Explained | The threats of surging seas

• UN Secretary-General Antonio Guterres issued a “global SOS” from Tonga in the Pacific, urging governments to step up climate action to “Save Our Seas” as two new reports revealed how rising sea levels are threatening the vulnerable region and beyond. 

• Guterres has been visiting the Pacific Ocean nations, Tonga and Samoa, where sea level rise has been one of the key issues he has been discussing with the communities he has met.

• The UN chief said global average sea levels are rising at rates unprecedented in the past 3,000 years.

• While global-mean sea level has risen over 10 centimetres since 1993, the situation is even worse in the Pacific, where some locations exceed 15 centimetres.

• Hazards related to sea-level rise, such as land erosion, flooding and freshwater salinisation, have implications for water access, food security and health.

• The study by the World Meteorological Organisation (WMO) on the State of the Climate in the South West Pacific, alongside a report by the UN Climate Action Team on Surging seas in a warming world, shows that changes to the ocean are accelerating, with devastating impacts.

• Together, they outline how monthly sea temperatures continue to shatter records. 

• Without new adaptation and protection measures, the economic damage could amount to trillions of dollars, the UN chief said.

Acceleration in sea-level rising

• Global-mean sea level is rising and accelerating as a direct consequence of human-induced global warming. 

• This rise in sea level is driven primarily by two factors: the melting of land ice and the expansion of seawater as it warms.

• Their combined effects have led to mean sea-level rising across most of the world’s oceans.

• Land ice — glaciers and ice sheets — contain a large portion of the Earth’s freshwater. As temperatures rise, especially in the polar regions, glaciers and ice sheets are melting, adding water to the oceans. 

• At the same time, the ocean has absorbed more than 90 per cent of the excess heat that has accumulated in the Earth system since 1971 due to rising greenhouse-gas emissions. As this heat is absorbed, ocean temperatures increase and water expands, leading to SLR.

• Research on climate ‘tipping points’ and ice-sheet dynamics is raising alarm among scientists that future sea-level rise (SLR) could be much larger and occur sooner than previously thought.

• 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.

• The average rate of SLR was 0.13cm [0.06–0.21cm] per year between 1901–1971, increasing to 0.19cm [0.08–0.29cm] per year between 1971–2006, and further increasing to 0.37cm [0.32–0.42cm] per year between 2006–2018.

• According to the World Meteorological Organisation (WMO), the rate of SLR in the past ten years has more than doubled since the first decade of the satellite record, increasing from 0.21cm per year between 1993–2002 to 0.48cm per year between 2014–2023.

• The recent acceleration in SLR is primarily due to increasing rates of ice loss from the Greenland and Antarctic ice sheets, which are losing ice mass at average rates of around 270 and 150 billion tonnes per year, respectively. The seven worst years of ice loss on record all occurred in the last decade.

• Ocean-warming rates have shown a particularly strong increase in the past two decades. In 2023, sea-surface temperatures and ocean-heat content reached their highest levels in the observational records.

• It is expected that the upper 2,000 meters of the ocean will continue to warm due to excess heat that has accumulated in the Earth system from global warming.

Global impacts of sea-level rise

• Much of the world’s population, economic activities, and cultural world heritage sites are concentrated near the sea.

• Accelerated SLR has the potential to redefine the coastlines of the 21st century. It can pose major risks to the safety, security, and sustainability of many low-lying islands, populous coastal megacities, large tropical

agricultural deltas, and Arctic communities.

• The low-elevation coastal zone (LECZ), which comprises continental and island areas connected to the sea no more than 10 meters above mean sea level, includes a wide diversity of systems, from small islands to megacities, from the Tropics to the Poles, in both the Global North and Global South. 

• The LECZ generates around 14 per cent of global GDP and hosts almost 11 per cent of the global population (around 900 million people) — a figure projected to increase beyond 1 billion by 2050.

• SLR is a major threat for countries with large coastal populations, especially in developing countries. For Small Island Developing States (SIDS), SLR erodes land, destroys infrastructure, disrupts lives and livelihoods, and threatens habitability due to the concentration of people, assets, and infrastructure in the LECZ. 

• Relative SLR also threatens dozens of coastal megacities on all continents, including, but not limited to, Bangkok, Buenos Aires, Dhaka, Guangzhou, Jakarta, Lagos, London, Los Angeles, Miami, Mumbai, New Orleans, New York City, Rio de Janeiro, Shanghai, and Tokyo.

• By 2050, the majority are projected to experience additional SLR of more than 15cm, with six cities (Atlantic City, Boston, New Orleans, New York City, Osaka, and Shanghai) potentially seeing 24–41cm.

• SLR poses an alarming threat to human life and socio-economic livelihoods and the implications for adaptation and loss and damage are profound and far-reaching.

• Climate-driven coastal hazards and risks come not only from SLR itself but also from its amplification of storm surges, tides, and waves. Coastal-flood hazards and associated risks are also expected to increase as a result of local land sinking (subsidence) because of human activities such as building dams or groundwater and fossil fuel extraction.

• Their combined effects can lead to infrastructure damage due to coastal flooding, saltwater intrusion into groundwater and rivers, shoreline retreat, and change to or loss of coastal ecosystems and economic sectors.

• Such impacts are already or are likely to create risks to livelihoods, settlements, health and well-being, and food and water security. Impacts can also reach far beyond coastal communities. For example, climate-induced, involuntary displacement and migration from coastal areas may lead to population movements to inland areas, while loss of economic activities such as fisheries or agriculture and damage to ports can severely compromise global food systems, supply chains, and maritime trade, with local-to-global geopolitical, economic, and security ramifications.

• Small rises in relative sea-level can disproportionately increase coastal flood frequency. The frequency of present-day, extreme-butrare sea-level events is projected to increase substantially in most regions.

• SLR can hamper the ability of coastal communities to adapt to climate impacts through its destruction of natural coastal defences and ecosystems. Mangroves, corals, saltmarshes, and seagrass meadows currently protect hundreds of millions of people worldwide against storm surges and waves. A 1m loss in coral reef height is projected to more than double the global area flooded during a 100-year event in 2100.

• Tourism revenues, a key economic driver especially in many nations, can suffer as beaches, resorts and other tourist attractions like coral reefs are damaged.