A paper published this week in the journal Science supports the hypothesis that heat transfer by ocean currents – rather than global heating or cooling – may have been responsible for the global temperature patterns associated with the abrupt climate changes seen in the North Atlantic during the past 80,000 years.
Authored by the University of Bremen's Frank Lamy and colleagues, the paper provides new evidence that Southern Hemisphere climate may not have changed in step with Northern Hemisphere climate. Though these new measurements of ocean surface temperature off Chile are consistent with information from Antarctic ice core samples, they still contradict measurements made on land in the Southern Hemisphere – suggesting additional research will be needed to resolve the issue.
Scientists have found evidence of rapid and dramatic climate change that took place in a matter of decades during cool periods of the last 80,000 years in the North Atlantic. Knowing whether climate changes took place simultaneously in the Northern and Southern Hemispheres is vital to understanding the mechanism involved – and assessing whether similar abrupt climate change could be a threat today.
"People are very interested in these dramatic climate changes because they occur on very human time scales," said Jean Lynch-Stieglitz, associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology and author of a "Perspectives" article accompanying the Lamy paper in Science. "It's really important to understand what is causing them and what conditions are necessary for the climate to rapidly transition from cold to warm and back again."
To understand past climate conditions, scientists study ice cores taken from frozen areas such as Greenland and Antarctica, and sediment cores taken from the ocean floor. The Northern Hemisphere has been well studied, but comparatively little data exists about the Southern Hemisphere, which has more open ocean area which provides scant data.
And the information that exists about the Southern Hemisphere is contradictory, with pollen samples and land-based data from southern Chile and New Zealand suggesting climate change synchronized with the Northern Hemisphere – and Antarctic ice cores suggesting the opposite.
Lynch-Stieglitz, who co-authored an earlier paper based on less detailed South Atlantic data, believes the new paper represents progress toward understanding Southern Hemisphere climate change.
"The real significance of this paper is that it gets us closer to understanding the mechanism causing these rapid climate changes," she said. "Earlier sediment core work at lower resolution has suggested that the Southern Hemisphere has been doing its own thing. The record from Antarctica is nicely resolved and shows that the Southern Hemisphere is not participating either in magnitude or timing with the climate changes that have occurred in the North Atlantic."
The Lamy researchers studied sediment cores taken from a location off the coast of southern Chile where sediment builds up rapidly, providing detailed information about climate change with good time resolution. Their 50,000-year record is consistent with Antarctic ice core data showing that Southern Hemisphere climate change did not occur at the same or in the same magnitude as Northern Hemisphere change.
"What this paper suggests is that that when it was really cold off Greenland in the North Atlantic, it was actually a bit warm off Chile," said Lynch-Stieglitz. "That's very similar to the record in Antarctica. The fact that the ocean off Chile looks so much like what has been going on in Antarctica gives us hope that there may be a consistent response throughout the Southern Hemisphere."
Knowing what was happening in the Southern and Northern Hemispheres is important because the mechanisms that could have caused synchronized change differ dramatically from those that could have caused unsynchronized change.
Both hemispheres warming and cooling at the same time would imply global changes caused by rising levels of greenhouse gases. But one hemisphere cooling while the other warmed would suggest simple heat transfer, accomplished by changes in ocean or atmospheric currents.
"You can make the climate cool in certain places just by redistributing the heat through changes in ocean currents, atmospheric circulation or both," said Lynch-Stieglitz. "The most fully developed theory to account for these rapid climate changes is that they do represent changes in the transport of heat into the North Atlantic by what we call overturning circulation of the ocean."
In that scenario, warm water flows northward from the Southern Hemisphere into the North Atlantic, where it gives up its heat. Being denser, the cooled water then sinks and flows back south. The scenario accounts for both heating in the north and cooling in the south.
It's possible, Lynch-Stieglitz notes, that both global warming and changes in ocean heat transport occurred simultaneously, though records of carbon dioxide concentrations do not show concentration increases that would be enough by themselves to account for the climate change.
The Lamy paper provides the best measurement yet of ocean surface temperature in the Southern Hemisphere, but its information alone will not resolve the question of whether synchronized climates changes have occurred. Ultimately, geologists will have to find other ocean locations with sufficient sedimentation to settle the issue.
"Because so much of the Southern Hemisphere is open ocean, the sediments are accumulating very slowly in most locations," she added. "We've got to find other locations where sediments are accumulating rapidly."
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