Sea level rise: Everything you need to know

This article was first published in September 2022 and most recently updated in September 2024.

“Since the start of the 20th century, global-mean sea level has risen faster than over any prior century in at least the last 3,000 years, and the rate of increase is accelerating,” warns ‘Surging Seas in Warming World’ - a new United Nations (UN) report on the current and future impacts of sea level rise.

Homes, livelihoods and, ultimately, lives are under threat from rising sea levels.

Here’s what you need to know.

Sea level rise refers to the increase in the average height of the ocean’s surface, measured from the center of the Earth. This phenomenon is primarily driven by two main factors: the melting of glaciers and ice sheets, and the thermal expansion of seawater as it warms. As global temperatures rise due to climate change, ice sheets in Greenland and Antarctica are melting at an accelerated rate, contributing significantly to sea level rise. Additionally, as seawater warms, it expands, further increasing the sea level. This rise in sea level is a critical indicator of climate change, with far-reaching impacts on coastal communities, ecosystems, and economies worldwide.

For the first time, the UN General Assembly’s High-Level Week 2024 will feature a dedicated meeting on Sea-Level Rise on September 25, encompassing four multi-stakeholder thematic panel discussions. Collectively, they will address the legal, financial, socio-economic and scientific aspects of sea-level rise, covering its impacts on livelihoods, adaptation strategies and decision-making processes. The meeting will also address how global climate change contributes to local sea level rise, affecting different regions in unique ways.

Climate change, nature and the energy transition will also be under the spotlight at the World Economic Forum’s annual Sustainable Development Impact Meetings (SDIM24) in New York 23 to 27, which coincide with UNGA’s General Debate.

Uniting leaders from politics, business and civil society, SDIM aims to drive innovation on the UN Sustainable Development Goals, focusing on climate action, digital literacy, and economic inclusion. Only 17% of the SDGs are on track, underscoring the urgent need for accelerated progress.

Now satellites carry out this task by bouncing radar signals off the sea surface to measure changes in sea level. The Forum’s work on Amplifying the Global Value of Earth Observation highlights monitoring changing sea levels as a key application of the technology to support vulnerability analysis.

Because local weather conditions and other factors can affect sea level, measurements are taken globally and then averaged out.

In 2021, scientists discovered a sea-level "fingerprint" from the Greenland ice sheet, confirming fears about the extent to which ice is melting.

These fingerprints are "detectable patterns of sea level variability around the world resulting from changes in water storage on Earth’s continents and in the mass of ice sheets", according to NASA.

The Greenland ice sheet is now losing around 30 million tonnes of ice an hour, reports The Guardian, and a recent study predicts its total collapse could happen by 2025.

With the ice sheet at “a tipping point of irreversible melting”, scientists currently expect an unavoidable sea level rise of 1-2 metres.

Global sea levels have already risen by over 10cm between 1993 and 2024, according to NASA, which says sea levels have been rising at unprecedented rates over the past 2,500 years.

While global sea levels have risen by over 10cm between 1993 and 2024, relative sea level rise can vary significantly depending on local factors such as land subsidence and ocean currents.

The global sea level has risen by about 21cm since records began in 1880. While measuring in centimetres or even millimetres might seem small, these rises can have big consequences. This is particularly true where storm surges sweep further inland than they would have previously.

Two main factors cause sea level rise. These are melting ice from glaciers, and seawater expanding because of rises in global temperatures, explains the US National Oceanic and Atmospheric Administration.

Greenland and Antarctica are losing around 270 billion and 150 billion tonnes of ice a year,

There are also likely negative feedback loops that could speed up glacier ice melt. For example, the Thwaites Glacier in Antarctica is disintegrating more quickly than anticipated. It’s nicknamed the ‘doomsday glacier’ because sea levels could rise more than three metres without it and its supporting ice shelves.

Heat stored in the ocean is responsible for between a third and half of global sea level rise, NASA says. The past decade has been the ocean’s warmest since at least 1800, and ocean temperatures reached a new high in 2023.

As sea levels rise, the frequency of high tide flooding, also known as nuisance flooding, is expected to increase, impacting coastal infrastructure and ecosystems.

Since 1971, oceans have absorbed over 90% of excess heat in the Earth system caused by rising greenhouse gas emissions, the UN’s Surging Seas report reveals.

Climate change is the primary driver of global sea level rise.

As the Earth’s temperature increases, the polar ice caps and glaciers melt, releasing vast amounts of water into the oceans. This melting ice contributes directly to the rising sea levels.

Furthermore, the warming of the ocean causes the water to expand, a process known as thermal expansion, which also contributes to sea level rise.

The relationship between sea level rise and climate change is complex and multifaceted. Understanding this connection is crucial for predicting future sea level rise scenarios and developing strategies to mitigate its impacts.

As global warming continues, the rate of sea level rise is expected to accelerate, posing significant challenges for coastal regions around the world.

The effects of global sea level rise are profound and multifaceted, impacting both human and natural systems. Rising seas threaten infrastructure, including roads, bridges, and buildings, leading to increased costs for maintenance and repair.

Coastal flooding becomes more frequent and severe, exacerbating erosion and causing saltwater intrusion into freshwater sources, which can compromise drinking water supplies and agricultural productivity.

Additionally, sea level rise poses a significant threat to coastal ecosystems such as mangroves, coral reefs, and salt marshes, which provide critical habitat for numerous species.

The displacement of people living in low-lying areas due to rising seas can lead to social and economic challenges, including loss of property, livelihoods, and increased pressure on social services.

Addressing these impacts requires comprehensive adaptation strategies to protect vulnerable communities and ecosystems.

The economic and social impacts of sea level rise are significant and far-reaching. Rising seas can lead to increased costs for coastal protection measures, such as building sea walls and surge barriers, and repairing damage to infrastructure.

The loss of property and livelihoods due to coastal flooding and erosion can have devastating effects on communities, particularly in vulnerable regions. Additionally, sea level rise can exacerbate social and economic challenges by displacing people, disrupting economic activities, and straining social services.

For example, communities that rely on tourism, fishing, and agriculture may face significant economic losses as rising seas threaten their way of life.

Understanding these economic and social impacts is critical for developing effective adaptation strategies and mitigating the effects of sea level rise on vulnerable populations.

Throughout Earth’s history, sea levels have fluctuated significantly, with major changes occurring during the last ice age and the subsequent warming period. However, the current rate of sea level rise is unprecedented.

Since 1900, the global average sea level has risen by approximately 15-20 cm, a rate much faster than historical averages. This rapid increase is largely attributed to human-induced climate change, driven by the burning of fossil fuels and the resulting increase in greenhouse gas emissions.

Understanding past sea level rise and its historical context is essential for predicting future changes and developing effective adaptation strategies.

By studying historical data, scientists can better understand the natural variability of sea levels and the extent to which current trends are influenced by human activities.

Bangladesh, China, India and the Netherlands were singled out by the UN in 2023 as being at high risk from rising sea levels, with nearly 900 million people living in low-lying coastal areas in acute danger.

In its Surging Seas report, the organization highlights the dangers facing the communities of the Pacific Small Island Developing States: "The Pacific SIDS, especially those in the western tropical Pacific (e.g., Kiribati, Tuvalu, and the Republic of the Marshall Islands), are particularly vulnerable to SLR [sea level rise] because of: (i) high exposure to tropical cyclones and other tropical storms; (ii) high shoreline-to-land area ratios; (iii) high sensitivity to changes in sea level, waves, and currents; and (iv) its many low-lying coral atolls or volcanically-composed islands."

According to Reuters, sea levels around Tonga are rising at almost twice the global average rate.

In Europe, sea level rise is expected to go above 10cm "prior to 2050", says the European Environment Agency.

While recent research in the US has found that almost 1,100 critical buildings in coastal communities could be at risk of monthly flooding by 2050. Some communities could become unliveable within two to three decades, the report says.

In its Global Risks Report 2024, the World Economic Forum added the category 'Critical change to Earth systems' as one of the top two threats to the world in the coming decade - and sea level rise from collapsing ice sheets is identified as a key contributing factor.

Adaptation is vital, it says, but "efforts are falling short", with a finance gap currently estimated at $194 billion to $366 billion a year.

Countries and cities around the world are nevertheless putting strategies into action. In New Zealand, climate adaptation policies are being designed to ensure public housing is not built near areas prone to climate hazards.

Sea walls, surge barriers and other coastal defences are being built and strengthened in several countries including Denmark, Germany and the United Kingdom.

South Korea and the islands of the Maldives in the Indian Ocean are experimenting with floating homes, while China, India and other nations are finding ways to absorb and store storm water for reuse.

More drastic action is taking place in Fiji, where government officials are making plans to relocate whole villages because of rising sea levels - 42 villages have been recommended for relocation in the next five to 10 years, while six have already been moved to safer ground, The Guardian reports.