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How Much Do Oceans |
The chill, overcast ocean between Antarctica and New Zealand seems an incongruous place to investigate global warming. This is, after all, iceberg country.
Earlier in the spring, air temperatures hovered near freezing and the Seattle-based ship Discoverer heaved up and down in sustained winds of 25 knots as its captain fretted while holding position in the drifting ice.
Out on the tilting deck, scientists from the Pacific Marine Environmental Laboratory at Sand Point, a division of the National Oceanographic and Atmospheric Administration, hauled up water from more than two miles deep. Because of its salt content, which discourages freezing, the water that splashed their hands as they filled lab bottles was actually chillier than 32 degrees.
Yet the cold was a clue to what the researchers were doing there. Cold polar water that becomes dense and salty sinks to the bottom of the world's ocean basins and slowly migrates toward the equator, taking carbon dioxide from the atmosphere with it.
That exchange helps delay the onset of global warming, in ways scientists are still trying to understand.
"The payoff is looking at your data," explained Marilyn Roberts, one of the Seattle scientists. She was one of 27 researchers, from four countries, on board the Lake Union ship.
One technique that produced new information was John Bullister's use of human-made chlorofluorocarbons - the same CFCs blamed for creating the seasonal ozone hole over the South Pole - to determine for the first time how long it takes cold water to make the slow migration to the sea bottom at the equator. CFCs not only migrate upward to the ozone layer, they are also absorbed by the ocean. Their concentration in sinking water, starting when they were introduced about 70 years ago, can now be used as a tracer of deep ocean currents.
The time needed for Antarctic bottom water to reach the equator? About 40 to 50 years, Bullister found.
There is no question that by burning fossil oil and coal rich in carbon, humans are changing the concentration of carbon dioxide in the world's atmosphere. Since the start of the Industrial Revolution, it has risen from 280 parts per million to 356 parts per million and continues to climb at 1.5 ppm each year.
This trace gas helps keep some of the sun's heat from being lost to space, making the planet habitable. But too much carbon dioxide, methane, nitrous oxide and CFCs might warm the Earth enough to alter its climate in the next 100 years.
The latest projections call for the planet to warm about three to eight degrees within 50 years or so. But so far the world has warmed only a degree at most (some scientists don't think there is even convincing evidence of that), and about half the carbon dioxide that our fuels release into the air each year is absorbed, or "sinks," into the ocean and forests. Will this absorption protect us from the worst consequences of global warming? If so, for how long?
A massive scientific effort to answer that question is under way. It has resulted in 67 ship cruises to date, recruiting scientists from 16 countries, with the cost to the U.S. alone estimated at $40 million a year.
The Discoverer voyage from January to March was the last in the Pacific; Congress is now debating whether to finance completion of the studies in the Indian and Atlantic oceans.
"The cutbacks being proposed in the federal programs are going to have a major impact," explained Richard Feely, the Pacific Marine Environmental Lab's chief scientist for the cruise. "There is concern whether the program will be completed in the Atlantic." The Departments of Commerce and Energy, which are spearheading the research program along with the National Science Foundation, have been targeted in Congress for possible reductions or elimination.
The idea behind these voyages is simple enough. If we want to understand how much the ocean can clean up after ourselves, we have to understand how much carbon and heat it is absorbing and the fate of that enriched water.
Before the series of cruises started in 1990, there were only about 5,000 water samplings tracking carbon dioxide's fate from all the world's oceans: too few for computers to make accurate predictions.
That many samples now are typically taken on a single cruise. The Discoverer stopped 188 times in its track from Australia to the Antarctic waters and then back north to New Zealand and Pago Pago in Samoa, collecting 6,000 samples at depths exceeding three miles in places.
It took an average of four hours to raise and lower a large cage of cylinders that measure temperature to a thousandth of a degree, the water's salt content to two parts in a million, and pressure so accurately that scientists knew within a few feet how deep their instruments were.
The cylinders opened and closed at predetermined depths to collect water samples that were analyzed for carbon dioxide, CFC content and other chemistry, in portable labs mounted on the Discoverer's deck.
Then the ship steamed between 10 and 33 miles to the next sampling point.
This routine occurred over and over, almost around the clock, for 60 days, from chilly southern waters to 90-degree heat near the equator. And that was just one of scores of similar voyages, criss-crossing the world's oceans in huge grid patterns designed to provide data for massive computer simulations of how climate might change.
"If we don't have the data, we can't evaluate it," pointed out Feely. Crunching the numbers from hundreds of thousands of water samples will take another five years, he estimated.
One question being probed, for example, is how the formation of sea ice off Antarctica affects the rate at which carbon dioxide from oil and coal burning is absorbed into ocean waters. In the 1970s a large area of open water called a polynya opened in the Weddell Sea, allowing exposure of the sea water to cold air and allowing it to cool and sink faster, carrying carbon with it.
The polynya has since closed. Has that affected the rate at which the Weddell Sea absorbs carbon dioxide? And could such changes ultimately affect forecasts of future temperature increases? What is the fate of that cold 1970s water? How did it move through the oceans, and where is it now?
Another example is how fast ocean water moves up and down. What the cruises have discovered is what could be called a dead zone near the equator: a large block of "old" water suspended between the surface and the bottom and which does not mix vertically.
When this stability persists, small organisms in the sea called plankton tend to bloom near the surface and take up more carbon dioxide. When winds or changing temperatures disturb the stable surface and the sea begins to mix, the plankton sink and decline and more carbon dioxide remains in the atmosphere. Again, understanding this relationship is key to predicting how much carbon dioxide will remain in our air, and how fast temperatures will rise.
Progress is being made at reducing the uncertainties in the climate debate. Other scientists focusing on the Arctic, Antarctic, the atmosphere and forests of the world are adding their own data. But it will still be years before the argument is resolved.
"We have a long ways to go with our models," Bullister said.
Originally appeared in The
Seattle Times, Tuesday, June 4, 1996. Reprinted in its entirety by
permission.
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