Change in ocean currents
Abrupt Climate Change: Should We Be Worried? ShareThis E-mail Print PDF
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THE GLOBAL OCEAN CONVEYOR—The global ocean circulation system, often called the Ocean Conveyor, transports heat throughout the planet. White sections represent warm surface currents. Purple sections represent deep cold currents. (Illustration by Jayne Doucette, WHOI)
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DRAMATIC CHANGES IN THE NORTH ATLANTIC—Subpolar seas bordering the North Atlantic have become noticeably less salty since the mid-1960s, especially in the last decade. This is the largest and most dramatic oceanic change ever measured in the era of modern instruments. This has resulted in a freshening of the deep ocean in the North Atlantic, which in the past disrupted the Ocean Conveyor and caused abrupt climate changes. (B. Dickson, et. al., in Nature, April 2002)
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DRAMATIC CHANGES IN THE NORTH ATLANTIC—Subpolar seas bordering the North Atlantic have become noticeably less salty since the mid-1960s, especially in the last decade. This is the largest and most dramatic oceanic change ever measured in the era of modern instruments. This has resulted in a freshening of the deep ocean in the North Atlantic, which in the past disrupted the Ocean Conveyor and caused abrupt climate changes. (B. Dickson, et. al., in Nature, April 2002)
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A LONG RECORD OF ABRUPT CLIMATE CHANGES—Ice cores extracted from the two-mile thick Greenland ice sheet preserve records of ancient air temperatures. The records show several times when climate shifted in time spans as short as a decade.
The Younger Dryas—about 12,700 years ago, average temperatures in the North Atlantic region abruptly plummeted nearly 5°C and remained that way for 1,300 years before rapidly warming again.
The 8,200-Year Event—A similar abrupt cooling occurred 8,200 years ago. It was not so severe and lasted only about a century. But if a similar cooling event occurred today, it would be catastrophic.
The Medieval Period—An abrupt warming took place about 1,000 years ago. It was not nearly so dramatic as past events, but it nevertheless allowed the Norse to establish settlements in Greenland.
The Little Ice Age—The Norse abandoned their Greenland settlements when the climate turned abruptly colder 700 years ago. Between 1300 and 1850, severe winters had profound agricultural, economic, and political impacts in Europe. (R.B. Alley, from The Two-Mile Time Machine, 2000)
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8,200 YEARS AGO—AN ABRUPTLY COLDER, DRIER EARTH—Rapid changes in ocean circulation are linked to an abrupt climate change 8,200 years ago that had global effects. Some regions turned significantly colder while others experienced widespread drought. (R.B. Alley, et al., in Geology, 1997)
Related Multimedia
The Ocean Conveyor
The Ocean Conveyor is propelled by the sinking of cold, salty (and therefore denser) waters in the North Atlantic Ocean (blue lines). That creates a void that pulls warm, slaty surface waters northward (red lines). The ocean gives up its heat to the atmosphere above the North Atlantic Ocean, and prevailing winds (large red arrows) carry the heat eastward to warm Europe.
Illustration and animation by Jack Cook, WHOI
» View Video (Quicktime)
If Too Much Fresh Water Enters the North Atlantic
If too much fresh water enters the North Atlantic, its waters could stop sinking. The Conveyor would cease. Heat-bearing Gulf Stream waters (red lines) would no longer flow into the North Atlantic, and European and North American winters would become more severe.
Illustration and animation by Jack Cook, WHOI
» View Video (Quicktime)
Abrupt Climate Change: Should We Be Worried? : Woods Hole Oceanographic Institution
Robert B. Gagosian
President and Director
Woods Hole Oceanographic Institution
Prepared for a panel on abrupt climate change at the
World Economic Forum
Davos, Switzerland, January 27, 2003
Are we overlooking potential abrupt climate shifts?
Most of the studies and debates on potential climate change, along with its ecological and economic impacts, have focused on the ongoing buildup of industrial greenhouse gases in the atmosphere and a gradual increase in global temperatures. This line of thinking, however, fails to consider another potentially disruptive climate scenario. It ignores recent and rapidly advancing evidence that Earth’s climate repeatedly has shifted abruptly and dramatically in the past, and is capable of doing so in the future.
Fossil evidence clearly demonstrates that Earthvs climate can shift gears within a decade, establishing new and different patterns that can persist for decades to centuries. In addition, these climate shifts do not necessarily have universal, global effects. They can generate a counterintuitive scenario: Even as the earth as a whole continues to warm gradually, large regions may experience a precipitous and disruptive shift into colder climates.
This new paradigm of abrupt climate change has been well established over the last decade by research of ocean, earth and atmosphere scientists at many institutions worldwide. But the concept remains little known and scarcely appreciated in the wider community of scientists, economists, policy makers, and world political and business leaders. Thus, world leaders may be planning for climate scenarios of global warming that are opposite to what might actually occur.1
It is important to clarify that we are not contemplating a situation of either abrupt cooling or global warming. Rather, abrupt regional cooling and gradual global warming can unfold simultaneously. Indeed, greenhouse warming is a destabilizing factor that makes abrupt climate change more probable. A 2002 report by the US National Academy of Sciences (NAS) said, “available evidence suggests that abrupt climate changes are not only possible but likely in the future, potentially with large impacts on ecosystems and societies.”2
The timing of any abrupt regional cooling in the future also has critical policy implications. An abrupt cooling that happens within the next two decades would produce different climate effects than one that occurs after another century of continuing greenhouse warming.