The Earth Gets Hotter as You Dig Down. Why Does the Ocean Get Colder?

Dig into the Earth and it gets hotter the deeper you go. Dive into the ocean and it gets colder. The two facts seem to contradict each other, since the bottom of the ocean sits on top of that warm Earth. The explanation is that the ocean's cold comes from above, and the Earth's heat is far too faint to change it.

The great ocean conveyor moves water around the globe. Credit: NOAA.

Cold water sinks

Cold water is denser than warm water. That is why the ocean gets colder with depth: cold, salty water sinks to the bottom of the ocean basins, beneath the lighter, warmer water near the surface. The sinking and spreading of this cold water at depth, together with the wind-driven flow of warm water at the surface, creates the complex pattern of circulation known as the global conveyor belt.

The Earth's heat is real, but tiny

The Earth grows hotter with depth mainly because energy from radioactive decay is leaking outward from the planet's core. Some of that geothermal energy does pass into the ocean along the seafloor. The effect is simply too small to measure directly.

The numbers show why. Heat flowing out of the Earth averages only about one tenth of a watt per square meter — very little compared with what it takes to warm the ocean. At that rate, and ignoring currents, it would take well over a year just to raise the temperature of the bottom meter of the ocean by one degree Centigrade.

And the water keeps moving

The ocean does not stand still long enough for even that to happen. Deep currents, driven by differences in temperature and salinity, constantly replace the bottom layer of water with colder water — carrying away whatever warmth the seafloor gives it.

Sources

Based on "Why does the ocean get colder at depth?" from NOAA's National Ocean Service, a work of the United States government in the public domain.