Over the past 50 million years, theEarth’s climate has been cooling(Fig. 1). Although Antarctica has

been glaciated for at least the past 35 millionyears1, large ice sheets did not appear in theNorthern Hemisphere until about 2.7 mil-lion years ago. Earth scientists largely agreethat overall climate cooling is associatedwith decreasing levels of carbon dioxide inthe atmosphere2,3, and that ice sheets canonly grow if sufficient moisture is availableand winter snow survives the summer heat4.But what triggered the onset of the ice ages 2.7 million years ago? Explanations havefocused on continental temperatures4,5, withidentification of potential moisture sourcesfrom the Atlantic5,6, but there remain manyopen questions7.

Haug et al.8 (page 821 of this issue) con-tribute an important piece to the ice-agepuzzle. Geochemical evidence suggests that,2.7 million years ago, the seasonal tempera-ture contrast of the subarctic Pacific Oceansea surface became larger as summerswarmed and winters cooled.Warmer summersea-surface temperatures result in a warmeratmosphere that can hold more moisture.Like a snow gun blasting away at ski slopes,westerly winds blow the moisture onto thecold North American continent where it falls as snow and accumulates as ice (Fig. 1,inset; Fig.2,overleaf).

Haug et al. have combined geochemicalexpertise with numerical modelling to pre-sent an integrated approach to the origin ofthe ice ages.Evidence comes from the floor ofthe subarctic Pacific Ocean, on which theremains of certain species of marine plank-ton (diatoms, coccolithophores and foram-inifera) have accumulated over time. Theprimary evidence for summertime warming2.7 million years ago stems from the bio-chemistry of coccolithophores, which variesaccording to temperature9. Augmenting this well-established index are the 18O/16Oratios in the siliceous tests of diatoms, acomparatively more complex measure ofpalaeotemperatures10.

At first glance, the results from these tworecorders contrast with other climate indica-tors in this region. Foraminiferal 18O/16Oratios — a classical indicator11 — from thesame deep-sea sediments suggest sea-surfacecooling 2.7 million years ago. This particularevidence is corroborated by perhaps themost intuitive indicator of climatic cooling,

an increase in the amount of debris of conti-nental origin delivered to the site by icebergs.

How can these apparently contradictoryobservations be reconciled? Haug et al.8

point to seasonal changes in the biologicalcommunities of the subarctic Pacific Oceanwhere, in modern times, different planktoncommunities populate the various seasons.Coccolithophores and those species ofdiatoms used for the geochemical analysesprefer the warm ocean surface of latesummer and autumn. The particular fora-miniferan species used for analysis, on theother hand, are more prolific during latewinter and spring when the sea surface is fertile due to mixing with deeper, colder,nutrient-rich water.

Thus the two seemingly opposing tem-perature trends 2.7 million years ago simplyreflect an increase in seasonality in the sub-arctic Pacific Ocean, which is consistent withother reconstructions of events in the NorthPacific Ocean7. This then provides the con-figuration on which to build an ice age:

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late winter cooling reflects climate cooling,allowing snow to accumulate; late summerwarming increases the atmosphere’s poten-tial to hold moisture and to load the snowgun (Fig.1, inset).

What,then,caused the sudden increase inlate summer temperatures? To answer thisquestion, Haug et al.8 refer to the physicalproperties of sea water itself.Water has a highheat capacity, which means that the surfaceocean remains warm long after overlying airand adjacent land masses have cooled. Ifthere is mixing of the surface ocean withdeeper and cooler water, however, surfacewaters cannot warm up. This was the situa-tion before 2.7 million years ago, evidencefor which comes from the high accumula-tion rates of diatom skeletal remains at thestudy site, implying vigorous diatom pro-ductivity in the overlying sea surface andtherefore a continuous supply of nutrientsfrom deeper waters.

At 2.7 million years ago, the abundance of diatom remains plummets, suggesting a

Snow maker for the ice agesKatharina Billups

In the Northern Hemisphere, large-scale glaciation was initiatedcomparatively recently. Paradoxically, it seems that the trigger was aseasonal warming of the sea surface in an upwind oceanic region.

Figure 1 Global climate change over the past 60 million years. This record, showing a mainly coolingtrend, is inferred from foraminiferal oxygen-isotope records from all major ocean basins1 with18O/16O ratios plotted as per mil deviation from a standard (�18O). The horizontal grey bars indicatethe relative extent of polar ice sheets — light grey, ice volumes less than half of the maximum extent;dark grey, ice volumes close to the maximum extent (after ref. 1). Haug et al.8 provide evidence thatthe initiation of the Northern Hemisphere ice ages, 2–3 million years ago (arrow), was linked to thedevelopment of a stratified sea surface in the subarctic Pacific, which resulted in warmer sea-surfacetemperatures in late summer. Inset, the warmer sea surface was a source of moisture for the overlyingatmosphere, with westerly winds loading the snow gun that produced large ice sheets on the NorthAmerican continent. The star indicates the authors’ study site8.

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decrease in the nutrient availability, like thatbrought about by the sea surface being cut offfrom the deeper ocean, at least on a seasonalbasis. At the same time, the reduction in ver-tical mixing allows the sea surface to warm.Thus the development of a seasonally lay-ered, or stratified, surface ocean 2.7 millionyears ago, which was probably a regionalresponse to the large-scale climatic changesat this time12, allowed late summer/autumnwarming of the sea surface and provided amoisture source for ice growth.

Haug et al.8 test the interpretations of thegeochemical records with a suite of numeri-cal computer-model experiments. The sim-ulated ocean is ‘stratified’ and ‘destratified’to determine whether this mechanism canaccount for the geochemically derivedchanges in temperature. And it can. Thestratified model state produces more extremeseasons and a larger North American icesheet than does the destratified model.

This is an exemplary study. The individ-ual climate indicators may not have with-stood the uncertainties and assumptionsthat limit each of them, but put together byHaug et al. they tell a cogent story of theorigin of the ice ages. ■

Katharina Billups is at the College of MarineStudies, University of Delaware, 700 PilottownRoad, Lewes, Delaware 19958, USA.e-mail: kbillups@udel.edu1. Zachos, J. et al. Science 292, 686–693 (2001).

2. Raymo, M. E. & Ruddiman, W. F. Nature 359, 117–122 (1992).

3. Pearson, P. N. & Palmer, M. R. Nature 406, 695–699 (2000).

4. Milankovitch, M. Serb. Akad. Beogr. Spec. Publ. 132 (1941).

5. Haug, G. H. & Tiedemann, R. Nature 393, 673–676 (1998).

6. Driscoll, N. W. & Haug, G. H. Science 282, 436–438 (1998).

7. Ravelo, A. C. et al. Nature 429, 263–267 (2004).

8. Haug, G. H. et al. Nature 433, 821–825 (2005).

9. Wefer, G., Berger, W. H., Bijma, J. & Fischer, G. in Use of Proxies

in Paleoceanography (eds Fischer, G. & Wefer G.) 1–68

(Springer, Berlin, 1999).

10.Brandiss, M. E., O’Neil, J. R., Edlund, M. B. & Stoermer, E. F.

Geochim. Cosmochim. Acta 62, 1119–1125 (1998).

11.Emiliani, C. J. Geol. 63, 538–578 (1955).

12.Haug, G. H. et al. Nature 401, 779–782 (1999).

Hearing

Aid from hair forceCorné Kros

Mammals hear with exquisite sensitivity and precision over a hugerange of frequencies; tiny amplifiers in the inner ear make this possible.New results challenge current thinking on how these amplifiers work.

Our ability to hear relies on cells in theinner ear called hair cells — namedafter the bundle of 100 or so hair-

like projections that protrudes from theirupper surfaces. Sound bends the hair bun-dles, causing small electrical (‘transducer’)currents to flow, which in turn makes thehair cells signal the reception of sound tothe brain. In mammals, the silent majorityof the hair cells (the outer hair cells) do nottalk to the brain, instead helping the innerhair cells — the true sensory receptors — todo so with more clarity than they could

achieve by themselves. But how is this done?For two decades scientists have sought

the answer in the extraordinary ability of theouter hair cells to change their length rapidlywhen stimulated. Now, however, Kennedy,Crawford and Fettiplace1 (page 880 of thisissue) and Chan and Hudspeth2 (in NatureNeuroscience) present provocative evidencethat the main component of the elusive‘cochlear amplifier’ may instead reside in thehair bundles of the outer hair cells.

Sound waves that reach the ear lead tovibrations inside the cochlea — a fluid-filled,

coiled tube forming the auditory part of theinner ear (Fig. 1a). The sensory hair cellsreside in a thin strip of tissue, the organ ofCorti, that is wedged between two mem-branes of the cochlea. The vibrations cause ashearing motion between these two mem-branes, which bends the hair bundles. Like arolled-up piano, one end of the organ ofCorti vibrates best at low frequencies and theother at high frequencies. In normal ears, thevibration is boosted and sharpened for softsounds by what has become known as thecochlear amplifier3.

Twenty years ago, a remarkable discoveryby Brownell and colleagues4 seemed to showwhat the cochlear amplifier is made of: theyfound that electrical stimulation of the outerhair cells made them lengthen and shortentheir cell bodies. The idea is that, in vivo, theelectricity produced by bending the hairbundles would drive this lengthening andshortening, or electromotility, as fast assound could vibrate the bundles. The strate-gic position of the outer hair cells wouldlocally boost the vibration of the organ ofCorti, and in this way stimulate, by fluidcoupling, the bundles on inner hair cells.

At a molecular level, this mechanism isthought to rely on a motor protein calledprestin, named from the musical term for avery fast tempo. The basolateral membranesof the outer hair cells are packed with thisprotein5, which changes shape as fast as youcan change the voltage across the membrane,over a range of frequencies up to at least 100kHz (ref. 6). But there is a snag: althoughprestin is quick, it is not clear whether thetransmembrane voltage in vivo changesmuch over the period of the sound wave, atsound frequencies greater than a few kilo-hertz. This is because the receptor potentialdue to the transducer currents is severelyattenuated at higher frequencies by theelectrical impedance of the cell7.

An alternative source of force that is notvoltage-dependent may thus be needed topower the cochlear amplifier. Kennedy andcolleagues1 report large forces generated bythe hair bundles of rat outer hair cells in vitro,when stimulated by a flexible glass fibre.Youwould expect the tip of the fibre to move lesswhen attached to the bundle than when it isfreely moving in the fluid.So there must havebeen disbelief in the lab when, in some cases,the fibre moved further when coupled to thehair bundles, implying that a force in thebundle drags the fibre along, instead of theother way round.

This force — an order of magnitude largerthan the force that is a necessary by-productof opening the ion channels through whichthe transducer current enters hair cells8 — isnot there at the moment the hair bundle is moved by the fibre, but develops within a fraction of a millisecond. Its time course is closely coupled to that over which thetransducer current adapts to a steady

Figure 2 Icy evidence in the Northern Hemisphere: a present-day ice-sheet on the Svalbard Islands.

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