Ancient Global Warming Flipped Ocean Circulation, May Do So Again

SAN DIEGO, California, January 5, 2006 (ENS) - For the first time, evidence that global warming triggered a reversal in the circulation of deep ocean patterns around the world has been uncovered by scientists affiliated with the Scripps Institution of Oceanography. While the changes they describe occurred 55 million years ago, the scientists say today's conditions are similar and could have similar drastic effects on ocean circulation.

In today's issue of the journal "Nature," scientists Flávia Nunes and Richard Norris describe how they examined a four to seven degree warming period that occurred some 55 million years ago during the closing stages of the Paleocene and the beginning of the Eocene eras.

"The Earth is a system that can change very rapidly," said Nunes. "Fifty-five million years ago, when the Earth was in a period of global warmth, ocean currents rapidly changed direction and this change did not reverse to original conditions for about 20,000 years."


Scientist Flávia Nunes on board an oceanographic research vessel (Photo courtesy Scripps)
The global warming of 55 million years ago, known as the Paleocene/Eocene Thermal Maximum (PETM), emerged in less than 5,000 years, an instant of geological time.

Modern carbon dioxide input to the Earth's atmosphere from fossil fuel sources is approaching the same levels estimated for the PETM period, say the scientists, which raises concerns about future climate and changes in ocean circulation.

They say the Paleocene/Eocene example suggests that changes produced by human activities may have lasting effects not only on global climate, but on deep ocean circulation.

Fossil records show that the global warming at the time of the PETM created changes ranging from a mass extinction of deep sea bottom dwelling marine life to migrations of terrestrial mammal species, as warm conditions may have opened travel routes frozen over when climates were colder. This time period is when scientists find the earliest evidence of horses and primates in North America and Europe.

Nunes and Norris base their findings on the chemical makeup of microscopic sea creatures that lived 55 million years ago.

The scientists analyzed carbon isotopes, or chemical signatures, from the shells of the one-celled animals called foraminifera, or "forams," that exist in vast numbers in a variety of marine environments.

"A tiny shell from a sea creature living millions of years ago can tell us so much about past ocean conditions," said Nunes. "We know approximately what the temperature was at the bottom of the ocean. We also have a measure of the nutrient content of the water the creature lived in. And, when we have information from several locations, we can infer the direction of ocean currents."

In the study, the scientists looked at a foram named Nuttalides truempyi from 14 sites around the world in deep-sea sediment cores retrieved via the Integrated Ocean Drilling Program, for which Joint Oceanographic Institutions, Inc., manages the U.S. component.

Nuttalides truempyi

An example of the single celled foram known as Nuttalides truempyi (Photo courtesy ODP)
Chemicals from the foram's shells were used as nutrient "tracers" to reconstruct changes in deep ocean circulation through the ancient time period.

Nutrient levels tell the researchers how long a sample has been near or isolated from the sea surface, giving them a way to track the age and path of deep sea water.

Nunes and Norris found that deep ocean circulation in the Southern Hemisphere abruptly stopped the conveyor belt-like process known as "overturning," in which cold and salty water in the depths exchanges with warm water on the surface.

Even as it was shutting down in the south, overturning appars to have became active in the Northern Hemisphere.

The researchers believe this shift drove unusually warm water into the deep sea, likely releasing stores of methane gas that led to further global warming and a massive die-off of deep sea marine life.


Richard Norris is a professor of paleobiology in the Geosciences Research Division at Scripps Institution of Oceanography. (Photo courtesy Scripps)
"Overturning is very sensitive to surface ocean temperatures and surface ocean salinity," said Norris, a professor of paleobiology in the Geosciences Research Division at Scripps. "The case described in this paper may be one of our best examples of global warming triggered by the massive release of greenhouse gases and therefore it gives us a perspective on what the long term impact is likely to be of today's greenhouse warming that humans are causing."

Overturning is a fundamental component of the global climate conditions we know today, said Bil Haq, program director in the National Science Foundation's division of ocean sciences, which funded the research.

Haq says overturning in the modern North Atlantic Ocean is a primary means of drawing heat into the far north Atlantic and keeping temperatures in Europe relatively warmer than conditions in Canada.

Today, deep water generation does not occur in the Pacific Ocean because of the large amount of freshwater input from the polar regions, which prevents North Pacific waters from becoming dense enough to sink to more than intermediate depths.

But in the Paleocene/Eocene, deep-water formation was possible in the Pacific because of global warming, the researchers say, adding that the Atlantic Ocean also could have been a significant generator of deep waters during this period.