist with setting and validating emissions reduction goals aligned with the latest climate science.
Whether you’re just getting started or already leading in sustainability, Greenly gives you the tools and expertise to go further.
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