Deep Pacific waters warmed in recent years.

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[degrees]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[degrees]C warmer in 1999 than it was in 1985. Instruments on both cruises could measure temperatures accurately within 0.001[degrees]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.

There aren't any chemical signs that the recent warming stems from increased undersea hydrothermal activity in the region, says Freeland. Also unlikely is that the warming results from increased radioactive decay of isotopes underneath the ocean floor, since the amount of heat generated from that process is essentially constant over eons.

The anomalous rise in temperature poses a challenge to theoretical models of ocean circulation, which hold that deep waters of the North Pacific have been flowing along the ocean floor undisturbed for hundreds of years. The new data suggest that at least some water warmed by rising global temperatures at the ocean's surface is mixing with deep currents, says Freeland.--S.P.