Sea levels are rising – so what?

If only that were the half of it. An introduction.

While the cause of the climate crisis occurs in the air, its effects and associated heat impact the surface of the Earth.

Drought, heatwaves, wildfires - three of the four classical elements have been the focus of discussion for years. The fourth element, water, is addressed in only one respect: rising sea levels. At the poles, ice sheets are melting, filling the basin with additional water. Sea levels could rise by several metres along the coasts of islands and continents, which (rightfully) causes concern.

Regrettably, this represents only one aspect of the "climate crisis and oceans" issue. The remaining consequences are no less severe, alarming and profound.

We wish to elaborate further on this topic here.

For completeness: The mechanisms behind sea-level rise

The underlying principle appears simple at first. Consider a cold drink: ice cubes float at the top to keep the liquid cool. Initially, these are not strictly necessary, as the beverage could simply be chilled and served cold. However, the average consumer does not drink it all at once – they leave it standing in ambient air or hold it in their hand. Instantly, the second law of thermodynamics applies: temperature equalises from the warmer to the colder area. The drink warms up.

The ice cube keeps the drink cold because energy is first required to warm the ice and subsequently melt it. This thermal energy is drawn from the beverage and the surroundings. A similar process occurs when ice melts in nature. Thermal energy is absorbed for the phase transition from ice to water. Melting sea ice therefore cools the ocean – and it melts when water and air temperatures exceed those of the ice.

At this point, a clear distinction must be made: there are ice masses located on land (ice sheets and glaciers) and ice masses floating on water (sea ice).

On Earth, there are two massive accumulations of land-based ice: Antarctica (the landmass around the Earth's South Pole, inhabited by penguins, among others) containing nearly 90%, and Greenland containing 10% – the remaining fraction of less than one percent comprises all glaciers combined (0.6%). Together, these currently cover approximately 15 million square kilometres of land surface.

On an annual average, sea ice covers one and a half times that area: about 22.5 million square kilometres of floating ice exists in roughly equal proportions in the Arctic region (the area around the Earth's North Pole, inhabited by polar bears, among others) and surrounding Antarctica. In both regions, these masses fluctuate significantly throughout the annual cycle – by 16 million square kilometres between summer and winter in the south, and by 10 million square kilometres in the north. These areas float entirely on seawater.

Ice-covered land surfaces are melted primarily by warming air – the meltwater then flows into the nearest sea. It arrives there at a relatively cold temperature, meaning these former ice masses do not significantly contribute to ocean warming, but actually counteract it slightly. However, these volumes of water increase the total volume of the ocean, analogous to pouring more cold liquid into a glass.

The situation differs with melting, floating sea ice: just as a melting ice cube does not overflow a glass (which usually goes unnoticed, as it is typically consumed beforehand), melting icebergs do not raise sea levels. Sea-level rise is driven exclusively by the melting of land-based ice.

In principle, the seasonal mass differences between the northern and southern polar regions balance each other out over the year – when more water melts in the northern summer (March to September), it is winter in the south, and substantial amounts of seawater freeze into ice. In the northern winter (September to March), the process reverses, so that these rhythmic cycles result in no net annual change in sea level. This changes when land-based ice masses melt permanently and fail to reform during the subsequent winter. This effect has intensified in recent years, leading to estimates that the Arctic Ocean could become ice-free during the polar winter within the next two decades. The water that no longer freezes must remain in the oceans as liquid water. Consequently, the overall ocean volume increases. Additionally, warmer water occupies more volume than cold water – if seawater warms by 1 °C, global sea levels rise by approximately 25 cm to 50 cm from thermal expansion alone.

The projected extent of global sea-level rise is calculated using various models. Statements such as "If all 24 million cubic kilometres of ice constituting the Arctic and Antarctic polar caps, glaciers and ice fields were to melt, sea levels would rise by more than 60 metres" are frequently encountered. This figure is accurate, but even under a business-as-usual scenario (the "worst-case" pathway), such an outcome would not occur for approximately 1,000 years.

Over the coming decades, projected increases are significantly lower – yet smaller values should not be underestimated. IPCC projections estimate a rise of 26 cm to 78 cm in the coming decades. This might suggest that raising dykes by a single metre would suffice for a considerable period. This is incorrect: statistically, all dykes must be raised by 1.5 times (and in some locations up to twice) the average sea-level rise to maintain current levels of coastal protection [Source 2]. Consequently, dykes would need to be raised by well over two metres – requiring immense volumes of soil and carbon-intensive concrete. This represents a substantial challenge.

Corals, for example, react sensitively to water temperatures exceeding 30 °C and to ocean acidification, which degrades the calcium carbonate skeletons of corals and other calcifying organisms. Recent studies demonstrate that even the skin and teeth of sharks are degraded by increasingly acidic water (lowered pH values), causing physiological stress to the animals [Sources 3 and 4].

Conclusion

The oceans are particularly threatened by the climate crisis – and ultimately, they represent perhaps the most critical tipping point of all: if the oceans fail as carbon sinks, a much larger proportion of ongoing anthropogenic CO₂ emissions will remain in the atmosphere, accelerating the greenhouse effect. Conversely, if ocean heat uptake slows, the additional solar energy retained by the greenhouse effect will remain in the atmosphere and on land, where human populations reside.

Numerous scenarios and models illustrate how uninhabitable the planet could become. We must collectively strive to mitigate these impacts and preserve a habitable planet.

We have compiled several approaches on this homepage. Additional references are available in the bibliography.

Concluding note: The global scientific consensus on this subject is expanding daily – indeed, few phenomena are as comprehensively researched and scientifically documented as the climate crisis.

We can only provide an overview here, alongside recommended literature for further scientific study. The sources section contains links to websites [Sources 15-22] where further detailed scientific evidence and comprehensive data can be found.