Warming up the seas.

In February, scientists unveiled compelling evidence that human activity is causing the oceans to heat up, a finding with troubling implications for life on both sides of the ocean's surface.

"Previous observations have shown that the oceans are warming, but we hadn't been able to attribute that to human forces" says David W. Pierce of the Scripps Institution of Oceanography in San Diego, where much of the research took place.

Pierce and his colleagues used two distinct computer models of climate to calculate human-produced warming over the last 40 years in the world's oceans, accounting for variations in volcanic and solar activity. Although the models were developed independently--one by British researchers, the other by the Department of Energy--the results of both models showed a stunning parallel to what has actually occurred, indicating that human forces are responsible for the heating of the seas.

And warming they are. Researchers at the World Data Center for Oceanography have painstakingly compiled 50 years of monitoring figures from oceanographic research institutes around the world, plus observations from naval vessels, submarines, merchant ships, and satellites--more than seven million observations in all. The results show that the upper 10,000 feet of the world's oceans have warmed by an average of .037 degrees Centigrade since 1955, a staggering figure if one considers the volume involved and water's considerable ability to store heat, according to the center's director, Sydney Levitus, an oceanographer with the National Oceanic and Atmospheric Administration.

"In some places the ocean is cooling, in others it's warming a great deal," he says, noting that the North Atlantic alone accounts for more than half the heat input. "The overall effect is consistent with what we would expect due to the increase in greenhouse gasses in the Earth's atmosphere since the industrial revolution."

Warmer ocean temperatures have had serious consequences for marine life. In Alaska's Bering Sea, capelin, herring and other coldwater fish have been in decline, while warmer water creatures like albacore and ocean sunfish have made their appearance. Similarly, around the Antarctic Peninsula, Adelie penguins, which winter on increasingly scarce sea ice, are being displaced by chinstrap penguins, which prefer open water, just as elephant seals are pushing aside their cold-loving cousins, the crabeater seals.

Polar bears, which spend most of their lives on the frozen sea, face an uncertain future. Around Hudson's Bay, sea ice has been vanishing two weeks earlier than it did as recently as the 1970s, reducing the bears' hunting season. As a result, the bears are getting thinner and reproducing less successfully. The longer-term outlook is dim: A comprehensive international study released last year predicted that half of the Arctic's summer ice will vanish by the end of the century, leaving the bears with precious little habitat.

Nor are the effects limited to polar regions. Coral reefs, the ecological foundations for most undersea life in the tropics, are particularly sensitive to temperature, becoming sickly when the sea becomes just a few degrees warmer than normal. The 1997 to 1998 El Nino event triggered a global coral disaster, killing a large proportion of the corals in many nations across the Caribbean, Indian Ocean, and Central Pacific. Seven years later, about half of the damaged reefs are bouncing back, but if such severe warming events become more commonplace, many corals may not survive the experience.

"If these events become more frequent because of the increase of carbon dioxide (CO2) in the atmosphere, corals won't have enough time to recover in between," says John Guinotte, a marine biogeographer at the Marine Conservation Biology Institute in Redmond, Washington.

MI that extra CO2 may be affecting marine life indirectly as well. Scientists estimate that about half of all the CO2 humans have put into the atmosphere since 1800 has wound up in the oceans--so much, in fact, that it has made the upper few thousand feet significantly more acidic. Corals and other calcifying organisms have been shown to have more difficulty growing their shells in such conditions.

Amongst the affected species are microscopic marine plants called coccolithophorids, which lie at the base of the marine foodchain and are part of the "meadows of the sea" on which most marine life ultimately depends. An international research team led by Ulf Riebesell of Germany's Alfred Wegener Institite for Polar and Marine Research found that under the sort of acidic conditions expected in the upper levels of many oceans by century's end, coccolithophorid shell growth was as much as 83 percent slower and marked by malformations.

"If these species are having a tough time building their shells under these more acidic conditions, the predators that depend on them will probably have trouble as well," says Guinotte.

If that wasn't enough to worry about, there's the matter of all that ice melting away in the Arctic and Antarctic, which has the potential to slow, rearrange or even stop current ocean circulation patterns, with potentially profound effects for life on Earth, above and below the waves.

Ocean currents like the Gulf Stream or the California Current serve as the planet's radiator, evening out the climate by delivering hot water from the Equator to places like the Northeastern U.S. or the British Isles--which would otherwise be far colder--and replacing it with upwellings of cold bottom water conveyed from the poles. But the system is largely driven by the annual freezing of polar sea ice, which drives large quantities of extremely cold, salty, dense water down to the ocean bottom. But as sea ice vanishes--and ice sheets and glaciers melt--there's less of this super-dense bottom water being produced and, scientists fear, that may be tantamount to turning off the pump that drives ocean circulation.

The process has already begun. The online edition of the Times of London reported last May that Peter Wadhams, a professor of ocean physics at Cambridge University, visited the Arctic ice cap on Royal Navy submarines and discovered "that one of the 'engines' driving the Gulf Stream--the sinking of supercooled water in the Greenland Sea--has weakened to less than a quarter of its former strength." Says Wadhams, "Until recently, we would find giant 'chimneys' in the sea where columns of cold, dense water were sinking from the surface to the seabed [1.8 miles] below, but now they have almost disappeared."

Weakening the Gulf Stream will have serious consequences for marine life. Most undersea creatures select habitats based on water temperature, salt-content and proximity to currents or upwelling areas that carry nutrients, prey and progeny from one place to another; all of these features would be radically altered if circulation slows or stops. That will prove disruptive to us as well, and not only because of the possible impoverishment of fisheries and other marine resources we depend on.

After all, many scientists think that a slowing of ocean circulation may be the switch that triggers the onset of Ice Ages. Warming up the oceans may, paradoxically, result in a dramatic cooling of Europe, the Northeastern U.S. and other places that have long relied on ocean currents to keep them warm. CONTACT: Woods Hole Oceanographic Institution Ocean and Climate Change Institute, [phone], www.whoi.edu/insti tutes/occi/currenttopics/ctabruptclimate.htm; World Data Center for Oceanography, [phone], www.nodc.noaa. gov/General/NODC-dataexch/NODC-wdca.html.

COLIN WOODARD is the author of Ocean's End: Travels Through Endangered Seas. He lives in Portland, Maine and has a website at www.colinwoodard.com.