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Is global warming preventing an Ice Age?

Dec. 17, 2008
Courtesy Uni­ver­s­ity of Wis­con­sin-Mad­i­son
and World Science staff

Al­though hu­man-caused glob­al warm­ing is po­ten­tially lead­ing the world in­to ec­o­log­i­cal ca­tas­tro­phe, it may al­so be spar­ing us from one of the Earth’s per­i­od­ic ice ages, some re­search­ers say.

The claim, even if cor­rect, by no means in­di­cates glob­al warm­ing is good: its fu­ture ef­fects are quite un­known, sci­en­tists say, whe­re­as ice ages, while cer­tainly un­pleas­ant, at least have prece­dents.

Al­though hu­man-caused glob­al warm­ing is po­ten­tially lead­ing the world in­to ec­o­log­i­cal ca­tas­tro­phe, it may al­so be spar­ing us from one of the Earth’s per­i­od­ic ice ages, some re­search­ers say. (Im­age cour­tesy USGS)
But as a mat­ter of sci­en­tif­ic cu­ri­os­ity, it’s worth not­ing that “in­creased gla­cia­tion... would probably be hap­pen­ing to­day” if hu­mans weren’t he­re, said John Kutz­bach, a cli­mate mod­el­er at the Uni­ver­s­ity of Wis­con­sin-Mad­i­son.

The theory is reminiscent of another re­cent piece of re­search, sug­gest­ing a bout of glo­bal warm­ing may have kept Earth from to­tally freez­ing over hun­dreds of mil­lions of years ago. Only now, re­search­ers, say, some­thing similar could be hap­pening to­day.

The con­tro­ver­sial idea—first pro­posed by Uni­ver­s­ity of Vir­gin­ia cli­ma­tolo­g­ist Wil­liam F. Rud­di­man—is based on the con­ten­ti­on that hu­man-induced glob­al warm­ing started long be­fore it’s gen­er­ally ac­cept­ed to have be­gun.

The com­mon wis­dom is that the ad­vent of the steam en­gine and the coal-fueled in­dus­t­ri­al age two cen­turies ago marked the be­gin­ning of hu­man in­flu­ence on glob­al cli­mate. But Kutz­bach and like­minded sci­en­tists con­tend it really started thou­sands of years ago with large-scale ag­ri­cul­ture in Asia and ex­ten­sive de­for­esta­tion in Eu­rope.

Al­though these pro­cesses would have been a much weaker in­flu­ence on cli­mate than in­dus­t­ri­al ac­ti­vity, their ef­fect be­comes im­por­tant be­cause of the long­er time per­i­od in­volved, said Ste­phen Vav­rus, a cli­ma­tolo­g­ist at the uni­ver­s­ity.

Both an­cient and mod­ern glob­al warm­ing would have had the same source: the re­lease in­to the atmosphere of so-called green­house gas­es that act like a blan­ket, trap­ping heat on Earth.

Green­house gas­es would have tak­en the form of meth­ane from ter­raced rice pad­dies in Asia and car­bon di­ox­ide from burn­ing forests in Eu­rope. The re­sult­ing warm­er atmosphere would have heat­ed the oceans, mak­ing them much less ef­fi­cient store­hous­es of car­bon di­ox­ide, re­in­forc­ing glob­al warm­ing, ac­cord­ing to Kutz­bach and Vav­rus.

The pa­ir pre­sented their re­search along with Gwe­naëlle Phi­lip­pon of the Sac­lay Cen­ter of Stud­ies in L’Orme des Me­ri­siers, France, at a meet­ing of the Amer­i­can Geo­phys­i­cal Un­ion in San Fran­cis­co Dec. 17.

“No one dis­putes the large rate of in­crease in green­house gas­es with the In­dus­t­ri­al Revoluti­on,” Kutz­bach notes. “The large-scale burn­ing of coal for in­dustry has swamped eve­ry­thing else” in the rec­ord, he added.

But look­ing ear­li­er, us­ing cli­mat­ic ar­chives such as 850,000-year-old ice from Ant­arc­ti­ca, sci­en­tists are teas­ing out ev­i­dence of past green­house gas­es in the form of fos­sil air trapped in the ice, the group said. That an­cient air, the re­search­ers said, con­tains the sig­na­ture of in­creased lev­els of at­mos­pher­ic meth­ane and car­bon di­ox­ide be­gin­ning thou­sands of years be­fore the in­dus­t­ri­al age.

“Be­tween 5,000 and 8,000 years ago, both meth­ane and car­bon di­ox­ide started an up­ward trend,” ex­plains Kutz­bach.

Ice ages, or gla­cial per­i­ods, have oc­curred at reg­u­lar 100,000-year in­ter­vals dur­ing the last milli­on years. Each per­i­od has been paced by reg­u­lar and pre­dict­a­ble changes in the or­bit of the Earth known as Mi­lankovitch cy­cles, a mech­an­ism thought to kick start gla­cial cy­cles, Kutzbach and col­leagues ex­plained.

“We’re at a very fa­vor­a­ble state right now for in­creased glacia­tion,” said Kutz­bach. “Na­ture is fa­vor­ing it at this time in or­bital cy­cles.” Im­por­tant­ly, the new re­search un­der­scores the key role of green­house gas­es in in­flu­enc­ing Earth’s cli­mate, he added. Whe­reas de­creas­ing green­house gas­es in the past helped in­i­ti­ate glacia­tions, the early ag­ri­cul­tur­al and re­cent in­dus­t­ri­al in­creases in green­house gas­es may be fore­stalling them, say Kutz­bach and Vav­rus.

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Although human-caused global warming is potentially leading the world into ecological catastrophe, it may also be sparing us from one of the Earth’s periodic ice ages, some researchers say. The claim, even if correct, by no means indicate global warming is good: its future effects are quite unknown, scientists say, whereas ice ages, while certainly unpleasant, at least have precedents. But as a matter of scientific curiosity, it’s worth noting that “increased glaciation... would probably be happening today” if humans weren’t here, said John Kutzbach, a climate modeler at the University of Wisconsin-Madison. The controversial idea—first proposed by University of Virginia climatologist William F. Ruddiman—is based in turn on another unusual contention, Kutzbach said. This is that human-induced global warming started many years before it’s generally accepted to have begun. The common wisdom is that the advent of the steam engine and the coal-fueled industrial age two centuries ago marked the beginning of human influence on global climate. But Kutzbach and likeminded scientists contend it really started thousands of years ago with large-scale agriculture in Asia and extensive deforestation in Europe. Although these processes would have been a much weaker influence on climate than industrial activity, their effect becomes important because of the longer time period involved, said Stephen Vavrus, a climatologist at the university. Both ancient and modern global warming would have had the same source: the release into the atmosphere of so-called greenhouse gases that act like a blanket, trapping heat on Earth. Greenhouse gases would have taken the form of methane from terraced rice paddies in Asia and carbon dioxide from burning forests in Europe. The resulting warmer atmosphere would have heated the oceans, making them much less efficient storehouses of carbon dioxide, reinforcing global warming, according to Kutzbach and Vavrus. The pair presented their research Dec. 17, along with Gwenaëlle Philippon of the Saclay Center of Studies in L’Orme des Merisiers, France, at a meeting of the American Geophysical Union in San Francisco. “No one disputes the large rate of increase in greenhouse gases with the Industrial Revolution,” Kutzbach notes. “The large-scale burning of coal for industry has swamped everything else” in the record, he added. But looking earlier, using climatic archives such as 850,000-year-old ice from Antarctica, scientists are teasing out evidence of past greenhouse gases in the form of fossil air trapped in the ice, the group said. That ancient air, the researchers said, contains the signature of increased levels of atmospheric methane and carbon dioxide beginning thousands of years before the industrial age. “Between 5,000 and 8,000 years ago, both methane and carbon dioxide started an upward trend,” explains Kutzbach. Ice ages, or glacial periods, have occurred at regular 100,000-year intervals during the last million years. Each period has been paced by regular and predictable changes in the orbit of the Earth known as Milankovitch cycles, a mechanism thought to kick start glacial cycles, Kutzbach and colleagues explained. “We’re at a very favorable state right now for increased glaciation,” said Kutzbach. “Nature is favoring it at this time in orbital cycles.” Importantly, the new research underscores the key role of greenhouse gases in influencing Earth’s climate, he added. Whereas decreasing greenhouse gases in the past helped initiate glaciations, the early agricultural and recent industrial increases in greenhouse gases may be forestalling them, say Kutzbach and Vavrus.