Human-caused greenhouse gases continue to accumulate in the atmosphere, but in the last decade, scientists say average surface temperatures have not risen as much as that accumulation was expected to generate.
A new study from NASA’s Jet Propulsion Laboratory in Pasadena discovered that this so-called "missing heat" is not hiding in the deep ocean.
JPL climate scientist Josh Willis co-authored the study and said if the deep-sea were the repository of the missing heat, the water would be expanding as it warmed. That in turn would lead to an increase in global sea levels.
Willis and his team used data from satellites and ocean probes to see if deep ocean waters were warming enough to have an effect on rising sea levels.
"And the answer from our point of view is no," Willis said. "There’s a small amount of heat going down there, but it’s not big enough to raise sea level."
This is not surprising since models of ocean current flow suggest it can take up 1000 years for water at the surface to cycle down to the bottom.
Still, he said the upper layers of the world's oceans were warming enough to cause sea water to expand and levels to rise, though not to the extent they would have with a warmer deep layer.
Willis said there are other theories about why average surface temperatures over the past ten years haven't risen as much as scientists thought they would.
He said it could be due to natural El Niño and La Niña cycles temporarily blunting the increases.
Or it could be related to the dimmer solar cycle the sun is currently experiencing, something it does every decade or so.
But Willlis is quick to point out, just because researcher can’t account for all the heat doesn’t mean global warming isn’t a very real threat to the planet.
"We're talking about the nitty gritty details of what's going on with global warming," he said. "But global warming is still happening. The earth is still heating up, people are still causing it, and sea levels are still rising."