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Three thousand scientific robots that are plying the ocean have sent home a puzzling message. These diving instruments suggest that the oceans have not warmed up at all over the past four or five years. That could mean global warming has taken a breather. Or it could mean scientists aren't quite understanding what their robots are telling them.

NPR's Richard Harris looked into this mystery.

RICHARD HARRIS: Up here on the surface of the Earth, the years since 2003 have been some of the hottest on record. But Josh Willis at NASA's Jet Propulsion Laboratory says if you really want to measure what's happening with global warming, you need to know what's happening inside the world's oceans.

Dr. JOSH WILLIS (NASA Jet Propulsion Laboratory): Even though we live in the atmosphere, that's where we care about the temperatures warming. Actually, the heat is by and large going into the ocean.

HARRIS: In fact, 80 to 90 percent of global warming involves heating up ocean waters. They hold much more heat than the atmosphere can. So Willis has been studying the ocean with a fleet of robotic instruments called the Argo system. These buoys can dive 3,000 feet down and measure ocean temperature. And since the system was fully deployed in 2003, it has seen no warming of the global oceans.

Dr. WILLIS: There's been a very slight cooling, but not anything really significant. So they really haven't warmed or cooled very much in the past few years.

HARRIS: Does that mean global warming's on a bit of a hiatus?

Dr. WILLIS: Well, it may be. You know, global warming doesn't mean that every year will be warmer than the last. And it may be that we are in a period of less rapid warming.

HARRIS: During that time, heat has been flowing out of the oceans and into the air. That's part of the weather phenomenon known as El Nino. So it's quite possible the air has warmed, but the oceans have not. But it's also possible that something more mysterious is going on.

Consider global sea level. Sea level rises when oceans get warm because warm water expands. This accounts for about half of global sea level rise. So with the oceans not warming, you'd expect to see less sea level rise. Instead, sea level has risen about half an inch in the past four years.

Dr. WILLIS: Sounds like a very small amount, but, in fact, globally, this is a huge volume of water. So it's very significant.

HARRIS: Willis says some of this water is apparently coming from a recent increase in the melting rate of glaciers in Greenland and Antarctica.

Dr. WILLIS: But, in fact, there's a little bit of a mystery. We can't account for all of the sea level increase that we see over the last three or four years.

HARRIS: One possibility is that the sea has warmed and expanded - and scientists are misinterpreting the data from the diving buoys. But assuming extra heat is not going into the oceans right now, where is it going? Probably back out into space.

Kevin Trenberth at the National Center for Atmospheric Research says the Earth has a number of natural thermostats, including clouds, which can either trap heat and turn up the temperature, or reflect sunlight and help cool the planet.

Dr. KEVIN TRENBERTH (National Center for Atmospheric Research): And, unfortunately, we don't have adequate tracking of clouds to fully determine exactly what role they've been playing during this period. At least not yet.

HARRIS: Trenberth says it's also possible that some of the heat has gone even deeper into the ocean. Or it's possible that scientists need to correct for some other feature of the planet they don't even know about. It's an exciting time, though, with all this new data about global sea temperature, sea level and other features of our climate.

Dr. TRENBERTH: And I suspect that we'll able to put this together with a little bit more perspective and further analysis. But what this does is highlight some of the issues and send people back to the drawing board.

HARRIS: Trenberth and Willis agree that a few mild years have no effect on the long-term trend of global warming. But they still have things to learn about how our planet copes with the heat.

Richard Harris, NPR News.

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