What if the whole Earth were covered by ocean?

Most of Earth is covered by sea. But there is land too.

What if there were no land at all? What if the whole Earth were one big ocean?

The U.S. Geological Survey says about 71 percent of Earth's surface is covered by water. The rest is land: deserts, forests, mountains and fields. Land and water behave very differently in sunshine.

So what if the whole planet were ocean? One thing we can explore with a model is temperature: how hot the days get and how cold the nights get. In this lab you can change the ground under one spot on the equator from damp soil to ocean.

About 71 percent of Earth's surface is water-covered, and the oceans hold about 96.5 percent of all Earth's water (USGS). Land and sea respond differently to the same sunshine, which shapes local climates.

This lab isolates one property, heat storage, and asks how it changes the daily temperature cycle. It uses a simple energy-balance model of a single patch on the equator at an equinox, and raises the surface's effective heat capacity from damp soil to ocean.

Ground warms up in the day. At night it cools down.

In Olly's lab, damp ground is about 29 degrees in the day and 21 degrees at night.

During the day, sunshine warms the ground. At night, the ground loses that heat to the sky and cools down. The difference between the warmest and coldest times is called the day-night swing.

In our model, a patch of damp soil on the equator, with air like Earth's, warms to about 29 °C in the afternoon and cools to about 21 °C before dawn: a swing of about 7 degrees. Dry sand, which stores less heat, swings much more, from about 37 °C down to 13 °C.

The model integrates C·dT/dt = (1 − α)S₀·max(0, cos 2πt/S) − εσT⁴ + k(T̄ − T) for one surface layer, with Earth-like air (albedo 0.30, effective emissivity 0.61, heat sharing toward 288 K).

With C = 2 MJ/m²·K (the model's damp soil), the repeating day runs from 28.5 °C to 21.3 °C, a 7.2 K swing (readouts: 29, 21, 7). With dry sand, C = 0.6, the swing grows to 23.7 K (37 to 13 °C). The less heat a surface stores, the more each hour of sunshine or darkness changes its temperature.

Now Olly turns the ground into ocean.

The days are not so hot. The nights are not so cold. It stays mild all the time.

Now change the surface to ocean. The model's ocean stores five times as much heat as the damp soil for every degree it warms.

The result: the afternoon only reaches about 26 °C and the night only drops to about 24 °C. The day-night swing shrinks from about 7 degrees to about 1. The sun shines just as strongly, but the water soaks up the heat without getting much warmer, then gives it back slowly through the night.

With C = 10 MJ/m²·K the same sunshine produces a maximum of 25.6 °C and a minimum of 24.2 °C, a swing of 1.4 K (readouts 26, 24, 1). Doubling again to the slider's maximum of 20 gives 0.7 K.

The swing falls roughly in proportion to 1/C because the daily energy surplus and deficit are fixed by the sunshine, and a larger heat capacity turns the same energy into a smaller temperature change. Note that the model's damp soil, sand and ocean values are rough stand-ins chosen for the model, not measurements of any one place.

Water needs lots of heat to get warm. Then it keeps that heat for a long time.

So water warms slowly and cools slowly. That keeps it mild.

Water has a high specific heat: it takes a lot of energy to warm it up. The U.S. Geological Survey says it takes 4,184 joules to warm one kilogram of water by 1 °C, but only 385 joules to warm a kilogram of copper by the same amount.

Our model is very simple. It looks at one spot on the equator on one day of the year. It leaves out clouds, rain, winds that change, seasons and the way a real ocean mixes, so it can only show how heat storage changes the daily swing.

Specific heat measures the energy per kilogram per degree; water's 4,184 J/(kg·°C) is higher than that of many other substances (copper: 385, per USGS). For a surface layer, what matters in the model is the heat capacity per square meter, C: how much energy the layer that warms and cools each day absorbs per degree.

Model limits, stated in its documentation: the temperatures are a teaching model, not a climate prediction. Clouds, rain, seasons, real winds and ocean currents are absent; heat sharing toward 288 K is fixed; only the equator at an equinox is modeled. A real ocean planet would also differ in brightness, clouds and humidity, none of which this model changes.

Towns by the sea have milder weather. The sea keeps them from getting too hot or too cold.

A pond stays about the same warmth day and night. Fish like that!

We can see the ocean's calming effect on Earth today. The U.S. Geological Survey explains that water near cities takes longer to heat up and longer to cool down than land, so cities near the ocean tend to have less extreme temperatures than inland cities.

For example, USGS says Nebraska, in the middle of the United States, has colder winters and hotter summers than Oregon, even though Oregon is farther north, because Oregon has the Pacific Ocean next door. A whole-ocean planet might be mild everywhere, but scientists would also need to know about its clouds, winds and ice, which our model leaves out.

USGS describes how water's high specific heat moderates nearby air: seasonal changes are gradual near the oceans, coastal cities have smaller and less extreme temperature ranges than inland ones, and Nebraska has colder winters and hotter summers than Oregon, which lies at a higher latitude beside the Pacific. It also notes that pond water stays relatively constant in temperature from day to night.

Extending that to a planet with no land is an idea, not a model result. A real ocean world's climate would depend on its clouds, winds, sea ice, atmosphere and distance from its star; this energy-balance patch captures only the heat-storage part.