Studying Deep Ocean Currents for Clues to Climates

November 9, 1999 New York Times

By WILLIAM K. STEVENS

[List of verbs in the reading: appear, be, become, believe, conclude, cool off, create, discover, establish, have, make, mean, occur, play, postulate, rush, say, show, suggest, transport.

Notice how the verbs change as the purposes shift: (1) generalizations about the science in simple present tense, (2) connections between the past and the present with present perfect, and (3) simple past tense for the historical background.]

El Nino and La Nina, the warmer and colder phases of a perpetual oscillation in the surface temperature of the tropical Pacific Ocean that drastically affects weather around the world, _____ in recent years _____ [become] familiar agents of short-term climate change.

Now some scientists _____ [believe] they may have identified a comparable kind of oscillation involving deep ocean currents. They _____ [say] this oscillation could warm and cool substantial parts of the globe on longer time scales of centuries to millenniums.

The deep ocean currents, in effect, _____ [create] a global conveyor belt that transports great amounts of heat around the world. When the currents run strongly, for example, Western Europe and the North Atlantic region _____ [be] relatively warm; when they _____ [be] weak, they _____ [cool off].

One instance of this cooling, some experts now suspect, _____ [be] what is called the little ice age, an intermittent cold spell from the 14th to 19th centuries that at its coldest sent glaciers coursing down mountain slopes and condemned much of Europe to famine by freezing harvests.

Two years ago, researchers at the Lamont-Doherty Earth Observatory of Columbia University _____ [discover] that cold spells lasting several centuries, much like the little ice age, have occurred regularly, about every 1,400 to 1,500 years, since the end of the last full ice age some 10,000 years ago.

Much bigger temperature swings, they found, _____ [occur] at the same intervals during the ice age itself. They _____ [discover] this by analyzing climate signals contained in ocean sediments in the North Atlantic.

Now, by putting together different sorts of evidence, other researchers led by Dr. Wallace S. Broecker, a marine geochemist at Lamont-Doherty, _____ [postulate] that the oscillations in deep-ocean currents may be responsible for the 1,500-year cold cycles. The study by Broecker and Stewart Sutherland of Lamont-Doherty and Tsung-Hung Peng of the National Oceanic and Atmospheric Administration _____ [appear] in the current issue of the journal Science.

While there is a long way to go before Broecker's postulate can be proved or disproved, it _____ [appear] to provide a possible new explanation for what Dr. Richard Alley, a climate expert at Pennsylvania State University, called "a fundamental beat to the climate."

He said Broecker had "put together a very possible story."

"We need good hypotheses," Alley said, "and he's given us one."

Discerning and explaining the climate's fundamental beat _____ [be] important to a central climate question of the day: whether the atmosphere is warming because of natural changes or human-induced changes in the atmosphere, chiefly emissions of heat- trapping waste industrial gases, like carbon dioxide.

Broecker _____ [be] a primary advocate of the concept of the ocean conveyor belt as a climate regulator. The conveyor _____ [believe] to be driven by changes in the salt content of the ocean's surface.

When salt builds up in a particular spot in the North Atlantic, it eventually _____ [make] the water dense enough to sink to the ocean depths. Other water _____ [rushe] in to replace it at the surface. This combination _____ [be] what sets the conveyor moving. The bigger and stronger the main conveyor current, the more heat it _____ [transport].

Aside from the North Atlantic, there _____ [be] one other place here the conveyor gets a boost from sinking water (scientists call this production of deep water): the Weddell Sea near Antarctica.

By combining the results of a number of recent chemical and physical studies by several scientists, Broecker _____ [conclude] that the two sites do not produce roughly the same amount of deep water all the time, as had been thought. Rather, he _____ [say], the southern and northern sites alternate as strong producers: when one is giving a strong boost to the conveyor, the other is gving a weak one, with widespread consequences for climate.

A number of studies of past climates _____ [show] that when the North Atlantic region has been cold, as in the end of the last ice age, Antarctica has been relatively warm. And a recent study by Dr. Gary Clow, a geophysicist at the United States Geological Survey in Denver, based on an analysis of ancient Antarctic ice, _____ [suggest] that since the end of the last glaciation, Antarctica was warmer whenever Europe was colder, as in the little ice age. Finally, chemical evidence _____ [show] that the production of deep water off Antarctica has slowed to about a third of its rate a century or so ago, when the little ice age was ending, while production has remained strong in the North Atlantic. That _____ [mean] the conveyor belt in that region continues to draw heat from the south to warm the North Atlantic region, especially Western Europe.

No firm cause-and-effect link _____ [establish] between the conveyor's supposed oscillations and the 1,500-year recurrences of cold spells like the little ice age. But Broecker _____ [say] there is a clear relationship between the conveyor's behavior and climate oscillations in the last ice age. And he says that if the conveyor has indeed oscillated since then, as now seems the case, it suggests that the link may have persisted.

It _____ [be], he says, "consistent with the idea that changes in the large-scale circulation in the ocean can have very important effects on the atmosphere," and that "we’re playing with a system that we still don't understand."