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Many scientists say the oceans are heating up, as part of global warming. Still, it's been quite a challenge to document the changes in ocean temperatures over the past century. NPR's Richard Harris talked to a government climate researcher who's taking a novel approach. He's been tracking down the logs of old ships from all over the world, including some from America's wartime enemies.

RICHARD HARRIS: Sydney Levitus works in an ordinary government office building in the ordinary town of Silver Spring, Maryland, but his work frequently transports him - at least figuratively - back onto the helms of ships that plied the seas many decades ago. He's looking at temperature readings from the ship's logbooks.

Mr. SYDNEY LEVITUS (National Oceanic and Atmospheric Administration): The data are priceless because you can't go back in time, obviously.

HARRIS: And scientists would dearly like to know how the temperature of the seas has changed since humans started adding carbon dioxide to the atmosphere. So, starting back in 1993, Levitus headed up an international effort to gather whatever historical records he could find. And surprisingly enough, new treasures still come across his transom.

On this day, he's thumbing through a stack of index cards sent along by the British Royal Navy.

Mr. LEVITUS: They thought they had cleaned out their basement a couple years ago and sent us everything, but they just discovered these boxes with additional data. So, we're now digitizing those data that we don't have.

HARRIS: Levitus picks up a random stack of cards.

Mr. LEVITUS: Let's see. This is from 1956, '52, 1948...

HARRIS: Each card has not only a date, but the latitude and longitude, salinity measurements, temperatures and depth of each data point. He reaches into another big box of cards marked USS Triton.

Mr. LEVITUS: This is pretty interesting. I wasn't aware of this.

HARRIS: The cards in his hands contain data collected by an American nuclear submarine on a historic mission it undertook in 1960.

Mr. LEVITUS: It's the first submerged circumnavigation of the Earth. It looks like these were all taken pretty close to the surface. What a very exciting set of observations.

HARRIS: Over the years, Levitus says they've managed to compile literally millions of data points from all around the world, and from friend and erstwhile foe alike. For example, Japan turned over copious records its warships collected during World War II.

Levitus tells one story of how he traveled from his office at the National Oceanic and Atmospheric Administration to look for temperature records in the Russian seaport of Murmansk. Colleagues there immediately took him to a neatly tended graveyard where Americans and other Allied troops had been buried during World War II.

Mr. LEVITUS: What they also did is showed me a picture of Murmansk at the end of World War II, and it was an aerial photograph. There was only one building left standing.

HARRIS: They pointed to the photo and told him, sorry, but you can see why you won't find any old ocean data here.

Mr. LEVITUS: But the fact is that there is - or was ocean data measured during World War II there again. And one - or another of our colleagues in Russia found this data and digitized it and sent it to us. So, even people within a country don't know what's there in their archives all the time.

HARRIS: Levitus says old ocean temperature measurements have been snatched from the jaws of mice, saved from mildew, leaky pipes, fires and flood. He has patiently compiled it, shared it with colleagues from all around the world, and built an increasingly clear picture of our warming oceans.

Richard Harris, NPR News.

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