Explain the meaning of global warming

But all this leaves a big question, one that scientists have been trying to answer: If the atmosphere is warming more slowly than projected, where did the heat go?

There are two ways to create a global-warming hiatus: The heat can go somewhere other than the atmosphere, or there might be less heat in the climate system than scientists predicted.

Global warming estimates usually center on measures of air temperature. But the climate system also can move heat from the atmosphere to somewhere else, like melting ice.

The most obvious culprit is the ocean, which absorbs 90 percent of the heat added to the climate system. With the oceans holding so much heat, the focus on mean surface temperature as the measure of global warming misses much of the point. Minor shifts between the oceans and the air could keep the planet heating up, even while slowing the pace of atmospheric warming. And that wouldn’t necessarily be good news, since warmer oceans would raise sea levels, change the climate, and hurt the ocean’s ecosystem.

But sea surface temperatures and the upper ocean heat content didn’t increase over the last decade, not by enough to account for what Trenberth called “missing heat”—the heat that greenhouse gas emissions should have trapped in the Earth’s climate system, but couldn’t be found.

Scientists speculated that the heat might be hidden in the deep ocean, beneath 700 meters, where there are not reliable temperature measurements. Without good data, they couldn’t prove heat was going into the deep oceans. So Meehl and his colleagues turned to computer models. Their approach was straightforward: Look at naturally occurring hiatus decades in climate models, and see where the computers put the heat. In the simulations, the deep ocean warmed by 19 percent more during hiatus periods, even as sea surface temperatures cooled in the tropical Pacific, creating a pattern similar to the prolonged La Nina-like pattern of the last decade. They then plugged in the conditions of the last decade and found that “the models produced roughly 20 percent less warming than the free running models.”

More proof of the deep ocean hypothesis: Over the last decade, more than 3, 000 Argo floats—basically, seaborne thermometers—were deployed across the world’s oceans to measure oceanic heat content. The floats can measure temperatures down to 2000 meters. They found that more than 30 percent of ocean warming over the past decade occurred beneath 700 meters—potentially covering most of the missing heat.

But analysis was limited by the absence of historical data: Without knowledge of how much heat was trapped in the deep ocean in the past, scientists couldn’t prove much. Just last month, an intriguing paper in the journal Geophysical Research Letters suggested a way around this problem. Using a model that projects ocean temperatures back to 1961, its trio of authors found that the surge in deep ocean heat intake over the last decade was indeed historic.

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