Oceanographic data gathered across the North Pacific in 1985 and again in 1999 indicate that the deepest waters there have been heating up.
In trans-Pacific research cruises, scientists measured the ocean's temperature, salinity, and other properties at more than 100 sites along latitude 47°N. That route stretches from Russia's Kuril Islands to the coast of Washington State, says Howard Freeland of the Institute of Ocean Sciences in Sidney, British Columbia.
A comparison of the two data sets showed that the average temperature of the water at depths below 5,000 meters was about 0.005°C warmer in 1999 than it was in 1985. Instruments on both cruises could measure temperatures accurately within 0.001°C, so the temperature increase—which was noted at points all across the ocean basin—almost certainly is real, says Freeland. He and his colleagues report their findings in the Feb. 26 Nature.