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Ocean current changes predicted to be gradual

July 18, 2009
Courtesy Oregon State University
and World Science staff

In a rare bit of hopeful news linked to glob­al warm­ing, find­ings of a ma­jor new study are con­sist­ent with grad­u­al changes of cur­rent sys­tems in the North At­lan­ti­c Ocean, rath­er than a more sud­den shut­down.

The lat­ter, more fright­en­ing sce­nar­i­o would probably trig­ger rap­id cli­mate changes in Eu­rope and else­where, sci­en­tists say. 

They stress that the find­ings, while en­cour­ag­ing, don’t change broader con­cerns about glob­al warm­ing.

The re­search, based on a giant computer sim­u­lat­ion of Earth’s cli­mate for 21,000 years back to the height of the last Ice Age, in­di­cates ma­jor changes in im­por­tant ocean cur­rent sys­tems can oc­cur, but they may take place more slowly and grad­u­ally than had been sug­gested.

The find­ings, pub­lished Fri­day in the re­search jour­nal Sci­ence, are con­sist­ent with oth­er re­cent stud­ies that are mov­ing away from the the­o­ry of an ab­rupt “tip­ping point” that might cause dra­mat­ic at­mos­pher­ic tem­per­a­ture and ocean cir­cula­t­ion changes in as lit­tle as 50 years.

“For those who have been con­cerned about ex­tremely ab­rupt changes in these ocean cur­rent pat­terns, that’s good news,” said Pe­ter Clark, a geo­scien­tist at Or­e­gon State Un­ivers­ity. 

“In the past it ap­pears the ocean did change ab­ruptly, but only be­cause of a sud­den change in the forc­ing,” he said. “But when the ocean is forced grad­u­ally, such as we an­ti­cipate for the fu­ture, its re­sponse is grad­u­al. That would give ecosys­tems more time to ad­just to new con­di­tions.”

Glob­al tem­per­a­tures are still pro­jected to in­crease about four to 11 de­grees by the end of this cen­tu­ry, Clark said, and the study ac­tu­ally con­firms that some of the most soph­is­t­icated cli­mate mod­els are ac­cu­rate.

“The find­ings from this stu­dy, which al­so match oth­er da­ta we have on recorded cli­mate change, are an im­por­tant val­ida­t­ion of the glob­al cli­mate mod­els,” Clark said. “They seem to be ac­cu­rately re­flect­ing both the type and speed of changes that have tak­en place in the past, and that in­creases our abil­ity to trust their pre­dic­tions of the fu­ture.”

The in­tens­ity of com­puta­t­ion on this ex­pe­ri­ment, in­volv­ing a quad­ril­lion cal­cula­t­ions each sec­ond, was so great that it took more than a year to run, Clark said. It was the longest such study of its type that ev­er ex­am­ined past cli­mate in such de­tail and com­plex­ity. The re­search was sup­ported by the Na­tional Sci­ence Founda­t­ion and oth­er agen­cies.

It in­clud­ed the height of the last Ice Age about 21,000 years ago, the emer­gence of the Earth from that Ice Age around 14,000 years ago, and some oth­er fairly sud­den warm­ing and cool­ing events dur­ing those pe­ri­ods that are of in­ter­est to re­search­ers.

The pe­ri­od when the Earth emerged from its last Ice Age ac­tu­ally had amounts of nat­u­ral warm­ing si­m­i­lar to those that may be ex­pected in the next cen­tu­ry or two, with some of the same ef­fect­s—melt­ing of ice sheets, sea lev­el rise, in­creases in at­mos­pher­ic car­bon di­ox­ide. Stud­ies of those pe­ri­ods, re­search­ers say, will pro­vide val­u­a­ble in­sights in­to how the Earth may re­spond to its cur­rent warm­ing.

A par­tic­u­lar con­cern for some time has been the opera­t­ion of an ocean cur­rent pat­tern called the At­lan­ti­c me­rid­i­o­nal over­turn­ing cir­cula­t­ion. This cur­rent sys­tem is part of what keeps Eu­rope much warm­er than it would oth­erwise be, giv­en its far north­ern lat­i­tudes, and there is ev­i­dence that it has “shut down” with some reg­u­lar­ity in Earth’s past—ap­par­ently in re­sponse to large in­fluxes of fresh wa­ter, and some­times quite rap­idly.

“Our da­ta still show that cur­rent is slow­ing, and may de­cline by 30 per­cent by the end of this cen­tu­ry,” Clark said. “That’s very sig­nif­i­cant, and it could cause substan­ti­al cli­mate change. But it’s not as ab­rupt as some con­cerns that it could shut down with­in a few dec­ades.”

Cli­mate changes, Clark said, are ac­tu­ally con­tin­u­ing to oc­cur some­what more rap­idly than had been pre­dicted in re­cent years. Arc­tic Sea ice is both thin­ning and shrink­ing, and at­mos­pher­ic car­bon di­ox­ide lev­els are go­ing up faster than had been pro­jected by the In­ter­gov­ern­ment­al Pan­el on Cli­mate Change.

* * *

In a rare bit of good news linked to global warming, findings of a major new study are consistent with gradual changes of current systems in the North Atlantic Ocean, rather than a more sudden shutdown. The latter, more frightening scenario would probably trigger rapid climate changes in Europe and elsewhere, scientists say. They stress that the findings, while encouraging, don’t change broader concerns about global warming. The research, based on the longest experiment of its type ever run on a “general circulation model” that simulated the Earth’s climate for 21,000 years back to the height of the last Ice Age, indicates major changes in important ocean current systems can occur, but they may take place more slowly and gradually than had been suggested. The findings, published Friday in the research journal Science, are consistent with other recent studies that are moving away from the theory of an abrupt “tipping point” that might cause dramatic atmospheric temperature and ocean circulation changes in as little as 50 years. “For those who have been concerned about extremely abrupt changes in these ocean current patterns, that’s good news,” said Peter Clark, a professor of geosciences at Oregon State University. “In the past it appears the ocean did change abruptly, but only because of a sudden change in the forcing,” he said. “But when the ocean is forced gradually, such as we anticipate for the future, its response is gradual. That would give ecosystems more time to adjust to new conditions.” Global temperatures are still projected to increase about four to 11 degrees by the end of this century, Clark said, and the study actually confirms that some of the most sophisticated climate models are accurate. “The findings from this study, which also match other data we have on recorded climate change, are an important validation of the global climate models,” Clark said. “They seem to be accurately reflecting both the type and speed of changes that have taken place in the past, and that increases our ability to trust their predictions of the future.” The intensity of computation on this experiment, involving a quadrillion calculations each second, was so great that it took more than a year to run, Clark said. It was the longest such study of its type that ever examined past climate in such detail and complexity. The research was supported by the National Science Foundation and other agencies. It included the height of the last Ice Age about 21,000 years ago, the emergence of the Earth from that Ice Age around 14,000 years ago, and some other fairly sudden warming and cooling events during those periods that are of interest to paleoclimatologists. The period when the Earth emerged from its last Ice Age actually had amounts of natural warming similar to those that may be expected in the next century or two, with some of the same effects—melting of ice sheets, sea level rise, increases in atmospheric carbon dioxide. Studies of those periods, researchers say, will provide valuable insights into how the Earth may respond to its current warming. A particular concern for some time has been the operation of an ocean current pattern called the Atlantic meridional overturning circulation. This current system is part of what keeps Europe much warmer than it would otherwise be, given its far northern latitudes, and there is evidence that it has “shut down” with some regularity in Earth’s past—apparently in response to large influxes of fresh water, and sometimes quite rapidly. “Our data still show that current is slowing, and may decline by 30 percent by the end of this century,” Clark said. “That’s very significant, and it could cause substantial climate change. But it’s not as abrupt as some concerns that it could shut down within a few decades.” Climate changes, Clark said, are actually continuing to occur somewhat more rapidly than had been predicted in recent years. Arctic Sea ice is both thinning and shrinking, and atmospheric carbon dioxide levels are going up faster than had been projected by the Intergovernmental Panel on Climate Change.