I all a dream.

        t sun inguisarsDid wander . . .

        —Byron, “Darkness”

        IN 1815 on t mountaiambora exploded spectacularly, killing a s blast andassociated tsunamis. It    volic explosion iimest St.    to sixty tom bombs.

        ravel terribly fast in times ran a small story—actually a letter from a merc—seven mo. But by time tambora’seffects . ty-six cubic miles of smoky as, and grit mospo cool.

        Sus    blearily colorful, an effect memorably captured by tist J. M.

        . turner, ly ted under anoppressive, dusky pall. It    inspired the Byron lines above.

        Spring never came and summer never summer. Crops everyed typy-five teen o Deats tinued until June andalmost no planted seed ock died or o be prematurelyslaug    certainly t for farmers iimes. Yet globally temperature fell by only about 1.5 degrees Fa. Eartural tat, as stists e instrument.

        teentury le Ice Age, as it ted all kinds of ry events—frost fairs on ting races along Dutc are mostly impossible no y ury geologists for beingsloo realize t t balmy pared    muc fair.

        t t. tteredic reindeer in tranded in improbable places—and ten came up ive but not terriblyplausible explanations. One Frencuralist named de Luc, trying to explain eboulders o rest one flanks of tains, suggestedt per t of apopgun. term for a displaced boulder is aic, but iury to apply more often to to the rocks.

        t Britis Arted t if James ton, ted Szerland,    oriations, telltale strand lines    point to passing ice ss. Unfortunately, ton    a traveler.

        But eve s, toed out of    up mountainsides byfloods—all ter in t make a boulder float, ed out—and becameone of t tue for ion. Unfortunately ice, andfor anotury most naturalists tio insist t ttributed to passing carts or even ts.

        Local    peasants,    uninated    by    stific ortter, uralist Jean de Cier told tory of rylaer o talking about tter matter-of-factly told    te some distaones ion,    ation: ‘transported t glacier extended in t as far as town of Bern.’ ”

        Cier    stific gat    friends uralist, Louis Agassiz, ial skepticism came to embrad eventually all but appropriate, theory.

        Agassiz udied under Cuvier in Paris and nouralory at tel in Szerland. Anotanistnamed Karl Scually t to    term ice age (in German Eiszeit ), in1837, and to propose t to s ice    just t over mu.    Agassiz es—tret it as Agassizincreasingly got t for imacy, was heory.

        Cier liketer enemy of , yetanot least partly in mind ages in stific discovery: first, people deny t it is true; t it isimportant; finally t the wrong person.

        At all events, Agassiz made t to uand tion,    everyo ts of t Alpine peaks, often apparently una eam    to climbtered an unyieldiao accept heories.

        urged o return to ise, fossil fis Agassiz was a man possessed by an idea.

        Agassiz’s t in Britain, uralists en couldn’t grasp t i bulk exerts. “Could scratc be due to ice ?” asked Roderick Mure at oing,evidently imagining t and glassy rime. to    incredulity at ts    for so mucy, endorsed t tion t ice could transportboulders presented “sucies” as to make it uny’s attention.

        Undaunted, Agassiz traveled tirelessly to promote o ameeting of tision for t of S Glasgo    Cy ofEdinburgion g t t be some general merit in t t certainly none of it applied to Scotland.

        Lyell did eventually e round.    of epip amoraine, or line of rocks, near ate in Scotland, ood if one accepted t a glacier ed, Lyell t    of t rating time fassiz. ly acg    of ier    speak to est living geologist offered support of only t tepid and vacillating kind.

        In 1846, Agassiz traveled to America to give a series of lectures and t last found teem    -rate museum, tive Zoology. Doubtless it    tled in Neain sympaterminable periods ofcold. It also    six years after    stific expedition to Greenlaed t nearly t semiti    like t one imagined in Agassiz’s t long last, o find a realfolloral defect of Agassiz’s t assistao e from an unlikely quarter.

        In tions in Britain began to receive papers onatics, electricity, and otific subjects from a James Croll of Anderson’sUy in Glasgoions i migated ice ages,    standard. So t a touc, urned out t Croll    an academic at ty, but a janitor.

        Born in 1821, Croll greed only to teen.    a variety of jobs—as a carpenter, insurance salesman, keeper of atemperance el—before taking a position as a janitor at Anderson’s (noy ofStrato do muco pass many quiet evenings in ty library teay, atics, and to produce a string of papers, icular empions ofEart on climate.

        Croll    to suggest t cyclical c, fromelliptical (o elliptical again, mig areat of ice ages. No one    before to sider an astronomicalexplanation for variations i eo Croll’s persuasivetain began to beore respoo tion t at some formertime parts of ty and aptitude    tland and widely honored:

        y in London and of ty of St. Andrews, among much else.

        Unfortunately, just as Agassiz’s t last beginning to find verts in Europe, aking it into ever more exotic territory in America. o find evidence flaciers practically everyually    iguised. None of ted supported suced try ature greil ly beloy.    it necessary to appoint to take hisplace.

        Yet, as sometimes ly out of faser e t ts may ion.”

        Part of t Croll’s putations suggested t t ret ice ageoccurred eigedt Eart of dramatic perturbation mucly t.

        it a plausible explanation for    migime except t in tin Milankovitcial motionsat all—raining—developed an ued i in tter. Milankovitc t t it    but t it oo simple.

        As Eart is subjeot just to variations in ts orbit, but also to rs in its angle of orientation to ts tilt and pitg tensity of sunligcicular it is subject to tion, knos obliquity,precession, and etricity, over long periods of time. Milankovitcbe a relationsy    t lengtely 20,000,40,000, and 100,000 years, but varying in eaco a fe t determining ts of interse over long spans of time involved a nearlyendless amount of devoted putation. Essentially Milankovitco    tion of ining solar radiation at every latitude oed for three ever-ging variables.

        of repetitive toil t suited Milankovitcemperament. For t ty years, even ables of    noed ina day or ter. tions all o be made in ime, but in1914 Milankovitc a great deal of t o ion as a reservist in t most of tfour years under loose    in Budapest, required only to report to t of ime     prisoner of war in ory.

        tual    oute    of     scribblings icalClimatology and tronomical tic Cc ttionsary    people    it    led to t eist, ladimir K?ppen—fateic friend Alfred egener—le, and rat.

        to be found in cool summers, not brutal ers.

        If summers are too cool to melt all t falls on a given area, more ining sunligive surface, exacerbating t and encimore snoo fall. tend to be self-perpetuating. As so an ice s, ting more ice to accumulate. As t G is not necessarily t of sno causes icess but t t snole, lasts.” It is t t an ice age could startfrom a single unseasonal summer. tover snos    and exacerbates t. “toppable, and omoves,” says McPhee. You have advang glaciers and an ice age.

        In t datis o correlateMilankovitc cycles es of ice ages as ts increasingly fell out of favor. o prove t . By time, e Joo find a geologist or meteist y.” Not until t of a potassium-argoing a seafloor sediments were ed.

        tougo explain cycles of ice ages. Many otors are involved—not least tion of tis, in particular t tly uood. It ed,    if you    t and inescapable ice ages. e are very lucky, itappears, to get any good    all. Even less ood are tive balminess erglacials. It is mildly unnerving toreflect t tory—t of farming, tionof toid ing and sd all t—aken placeypical patcerglacials ed as little as eigs ten th anniversary.

        t is, ill very muc’s just a some of t period of glaciation, aroundty t 30 pert of tenpert still is—and a furt is in a state of permafrost. ters of all ter o botuation t may be unique iory. t ters t glaciers even in temperate places suatural, but in fact it is a most unusual situation for t.

        For most of its ory until fairly ret times ttern for Earto be    iyed about fortymillion years ago, and o not bad at all. Ice ages tend to evidence of earlier ice ages, so tcuregro it appears t    seventeen severe glacial episodes in t 2.5million years or so—t cides us in Africafolloed culprits for t epo of t disrupting air flos. India, on island, ers into t forty-five million years, raising not only t alsot tibetan plateau be tonly cooler, but diverted    made to more susceptible to long-term c five millionyears ago, Panama rose from ting ts betlantid s of precipitation across at least    ofAfrica,    of trees and go looking for a nehe emerging savannas.

        At all events, is arranged as t appears t iceerm part of our future. Acc to Jo fifty mlacialepisodes    be expected, eag a haw.

        Before fifty million years ago, Eart o be colossal. A massive freezing occurred about 2.2 billion years ago, follo—se t some stists are noo t occurred as tion is more popularly knoh.

        “Snoions. tbecause of a fall in solar radiation of about 6 pert and a dropoff in tion (orretention) of greenially lost its ability to o its . Itbecame a kind of all-over Antarctica. temperatures plunged by as muc. tire surface of t may o    itudes and tens of yards tropics.

        t tes iceevery asfirmly t t er someosynt t, but as youo peer t, ice quickly bees opaque and after onlya feies ed. One is t alittle o er did remain exposed (pera    spot); t maybe t it remairanslut—a dition t does sometimes ure.

        If Eart question of    ever got    s so muc t it ay frozen forever. It appearst rescue may en interior. Once agaioteics for alloo be    s of    and gases t melted tmosperestingly, tburst—time event of life’s ory. In fact, it may not ranquil as all t. As Eart probably     o raise o ts of skyscrapersand rainfalls of indescribable iy.

        t all tubeo deep ovents undoubtedly    on as if not all ot ever o tirely. It ime ago and at tage    don’t know.

        pared burst, t times seem pretty smallscale, but of course tandards of anyto be found ooday. t,    a rate of about four    a year.

        a t must o be ts could benearly anding at t high.

        Be more ice, allest mountain summits poking tis sagged u of so mucertill rising bato place. ts didn’t just dribble outboulders and long lines of gravelly moraines, but dumped entire landmasses—Long Islandand Cape Cod and Nantucket, among ot along. It’s little    geologists befassiz rouble grasping tal capacity to reworklandscapes.

        If ice ss advanced again,    t Prince illiam Sound in Alaska, one of t glacial fields in Nort by tro earti. It measured 9.2 on ter scale. Along t liy feet. t, in fact, t it made er slos of pools in texas. And    did tburst    all. tsoaked it up a on moving.

        For a long time it    t o and out of ice ages gradually, over    t    been toice cores from Greenland e for somet is found t f. It s for most of its retory Eartable and tranquil place t civilization    ratly betal chill.

        to big glaciation, some toe rapidly, but tly plunged bato bitter cold for a t knoo sce as tit t to reize land after an ice s    it    so s t average temperatures leapt again, by as mucy years,erribly dramatic but is equivalent to exce ofSdinavia for t of terranean in just tures teen degrees in ten years, drastically altering rainfall patterns and groions. t tling enouged plaoday tty well unimaginable.

        is most alarming is t ural ply rattle Earter. As Elizabet, ing in ternal force, or even any t emperature bad fortly, and as often, as to be to be, s and terrible feedback loop,”

        probably involving tions of tterns of o circulation, butall tood.

        Oer to t tiness (and ty) of nortream to s to avoid a collision. Deprived of tream’s itudes returo s. But t begin toexplain veer as before. Instead, ranquility knoime in which we live now.

        to suppose t tretcic stability s much longer.

        In fact, some auties believe t     before. It isnatural to suppose t global    as a useful terendency to plunge bato glacial ditions.    ed out, uating and uable climate “t t todo is duct a vast unsupervised experiment on it.” It ed, y t first seem evident, t an ice age migually be induced by arise in temperatures. t a sligion rates andincrease cloud cover, leading in titudes to more persistent accumulations of snow.

        In fact, global o pohern Europe.

        Climate is t of so many variables—rising and falling carbon dioxide levels, ts of tis, solar activity, tately c it is asdifficult to pres of t as it is to predict ture. Mucake Antarctica. For at least ty million years after it settled over tarctica remained covered in plants and free of ice. t simply s havebeen possible.

        No less intriguing are te dinosaurs. tisStepes t forests itude of togreat beasts, including tyrannosaurus rex. “t is bizarre,” es, “for sucitude is tinually dark for t titudes suffered severe ers. Oxygen isotope studies suggest t te around Fairbanks, Alaska,    te Cretaceous period as it isnoyrannosaurus doing t migrated seasonally over enormousdistances or it spent mu tralia—time s orientation—a retreat to    possible. o survive in sus    only be guessed.

        Oo bear in mind is t if ts did start tain for    more er for to draime. t Lakes, less lakes of ada—t to fuel t ice age. tedby it.

        On t pory could see us melting a lot of ice rat. If all ts melted, sea levels of a ty-story building—and every coastal city in ted. Morelikely, at least in t term, is t Antarctic ice s. In t fiftyyears ters around it igrade, and collapses ically. Because of tty quickly—by beteen and ty feet on average.

        traordinary fact is t    knoeamy . Only oain: we live on a knife edge.

        In tally, ice ages are by no means bad uous ric freser lakest provide abundant nutritive possibilities for    as aspur to migration a dynamic. As tim Flannery ion you need ask of a ti in order to determie of its people: ‘Did you in mind, it’s time to look at a species of ape t trulydid.
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