Ocean: The Liquid Engine That Dominates Our Planet

Ocean: the liquid engine that dominates our planet

Professor Helen Czerski

30th October 2025

Seeing the Sea

Every map is a choice. There have been many proposed methods for representing the surface of a spherical planet on a flat piece of paper, but all of them involve subjective decisions about what to prioritise. The American geophysicist Athelstan Spilhaus identified one of the most common trade-offs (but one that is rarely stated explicitly): that to see the land, it is necessary to cut the ocean. He then pointed out the logical next step: that to see the ocean we must cut the land. This led to the “Spilhaus projection”, a map of the globe that cuts the land and relegates it to the edges, keeping the ocean intact, showing that the global ocean is a single connected water mass. This projection makes it easier to appreciate that we live on an ocean planet, and to see how large the Pacific ocean really is.

Figure 1 [see downloadable PDF]The Spilhaus Projection (created with ArcGIS)

More than meets the eye

A glass of seawater will almost always look the same to us, whatever its physical properties and chemical content. However, the invisible signature provided by two main properties – temperature and salinity – and then a long list of other characteristics (nutrients, oxygen, dissolved gases, trace metals and more) allow us to see that the ocean has a complex internal structure. The most important aspect of that structure is its layering. The ocean is stratified by density, which means that layers of less dense (warmer, fresher) water will lie on top of more dense (colder, saltier) water. In the depths, each layer can be one or two kilometres deep, but right at the surface there is a shallow “mixed layer” which is heated by sunlight. This can be perhaps 20 – 150 metres deep, depending on the weather conditions and season. When we look at maps of ocean surface temperature, we are looking at the temperature of this layer only. The weather is powered by heat that comes from the ocean surface and from land, but those winds may in turn push on the ocean, driving it to move.

The Restless Ocean

The ocean moves on many different timescales and size scales, and this complex engine sets the scene for our life on land. We may sometimes hear a news story stating that the water just off Cornwall is suddenly full of jellyfish, presented as though this is a random event due to a roll of the dice, but this human-focused perspective misses the bigger picture. The structured, dynamic ocean is constantly active and changing, but we are only directly exposed to the top surface of that complexity. That constant underwater movement may occasionally result in the right conditions for jellyfish next to Cornwall, but that is just the surface expression of a much deeper pattern. Human civilization has played out in the environmental context that the ocean has provided, and we rely directly on the dynamic ocean continuing to provide that context.

The wind provides energy for the ocean to move (by pushing on the ocean surface), but the actual outcome also depends strongly on the shape of coastlines, the spin of the Earth, and the small number of places where ocean surface water is decreasing in density and sinking. This “overturning” circulation is the largest and slowest pattern of ocean movement, and it is very important for moving heat around the planet.

Messengers

The “messengers” of the ocean are sound and light: energy and information that travel through the structure of the ocean. The physical importance of sunlight is that it is absorbed relatively quickly (the ocean is not very transparent), injecting energy into the water close to the ocean surface. Sound can travel very long distances through the ocean and can be an important source of information for both animals and humans. Experiments in the 1990s attempted to measure the global ocean temperature using sound from huge underwater speakers (the Heard Island feasibility test), but were considered too intrusive to continue. More recent work (Uzhansky et al, October 2025) has suggested that the natural background sound of the ocean could be used to monitor the change in average pH of the ocean, also known as ocean acidification.

Passengers

The “passengers” of the ocean are those chemical compounds or living organisms that drift around wherever ocean currents take them. There are many natural “passengers”, but we focus here on CFCs as an example of a passive tracer. CFCs do not exist naturally, and human production of them ramped up very quickly in the 1950s and 1960s. Once they were present in the atmosphere, they could also be taken up by the surface ocean, and in places where that surface water then sinks downwards into the depths, we can track how far the water has travelled by looking for CFC concentration. This provides direct evidence of the overturning circulation. We would not do this experiment deliberately, but this pollution has a useful silver lining.

Voyagers

The “voyagers” of the ocean are those organisms that deliberately move themselves through the structure of the ocean. This could be fish migrating to follow their food, whales migrating to breed, zooplankton migrating vertically to hide from predators, or human migrating across the ocean to fish, trade or explore. The larger ocean migrants are almost always navigating through and around ocean features, and bluefin tuna are a good example. Some bluefin tuna populations migrate across the Atlantic to seek out mesoscale eddies in the Gulf Stream, which are rotating water masses 50-100 km across that can remain intact for many months. These eddies provide a specific environment that creates a rich hunting ground for the tuna. This reinforces the message that “the ocean” is not just a large undifferentiated water mass, but one with features that are used by its inhabitants.

Conclusion

The ocean is a sophisticated and dynamic engine. It is the defining feature of Planet Earth not just because it covers a large proportion of the surface of our planet, but because it is essential for moving heat, water and much more besides around our planet. The way the ocean engine turns creates the environment that all land animals depend on. Earth is an ocean planet.

© Professor Helen Czerski 2025/6

References and Further Reading

Chen, J., Zhang, T., Tominaga, M. et al. Ocean Sciences with the Spilhaus Projection: A Seamless Ocean Map for Spatial Data Recognition. Sci Data 10, 410 (2023). https://doi.org/10.1038/s41597-023-02309-6

Monahan, Adam Hugh. "The probability distribution of sea surface wind speeds. Part I: Theory and SeaWinds observations." Journal of climate 19, no. 4 (2006): 497-520.

Munk, Walter H., Robert C. Spindel, Arthur Baggeroer, and Theodore G. Birdsall. "The Heard island feasibility test." The Journal of the Acoustical Society of America 96, no. 4 (1994): 2330-2342.

Udovydchenkov, Ilya A., et al. "Modeling deep ocean shipping noise in varying acidity conditions." The Journal of the Acoustical Society of America 128, no. 3 (2010): EL130-EL136.

Uzhansky, Ernst M., David R. Barclay, and Michael J. Buckingham. "On the measurement of ocean acidity with ambient sound." Journal of Geophysical Research: Oceans 130, no. 10 (2025): e2025JC022575.

Ilyina, Tatiana, Richard E. Zeebe, and Peter G. Brewer. "Future ocean increasingly transparent to low-frequency sound owing to carbon dioxide emissions." Nature Geoscience 3, no. 1 (2010): 18-22.

Cimoli, Laura, et al . "Annually resolved propagation of CFCs and SF6 in the global ocean over eight decades." Journal of Geophysical Research: Oceans 128, no. 3 (2023): e2022JC019337.