If every ship were taken out of the water, would sea levels change?

Imagine a world where every cargo liner, cruise ship, and fishing boat is lifted out of the ocean and placed on land. The image is striking, but does such a massive relocation of metal and steel actually affect the height of the seas? This question surfaced in New Scientist’s “Last Word” series, prompting readers to weigh in with scientific reasoning. Below we explore the physics, the numbers, and the broader implications of removing all ships from the water.

What Happened

When a ship sits in the ocean it displaces a volume of water equal to its weight, according to Archimedes’ principle. If the vessel were suddenly removed, that displaced water would flow back into the space the ship once occupied, causing a minute rise in sea level. The idea captured the imagination of readers, who offered a range of estimates—from a negligible change of a few micrometres to a more noticeable shift of a few millimetres. The consensus, however, points to an effect that is essentially imperceptible on a global scale.

In the discussion, contributors highlighted two key pieces of data: the total mass of the world’s merchant fleet (approximately 2 × 10^12 kg) and the total surface area of the world’s oceans (about 3.6 × 10^14 m²). By dividing the displaced volume by the ocean’s area, they arrived at a rise of roughly 0.005 mm—far below the detection limits of even the most precise satellite altimeters.

Key Details

The global commercial fleet, which includes container ships, tankers, and bulk carriers, is estimated to weigh around two trillion kilograms. Adding military vessels, fishing boats, and pleasure craft brings the total mass up to roughly three trillion kilograms. Because ships are built of steel, concrete, and other dense materials, their density is close to that of water, meaning they displace nearly the same volume as their own mass.

To calculate the sea‑level impact, we use the simple relation: Δh = (total displaced volume) / (ocean surface area). The displaced volume equals total mass divided by the density of seawater (~1025 kg m⁻³). Plugging in the numbers yields a volume of about 2.9 × 10^9 m³. Spread over 3.6 × 10^14 m², this translates to a rise of roughly 0.008 mm—well under one‑hundredth of a millimetre. Even if we double the estimate to include all small craft, the change remains under 0.02 mm.

Background

Sea‑level change is a topic of intense scientific scrutiny because it integrates contributions from melting ice sheets, thermal expansion of warming water, and changes in land water storage. The Intergovernmental Panel on Climate Change (IPCC) reports that global mean sea level has risen about 20 cm since the start of the 20th century, driven primarily by climate‑related factors. In this context, a shift measured in thousandths of a millimetre is effectively zero.

Archimedes’ principle, formulated in the 3rd century BC, tells us that any floating object displaces a volume of fluid equal to its weight. This principle underpins ship design and explains why removing a vessel does not “create” water; it simply allows the previously displaced water to flow back in. The principle also clarifies why the mass of the ship, not its shape, determines the volume of displaced water.

Why It Matters

Understanding the magnitude of this effect helps separate sensational speculation from scientific reality. Public discourse sometimes exaggerates the impact of human activities on sea level, leading to confusion about what actions are truly consequential. By quantifying the effect of removing all ships, we illustrate how even massive, visible structures contribute only a tiny fraction to the ocean’s total volume.

Moreover, the exercise demonstrates the power of back‑of‑the‑envelope calculations in science communication. Readers of New Scientist’s “Last Word” were able to take raw data, apply basic physics, and arrive at a robust answer. This kind of citizen‑science engagement builds trust in scientific methods and encourages critical thinking about environmental issues.

What Happens Next

Future discussions are likely to focus on more impactful contributors to sea‑level change, such as the melting of the Greenland and Antarctic ice sheets, which together hold enough water to raise global oceans by several metres. Researchers will continue refining satellite measurements, improving models of ice dynamics, and assessing regional variations that affect coastal communities.

In the meantime, the “ship removal” thought experiment remains a useful teaching tool. Educators can use it to illustrate concepts of buoyancy, volume displacement, and the relative scale of human-made structures versus the planet’s water reservoirs. It also serves as a reminder that while individual actions matter, the scale of some environmental challenges requires collective, systemic solutions.

In short, lifting every ship out of the sea would cause a sea‑level rise measured in fractions of a millimetre—an effect dwarfed by natural climate processes. The exercise underscores the importance of grounding environmental debates in quantitative analysis, ensuring that our focus stays on the factors that truly shape the future of our oceans.