Sea levels are rising – so what?

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

While the cause of the climate crisis occurs in the atmosphere, its warm effects impact the Earth's surface.

Drought, heatwaves, forest fires – 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. Ice sheets at the poles are melting, filling the bathtub with extra water. Oceans could rise by several metres along the coasts of islands and continents, which (justifiably) alarms humanity.

Unfortunately, this represents only one aspect of the "climate crisis and oceans" issue. The remainder is no less concerning, ominous, and profound.

Here, we wish to elaborate a little further.

For completeness: The mechanics of sea level rise

The concept initially appears quite straightforward. Imagine a cold drink: ice cubes float at the top to keep the liquid cool. Initially, these are not strictly necessary, as the beverage itself could be chilled and served cold. However, the average consumer does not drink it in a single gulp – but rather leaves it in the ambient air for a time or holds it in their hand. Immediately, the second law of thermodynamics takes effect: temperature equalisation from the warmer to the colder region. The drink becomes warmer.

To prevent this, the cooling effect of the melting ice cube is utilised, transferring its cold to the surrounding substance in accordance with the same principle. This substance is predominantly the drink – it is cooled by the ice cube, ideally at the same rate as it is warmed by the ambient air or hand. Under these conditions, the drink remains refreshing. Consequently, ice in the ocean cools the sea when it melts – and it melts when the water and air temperatures exceed that of the ice.

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

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

Sea ice covers an average of one and a half times that area annually: approximately 22.5 million square kilometres of floating ice exist, distributed roughly equally between the Arctic (the region around the Earth's North Pole, inhabited by polar bears, among other species) and surrounding Antarctica. In both regions, these masses fluctuate significantly throughout the annual cycle – by 16 million square kilometres in the south between summer and winter, 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 flows into the nearest sea. It arrives there at a relatively cool temperature, meaning these former ice masses do not significantly contribute to ocean warming, but actually counteract it slightly. However: these water volumes fill the bathtub, analogous to topping up the drink with cold liquid.

The situation differs with melting, floating sea ice: just as a melting ice cube does not overflow a drink (which usually goes unnoticed, as it is typically consumed by then), sea levels do not rise due to melting icebergs. In principle, the mass differences balance out over the year between the North and South Pole regions – when more water melts in the northern summer (March to September), it is winter in the south, and substantial volumes of seawater freeze into ice. In the northern winter (September to March), the process reverses, so that these rhythmic cycles ultimately result in no net change in annual average sea levels. This dynamic changes when ice masses melt permanently rather than refreezing during the subsequent winter. This effect has become increasingly pronounced in recent years, leading to projections that the Arctic Ocean could become ice-free during the polar winter within the next two decades. The water that no longer freezes must go somewhere: it remains in the oceans as liquid H2O. Consequently, the bathtub fills up. Added to this is the thermal expansion of water – if seawater warms by 1°C, global sea levels rise by approximately 25 cm to 50 cm.

The projected total sea level rise is calculated in various ways. Statements such as "If all 24 million cubic kilometres of ice making up the polar ice caps in the Arctic and Antarctic, the glaciers, and all 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 (worst-case scenario), it would not occur for approximately 1,000 years.

In the coming decades, the projected rise is significantly lower – yet smaller figures should not be underestimated. IPCC projections suggest a rise of 26 cm to 78 cm in the coming decades. It might therefore be assumed that raising dykes by a full metre would suffice for a considerable period. This is incorrect: statistically, all dykes would need to be raised by 1.5 times (in some areas up to double) the average sea level rise to maintain current safety standards [Source 2]. We would therefore need to raise all dykes by over two meters – requiring unimaginable quantities of soil and (highly carbon-intensive) concrete. This is a factor not to be underestimated.

Corals, for instance, are highly sensitive to water temperatures exceeding 30°C and to ocean acidification, which degrades the calcium carbonate skeletons not just of corals, but of all calcifying organisms. A recent study demonstrates that even the skin of sharks is damaged by ocean acidification (lower pH levels), causing distress to the animals [Sources 3 and 4].

Conclusion

The oceans are uniquely threatened by the climate crisis – and ultimately, they represent perhaps the most critical tipping point of all: if the oceans fail as a carbon sink, a much larger proportion of ongoing CO2 emissions will remain in the atmosphere, compounding the greenhouse effect. Conversely, if ocean warming ceases to absorb heat, the additional solar energy retained on Earth (amplified by greenhouse gases) will remain in the atmosphere and on land. Precisely where humanity attempts to live.

There are ample scenarios and visions detailing how inhospitable conditions could become. Let us work together to avert them and maintain our planet as a habitable environment for humanity.

We have summarised some initiatives to this end on this website. Further reading can also be found in the bibliography.

A concluding note: the collective scientific knowledge of humanity on this subject increases daily – indeed, it is often argued that no phenomenon on Earth has been as exhaustively researched and scientifically documented as the climate crisis.

We can therefore only provide a brief overview – paired with recommended literature for further in-depth study. In the sources, you will find links to website portals [Sources 15-22] offering more extensive information (including comprehensive scientific reference compendia).