Our rising Oceans

In what must rate as one of the worst forseeable disasters ever to confront humanity, the remorseless rise in the oceans will menace the homes and livelihoods of a billion people before the end of the century.

From 1900 to 2018, global sea levels rose by about 20cm (a long-term average of +1.7mm/yr), but measurements since 1993 (when global satellite data first became available) show the rate of global mean sea-level rise then doubled to +3.3 mm/yr - and has since accelerated to +4.4 mm/yr.

However world sea levels do not rise uniformly like water in a bathtub. In a recent study of worst case scenarios, scientists warned that some coastal communities may face total rises as high as 9 or 10 metres by 2100.

“By 2100 almost 45% of the global coastline would experience sea level increasess above the global mean of 4.2 m, with up to 9–10 m for the East China Sea, Japan and North European coastal areas.

“Up to 86% of coastal locations would face sea level rises above 3 m by 2100, compared to 33% currently,” they said.

Figure 1. Global sea level rise 1993-2024. Source: NASA

Sea level rise was listed in the Global Risks Report 2025 as one of the critical changes to the Earth System that together constitute the third greatest threat to humanity in the coming decade. The United Nations (UN) has rated it “a global crisis” that is already impacting around 1 billion people worldwide.

Around one third of global sea level rise is due to thermal expansion as ocean water heats and expands and roughly two thirds is due to faster melting of land-based ice, such as glaciers and ice sheets. The ocean is currently absorbing more than 90 percent of the increased atmospheric heat caused by human activity, turning much of this directly into rise.

Human activity also affects sea level rise by pumping out groundwater, which is released into rivers, adding to sea level rise while at the same time causing coastal cities to sink. Over the period 1993-2024, we pumped out an estimated 2.15 billion tonnes of groundwater a year, 80% of which ended up in the oceans. This has the compound effect of causing many cities to sink while seas rise.

Megacities which are physically sinking while oceans grow include major centres such as: Jakarta (pop 12m), Manila (16m), Dhaka (25m), Mumbai (23m), Bangkok (11m), Ho Chi Minh (11m), Tianjin (14m), Shanghai (31m) Yangon (7m), Istanbul (16m), Houston (2.3m) and Lagos (18m).

Dam building has a transient opposite effect by holding back freshwater from reaching the sea, chiefly in the dam-filling phase.

Just as the surface of the Earth is uneven, the surface of the ocean is also not level, being higher or lower in various places —in other words, sea levels do not change at the same rate globally. This means sea level rise is greater in some areas than others. Sea heights are also affected by transient climatic impacts such as El Nino. Furthermore, sea levels, measured at the shore, also depend on whether the landmass itself is rising or sinking due to plate tectonics. This results in a very uneven and complex picture of sea level rise globally, as the following map indicates:

Figure 2. Sea levels are rising faster in some places than others due to a combination of effects, both natural and man-made. Source: NOAA

In areas coated with ice, the melting of glaciers and ice caps causes the land surface to rebound upwards when relieved of the weight of frozen water. In these areas (blue down arrows) the sea level appears to recede as the land rises higher, though in fact the land locally is merely rising faster than the sea.

One of the most frightening, unknowable factors, in the equation is that sea level rise can change by metres in a matter of decades. It is well documented that, after the huge rise (120-140m+) at the end of the last Ice Age, the oceans actually reached a height 4 or 5 metres above where they are today, then settled back in a matter of centuries to their present trend.

However, British archaeologists, excavating and carefully dating several buildings from post-Roman (Dark Age) London, found evidence for a 4 metre sea level rise occurring over just 70 years, between AD 430 and 500. This indicates that sea levels can rise (and fall), unexpectedly and far more sharply than predicted by trends, over relatively short spans of time. The cause of such a sea surge might be something like the sudden collapse of an ice cliff holding back a major glacier. This is what scientists now fear in the cases of the Thwaites Glacier and West Antarctic Ice Sheet. Or it could be due to a major isostatic rebound caused by a melting glacier.

Currently at least 271 of the world’s 530 large (1m+) cities are located on coastlines, and around 15 per cent of the human population (1.25 billion people) lives within 15kms of the coast. Rather than fleeing the danger, humans – like lemmings – are in fact moving coastwards in ever-growing numbers.

The impacts of sea level rise are well understood, and many coastal authorities are taking steps to minimise them, whereas most national governments are taking too little action, too late. The impacts include:

• Increased height of storm surges and flooding during hurricanes, typhoons and storms

• Increased frequency of flooding in urban areas, affecting more homes and businesses, sewage systems etc

• Reduced coastal food production due to salination of land, flooding and salt intrusion into groundwater

• Greater damage to roads, bridges, buildings and other infrastructure, along with the refusal of insurance coverage in affected areas and higher taxes.

• Loss of mangroves, coral reefs and coastal wetlands which shield the land from storm damage

• Damage to homes, jobs and employment in coastal regions. Higher taxes to cover the cost of repair and prevention.

• Coastal erosion of beaches, silting of waterways, major changes in fish populations and other marine wildlife.

• Health risks from flooding of lowlying areas due to dirty water, more mosquitoes etc.

Predicting sea level rise is one of the hardest and most complex of scientific tasks – and disturbing new research indicates that it may have been significantly underestimated by the models it is based on. This would put 31–37% more land and 48–68% more people at risk than currently thought, say authors Katharina Seeger and Philip Minderhoud.

Meanwhile, Canute-like, Donald Trump continues to turn a blind eye and a deaf ear to the problem, despite the fact that his own residence, Mar-a-Lago, will be flooded for two thirds of the year by the 2040s – and under 4 metres of sea by the end of the Century. Trump’s reaction was to shut down scientific research that exposes such unpalatable facts and cynically claim that sea level rise meant “more oceanfront property”.

Sea level rise is not preventable in the short term, as this would require cooling the whole planet to post-Ice Age temperatures. However, the worst impacts can be reduced with sound local planning and actions.

It is another test of whether our civilization is fit to survive or not.