Cool­ing not worth shiv­er­ing about


Record cold in Amer­ica has brought tem­per­a­tures as low as mi­nus 44 de­grees Cel­sius in North Dakota, frozen sharks in Mas­sachusetts and igua­nas fall­ing from trees in Florida. Al Gore blames global warm­ing, cit­ing one sci­en­tist to the ef­fect that this is "ex­actly what we should ex­pect from the cli­mate cri­sis". Oth­ers beg to dif­fer: Kevin Tren­berth, of Amer­ica’s Na­tional Cen­tre for At­mo­spheric Re­search, in­sists that "win­ter storms are a man­i­fes­ta­tion of win­ter, not cli­mate change".

Forty-five years ago a run of cold win­ters caused a "global cool­ing" scare. "A global de­te­ri­o­ra­tion of the cli­mate, by or­der of mag­ni­tude larger than any hith­erto ex­pe­ri­enced by civilised mankind, is a very real pos­si­bil­ity and in­deed may be due very soon," read a let­ter to Pres­i­dent Nixon in 1972 from two sci­en­tists re­port­ing the views of 42 "top" col­leagues. "The cool­ing has nat­u­ral causes and falls within the rank of the pro­cesses which caused the last ice age." The ad­min­is­tra­tion replied that it was "seized of the mat­ter".

In the years that fol­lowed, news­pa­pers, mag­a­zines and tele­vi­sion doc­u­men­taries rushed to sen­sa­tion­alise the com­ing ice age. The CIA re­ported a "grow­ing con­sen­sus among lead­ing cli­ma­tol­o­gists that the world is un­der­go­ing a cool­ing trend". The broad­caster Mag­nus Mag­nus­son pro­nounced on a BBC Hori­zon episode that "un­less we learn oth­er­wise, it will be pru­dent to sup­pose that the next ice age could be­gin to bite at any time".

This alarm about global cool­ing has largely been for­got­ten in the age of global warm­ing, but it has not en­tirely gone away. Valentina Zharkova of Northum­bria Uni­ver­sity has sug­gested that a qui­es­cent sun presages an­other Lit­tle Ice Age like that of 1300-1850. I’m not per­suaded. Yet the ar­gu­ment that the world is slowly slip­ping back into a proper ice age af­ter 10,000 years of balmy warmth is in essence true. Most in­ter­glacial pe­ri­ods, or times with­out large ice sheets, last about that long, and ice cores from Green­land show that each of the past three mil­len­nia was cooler than the one be­fore.

How­ever, those ice cores, and oth­ers from Antarc­tica, can now put our minds to rest. They re­veal that in­ter­glacials start abruptly with sud­den and rapid warm­ing but end grad­u­ally with many thou­sands of years of slow and er­ratic cool­ing. They have also be­gun to clar­ify the cause. It is a story that re­minds us how vul­ner­a­ble our civil­i­sa­tion is. If we as­pire to keep the show on the road for an­other 10,000 years, we will have to un­der­stand ice ages.

The old­est ex­pla­na­tion for the com­ing and go­ing of ice was based on car­bon diox­ide. In 1895 the Swede Svante Ar­rhe­nius, one of the sci­en­tists who first cham­pi­oned the green­house the­ory, sug­gested that the ice re­treated be­cause car­bon diox­ide lev­els rose, and ad­vanced be­cause they fell. If this were true, then in­dus­trial emis­sions could head off the next ice age. There is in­deed a cor­re­la­tion in the ice cores be­tween tem­per­a­ture and car­bon diox­ide, but in­con­ve­niently it is the wrong way round: car­bon diox­ide fol­lows rather than leads tem­per­a­ture down­ward when the ice re­turns.

A Ser­bian named Mi­lutin Mi­lankovich, writ­ing in 1941, ar­gued that ice ages and in­ter­glacials were in­stead caused by changes in the or­bit of the Earth around the sun. These changes, known as ec­cen­tric­ity, obliq­uity and pre­ces­sion, some­times com­bined to in­crease the rel­a­tive warmth of north­ern hemi­sphere sum­mers, melt­ing ice caps in North Amer­ica and Eura­sia and spread­ing warmth world­wide.

In 1976 Ni­cholas Shack­le­ton, a Cam­bridge physi­cist, and his col­leagues pub­lished ev­i­dence from deep-sea cores of cy­cles in the warm­ing and cool­ing of the Earth over the past half mil­lion years which fit­ted Mi­lankovich’s or­bital wob­bles. Pre­ces­sion, which de­cides whether the Earth is closer to the sun in July or in Jan­uary, is on a 23,000-year cy­cle; obliq­uity, which de­cides how tilted the axis of the Earth is and there­fore how warm the sum­mer is, is on a 41,000-year cy­cle; and ec­cen­tric­ity, which de­cides how rounded or elon­gated the Earth’s or­bit is and there­fore how close to the sun the planet gets, is on a 100,000-year cy­cle. When these com­bine to make a "great sum­mer" in the north, the ice caps shrink.

Game, set and match to Mi­lankovich? Not quite. The Antarc­tic ice cores, go­ing back 800,000 years, then re­vealed that there were some great sum­mers when the Mi­lankovich wob­bles should have pro­duced an in­ter­glacial warm­ing, but did not. To ex­plain these "miss­ing in­ter­glacials", a re­cent pa­per in Geo­science Fron­tiers by Ralph El­lis and Michael Palmer ar­gues we need car­bon diox­ide back on the stage, not as a green­house gas but as plant food.

The ar­gu­ment goes like this. Colder oceans evap­o­rate less mois­ture and rain­fall de­creases. At the depth of the last ice age, Africa suf­fered long mega-droughts; only small pock­ets of rain­for­est re­mained. Cru­cially, the longer an ice age lasts, the more car­bon diox­ide is dis­solved in the cold oceans. When the level of car­bon diox­ide in the at­mos­phere drops be­low 200 parts per mil­lion (0.02 per cent), plants strug­gle to grow at all, es­pe­cially at high al­ti­tudes. Deserts ex­pand. Dust storms grow more fre­quent and larger. In the Antarc­tic ice cores, dust in­creased markedly when­ever car­bon diox­ide lev­els got be­low 200 ppm. The dust would have be­gun to ac­cu­mu­late on the ice caps, es­pe­cially those of Eura­sia and North Amer­ica, which were close to deserts. Next time a Mi­lankovich great sum­mer came along, and the ice caps be­gan to melt, the ice would have grown dirt­ier and dirt­ier, years of de­posited dust com­ing to­gether as the ice shrank. The darker ice would have ab­sorbed more heat! from the sun and a run­away process of col­laps­ing ice caps would have be­gun.

All of hu­man civil­i­sa­tion hap­pened in an in­ter­glacial pe­riod, with a rel­a­tively sta­ble cli­mate, plen­ti­ful rain­fall and high enough lev­els of car­bon diox­ide to al­low the vig­or­ous growth of plants. Agri­cul­ture was prob­a­bly im­pos­si­ble be­fore then, and with­out its hugely ex­panded en­ergy sup­ply, none of the sub­se­quent flow­er­ing of hu­man cul­ture would have hap­pened.

That in­ter­glacial will end. Today the north­ern sum­mer sun­shine is again slightly weaker than the south­ern. In a few tens of thou­sands of years, our de­scen­dants will prob­a­bly be strug­gling with volatile weather, dust storms and air that can­not sup­port many crops. But that is a very long way off, and by then tech­nol­ogy should be more ad­vanced, un­less we pre­vent it de­vel­op­ing. The key will be en­ergy. With plen­ti­ful and cheap en­ergy our suc­ces­sors could thrive even in a fu­ture ice age, grow­ing crops, wa­ter­ing deserts, main­tain­ing rain­forests and even melt­ing ice caps.


Salma Hayek, left, and Ash­ley Judd at­tend the 75th Golden Globe Awards in black, as part of a cam­paign about sex­ual ha­rass­ment.

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