story and design by Michon Scott · April 24, 2006

Coral reefs, octopi, eels, offshore oil rigs, El Niño, La Niña, the birthplace of life, tasty seafood, even assorted bottom feeders. All these things come to mind when we think of the world’s ocean. Researchers at NASA’s Jet Propulsion Laboratory and Goddard Institute for Space Studies have learned to think of the ocean as something else, something that might not occur to the rest of us. The ocean, they say, is Earth’s “biggest heat bucket.” And like a bucket placed under an overflowing sink, the ocean is filling up with the heat that increasing levels of greenhouse gases are preventing from escaping to space.

By comparing computer simulations of Earth’s climate with millions of measurements of ocean heat content collected by satellites and in-the-water sensors, a team of climatologists and oceanographers has provided what leading NASA climate scientist James Hansen calls the “smoking gun” of human-caused global climate change: a prediction of Earth’s energy imbalance that closely matches real-world observations.

Where Greenhouse Heat Hides

Like any planet, the Earth absorbs some radiation and emits some radiation back into space. If the amount of energy Earth emits matches the amount it absorbs, the planet’s energy budget is in balance, and its temperature remains steady. If the incoming and outgoing energy don’t match, the planet is either warming or cooling over time, even if the change isn’t immediately obvious. If greenhouse gases are forcing Earth to absorb more energy than it emits, why wouldn’t global surface temperatures increase right away?

It sounds reasonable that if excess greenhouse gases in the atmosphere are causing Earth to absorb more energy than it reflects back into space, that excess energy should heat up the atmosphere first. Something that many people find odd—but climate scientists have long known—is that most excess energy would really hide elsewhere.
 

Recent studies have shown that the world’s ocean holds more than colorful coral reefs. It also acts as a reservoir for excess energy. (Photo courtesy Photos.com.)

“It turns out that the atmosphere, the air, really can’t hold that much heat,” explains Josh Willis, an oceanographer with the California Institute of Technology working at NASA’s Jet Propulsion Laboratory. Heat capacity is the amount of energy that must be put into something to change its temperature, and air has a very low heat capacity. “If you put energy into the ocean, on the other hand, its temperature changes only very slightly.”

One reason the ocean heats more slowly than the atmosphere is the difference in their total mass. “The atmosphere only weighs a tiny fraction of what the ocean weighs,” Willis explains. “But there’s also a sort of intrinsic property of the air that makes it not quite as good at holding heat as the ocean. That property is called the specific heat. You probably have a feel for this if you’ve ever tried to boil a pot of water. You have to burn a lot of gas or wood to heat up the water. But if you had a similar quantity of air, it would take a lot less energy to heat it up to the same temperature. The water’s heavier, and it has a higher specific heat, and both of those things give it a much bigger heat capacity.”

What this means for planet Earth is that excess energy might not make itself immediately obvious by strongly warming the atmosphere. Instead, that energy might hide in the ocean, in the form of warmer ocean temperatures.

 Taking the Ocean’s Temperature

In his study of ocean heat storage, Josh Willis conducted experiments between New Zealand and Hawaii. (Photo courtesy NASA Jet Propulsion Laboratory.)