From cold, dark reaches of the North Atlantic are coming hints of potential large-scale changes in Earth's climate. Scientists studying the churning depths of subpolar waters were alerted to this possibility by an unexpected drop in their saltiness.
Though the change is small - a less than one-tenth of 1 percent decrease - it is enough to lead some researchers to believe that the ocean's mammoth circulation system may be slowly grinding to a halt. If so, such a trend might sharply affect Earth's climate over decades by helping warm its atmosphere, melt polar ice, and raise the sea level.
''We're really not sure how this change is taking place,'' admits physicist William Jenkins of the Woods Hole Oceanographic Institution here. ''But the flags should be going up because this is happening on a rapid time scale.''
The time scale is about 10 years - roughly the period between a previous expedition testing North Atlantic sea water and Woods Hole's Transient Tracers in the Ocean program last year, in which Dr. Jenkins and other scientists made the observations.
Jenkins says the salinity change in itself is not the focus of concern. What is worrisome is that it indicates a variation in the way ocean currents circulate. Dense Arctic waters, instead of being carried southward into the North Atlantic, have been collecting north of an ocean-floor sill extending from Greenland to Scotland. The normal circulation of these salt-laden waters has been slowing over the last decade, thus leading to fresher water in the North Atlantic.
But the ocean, a huge solar-powered ''heat engine'' that circulates water from the North Atlantic to the Pacific, can take from 1,000 to 2,000 years to complete a cycle. To scientists, the recent, more rapid change implies that the seemingly inexorable sea forces can be disrupted fairly quickly.
That is worrisome to scientists, because ocean circulation plays a critical role in maintaining the stability of climate, they say. Most of the carbon dioxide produced by the burning of fossil fuels is absorbed and disposed of by the oceans. The rate at which they can soak up the gas is limited partly by the rate at which the carbon-dioxide-laden surface water can be replaced by newly ventilated bottom waters.
If the ocean's ability to pick up carbon dioxide is impaired by the slowing of its circulation, that could raise the atmospheric levels of gas, which traps the sun's heat. Scientists suspect the result would be a ''greenhouse effect'' that could raise air temperatures by up to four degrees Fahrenheit.
That would cause further difficulties for the oceans since their circulation systems are sensitive to temperature. Colder water, being denser, slips under warmer water, setting up a cycle where warmer water flows toward the Arctic while colder water travels toward the tropics.
But a general climatic heating, observes Jenkins, would act as a huge ''thermal blanket,'' tending to lessen the temperature contrasts on which ocean circulation depends. ''Then we'd have a vicious cycle set in,'' he says.
It is still not known how severe this trend may be. ''The time scales we're talking about are so vast that it's sometimes hard to see where a natural variation ends and a major change starts,'' he adds. Nor are scientists sure what role man has played, if any, in the change.
''What is the important lesson in this is that the system is changing very rapidly,'' concludes Jenkins. ''However man may or may not affect it, we might have a whole new ball game in terms of predicting how things might come out.''