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over a very short interval of cosmic time. Iftotal hydrogen absorption at z ~ 6 is con-firmed with a higher signal-to-noise ratioobservation, it would signal a suddenchange in the properties of the intergalacticmedium, perhaps by the ‘switching on’ ofquasars and galactic star formation, both ofwhich produce strong ionizing ultravioletradiation.

The record for the highest redshiftobject in the known Universe is transientthese days, and the differences in cosmic‘look-back’ time for large jumps in redshiftare very small. The significance of this dis-covery lies more in the promise of a system-atic study of these most distant objects, ofwhich only a handful are known today.There are critical questions waiting foranswers. Most importantly, what are theseobjects and what is their evolutionary fate?Their spectra can be interpreted as revealingactive star formation at the rate of 4–25solar masses per year3. Such values are lowerlimits4 because of the obscuring effect ofintervening dust. Brighter objects have lowheavy-element content, implying that thestarlight comes from one of the earliestgenerations of star formation5. The typicalmass is still unknown, although it is likelyto be sub-galactic. The suspicion is thatthese objects appear luminous because ofstrong star formation.

We observe these objects as they were inthe past, but whether they will ultimatelybecome typical spiral galaxies like ourMilky Way, or the cores of massive ellipticalgalaxies is a mystery. The answer will helpus choose between competing theories ofgalaxy formation. High-redshift objects(z ¤ 3) may be regarded as the precursorsto large, ‘normal’ galaxies because theirapproximate space density at ~10% of thecurrent age of the Universe is consistentwith the density of luminous present-day

galaxies5. Observations favouring suchobjects as the ancestors of elliptical cores arethat many of the high-redshift galaxies arenearly spherical, and are unlikely to evolveinto flattened rotating disks.

An argument for these ancient galaxiesas precursors of spiral galaxies is that thereare few close neighbours, which would beexpected in the environment of a clusterwhere ellipticals are found6. Observed high-redshift objects are much more compactthan present-day galaxies, so they might besub-clumps that will ultimately merge. Theobjection to this picture is that you mightexpect to see small clusters of such objects,bound together and poised for coalescence.Typically, however, single objects have beendetected. Perhaps such sub-clumps light upwith newly formed stars at slightly differenttimes, so that only one star-forming regionper proto-galaxy is detectable at any veryearly epoch5,7.

The advent of near-infrared spec-troscopy with adaptive-optics — whichcompensate for the effects of the Earth’satmosphere — at ground-based telescopes,and stronger near-infrared capability inspace, should put the study of the earliestluminous objects, now at the limit of ourgrasp, onto a sound footing.Richard Green is at the Kitt Peak NationalObservatory, Tucson, Arizona 85726-6732, USA.e-mail: rgreen@noao.edu

1. Chen, H.-W., Lanzetta, K. M. & Pascarelle, S. Nature 398,

586–588 (1999).

2. Hu, E. M., Cowie, L. L. & McMahon, R. G. Astrophys. J. Lett.

502, L99–L103 (1998).

3. Steidel, C. C., Giavalisco, M., Pettini, M., Dickinson, M. &

Adelberger, K. L. Astrophys. J. Lett. 462, L17–L21 (1996).

4. Armus, L., Matthews, K., Neugebauer, G. & Soifer, B. T.

Astrophys. J. Lett. 506, L89–L92 (1998).

5. Lowenthal, J. D. et al. Astrophys. J. 481, 673–688 (1997).

6. Giavalisco, M., Steidel, C. C. & Macchetto, F. D. Astrophys. J. 470,

189–194 (1996).

7. Trager, S. C., Faber, S. M., Dressler, A. & Oemler, A. Jr

Astrophys. J. 485, 92–99 (1997).

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although many computer forecast modelspredicted 1997 would be warm in the tropi-cal Pacific up to three seasons in advance,none predicted the rapid development orultimate intensity of the event before itsonset2. Clearly we have much to learn fromthis experience.

El Niño, Spanish for ‘the child’ (andspecifically the Christ child), is the namePeruvian fishermen gave to coastal sea-temperature warmings that first appearedaround Christmas time. El Niño now moregenerally refers to a warming of the tropicalPacific basin that occurs roughly every threeto seven years in association with a weaken-ing of the trade winds. The flip side of El

Just under a year ago, a sharp drop inequatorial Pacific sea-surface tempera-tures heralded the end of the 1997–98 El

Niño. Called by some “the climate event ofthe century”, it was by several measures thestrongest on record.

Identifying why it was so strong chal-lenges our understanding of the physicalmechanisms responsible for El Niño. This ismore than simply an academic question:the 1997–98 El Niño severely disruptedglobal weather patterns and Pacific marineecosystems, and by one estimate caused$33 billion in damage and cost 23,000 livesworldwide1. There were warnings of animpending El Niño before it occurred. But

El Niño

The child prodigy of 1997–98Michael J. McPhaden

Niño, La Niña, is characterized by strongerthan normal trade winds and unusuallycold sea-surface temperatures in the tropi-cal Pacific. Both El Niño and La Niña areaccompanied by swings in atmosphericpressure between the eastern and westernPacific known as the Southern Oscillation.These phenomena are collectively referredto as ENSO (El Niño/Southern Oscillation),which also includes periods of near-normalconditions3. At the moment, a strong LaNiña is evident in the tropical Pacific, withseveral (but not all) forecast models pre-dicting a return to normal by the end of1999.

The general mechanisms underlyingENSO involve large-scale ocean–atmo-sphere interactions and equatorial oceandynamics4. But each El Niño and La Niña isunique in the combination of its strength,duration and pattern of development.Irregularity in the ENSO cycle can be seenin both the instrumental record dating backto the middle of the last century, and inproxy data (such as lake sediments, coralgrowth rings, tree rings and ice cores) goingback hundreds or even thousands of years5.So, in principle, it should not be surprisingthat an unusually strong El Niño occursevery so often.

Nonetheless, the 1997–98 El Niño wasunprecedented6. It developed so rapidlythat every month between June and Decem-ber 1997 set a new monthly record high forsea-surface temperatures in the easternequatorial Pacific7. December 1997 anom-alies (that is, deviations from normal) werethe highest ever recorded along the Equatorin the eastern Pacific. Moreover, before1997–98, the previous record-setting ElNiño occurred in 1982–83. These two‘super El Niños’ were separated by only 15years, compared with a typical 30–40 yeargap between such events earlier thiscentury8.

Several factors may have contributed tothe strength of the 1997–98 El Niño. Oneis chaos, which some theories invoke toaccount for the irregularity of the ENSOcycle. Nonlinear resonances involvingENSO and the seasonal cycle have receivedspecial attention, but other chaotic interac-tions may affect ENSO as well9. In 1997–98,events possibly conspired to produce anextraordinarily strong El Niño simply dueto the underlying tendency towards chaosin the climate system. A related issue is thatof weather ‘noise’. Weather phenomena,inherently unpredictable more than abouttwo weeks in advance, are a source of ran-dom forcing in the climate system. In thetropical Pacific, weather events occurring atthe right time, and on time and space scalesto which the ocean is sensitive, canmarkedly alter the evolution of the ENSOcycle10.

One notable source of weather in the

© 1999 Macmillan Magazines Ltd

tropics is the Madden–Julian Oscillation(MJO), a wave-like disturbance in theatmosphere with a period of 30–60 daysthat originates over the Indian Ocean11. Itcould have been that the ocean got a healthykick from the MJO at just the right time tosend it on a course towards record hightemperatures7. The tropical Pacific was pre-conditioned to the onset of an El Niño bythe build-up of excess heat in the westernequatorial Pacific due to stronger thannormal trade winds in 1995–96. However,beginning in late 1996, the MJO was par-ticularly energetic, and several cycles ofthe wave amplified through nonlinearocean–atmosphere interactions as theypassed over the western Pacific. This set inmotion a series of positive feedbacksbetween the ocean and the atmospherewhich reinforced initial MJO-inducedwarming.

Another possibility is that the ENSOcycle may be interacting with the PacificDecadal Oscillation (PDO) — which, as thename implies, is a naturally occurring oscil-lation of the coupled ocean–atmospheresystem in the Pacific basin with a period ofseveral decades12. In association with thePDO, sea-surface temperatures have gener-ally been higher in the tropical Pacific fromthe mid-1970s. Since then, there have beenmore El Niños than La Niñas, the early1990s was a period of extended warmth inthe tropical Pacific, and two super El Niñoshave occurred. The PDO may be one of the

reasons for the observed decadal modula-tion of the ENSO cycle, because it affectsthe background conditions on which ENSOevents develop. From that perspective, thestrength of the 1997–98 El Niño may bebut one manifestation of a linkage betweeninterannual and decadal climate variationsin the Pacific.

Global warming trends are yet anotherpossible influence on the ENSO cycle. Thewarmest years on record were, in order,1998 and 1997. The 1997–98 El Niñocontributed in part to these record highs,because global mean temperatures generallyrise a few tenths of a degree Celsius fol-lowing the peak El Niño warming as thetropical Pacific loses heat to the overlyingatmosphere13,14. Underlying these extremetemperatures, however, is a century-longwarming trend that may well be due toanthropogenic greenhouse-gas warming15.

Some computer models suggest thatglobal warming may be slowly heating upthe eastern equatorial Pacific Ocean, asobserved over the past 25 years16. Otherspropose that ENSO events may be strongeror more frequent in a warmer climate17.The superposition of ENSO variations onincreased warming due to CO2 and a warmphase of the PDO could produce tempera-ture fluctuations like those seen in the equa-torial Pacific since the mid-1970s, includingthe extreme temperatures associated withthe 1997–98 El Nino18. But computer mod-els used in global change studies are limited

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s

Figure 1 Taking the pulse of El Niño. The 1997–98 event was followed through the El Niño/SouthernOscillation (ENSO) Observing System, which was set up to monitor and predict ENSO variations.The Earth-based components, shown here, largely relay data in real time via satellites. The maincomponents are a volunteer ship programme (blue tracks); an island and coastal tide-gauge network(cream); and systems of drifting (orange arrows) and moored buoys (red). Complementing thisnetwork are satellites that provide data from space with near-global coverage. They include theUS/French TOPEX/Poseidon mission; the European Space Agency Earth Remote Sensing satellites;US Department of Defense satellites; and NOAA’s polar-orbiting weather satellites. Taken together,this ensemble of instrumentation delivers data on surface and subsurface temperature, wind speedand direction, sea level, and current velocity. The ENSO Observing System was completed in 1994 atthe end of the ten-year international Tropical Ocean Global Atmosphere programme. It is now beingcontinued in support of operational climate forecasting as well as research on ENSO dynamics.

100 YEARS AGOWhat constitutes the natural prey of thelion in his wild state is, I believe, adisputed point. The majority of people,probably, are of opinion that he isextremely fastidious in his tastes; others,again, assert that he will eat almostanything. Certainly, it is only reasonableto suppose that a lion sufficiently underthe impulse of hunger will eat “almostanything”! Years ago I was present onmore than one occasion when animateddiscussions on this point took placebetween two notable Africanecclesiastics — both since dead — BishopSmythies and Archdeacon Maples (hewas then), both of whom had travelled agood deal in Africa — Maples moreespecially — and had seen something ofthe habits of lions. Bishop Smythiesdefended the former theory; ArchdeaconMaples — a most talented andentertaining man — the latter, saying hehad known instances of lions killingporcupines, and adding that he believedthe porcupine to be specially endowedwith the power to propel his quills intohis assailant when so attacked. At thisjuncture, Bishop Smythies generally lostpatience and declined to continue theargument. Had Bishop Smythies lived, itwould have interested him … to knowthat in March last, at the Salt Stream,two days’ march N. W. of Kibwezi, I shota fine old lion in whose left fore-pawwere deeply buried the tips of threeporcupine quills.From Nature 13 April 1899.

50 YEARS AGOIt is announced that Prof. J. W. McBain,who has recently retired from the chair ofphysical chemistry at Leland StanfordUniversity, California, has been appointedthe first director of the National Chemicallaboratory of India. It is difficult to thinkof a better choice. While his manyEnglish friends have not seen so much ofhim since he left the University of Bristol,yet they have been able to follow hissteady development of the concept of themicellar structure of colloidalelectrolytes, a chapter in physicalchemistry which is particularly his own.… We must recollect that we are alsoindebted to him for coining the word‘sorption’, thus directing attention to thecomplexity of the interactions between agas and a solid.From Nature 16 April 1949.

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The mosquito-borne parasites (Plasmo-dium) that cause malaria have adapteda variety of devious molecular strate-

gies to enable them to survive and keep prop-agating in their immunologically aggressivehosts. Now, on page 618 of this issue, Preiseret al.1 reveal another clever new weapon thathas been developed by Plasmodium in orderto escape destruction in the molecular armsrace between host and parasite.

Malarial disease results from the cycles ofmultiplication of asexual Plasmodium para-sites in the blood of the host. This cyclicalpropagation is initiated by an invasive formof the parasite, the merozoite, when it entersthe red blood cells (erythrocytes). Onceinside, merozoites undergo transformationinto ring-shaped trophozoites, whichmature into large trophozoites before under-going schizogony, a form of haploid nucleardivision. This in turn generates between sixand 32 new merozoites, which burst out oftheir erythrocyte casement into the blood-stream.

Merozoites do not live for long outsidethe cell and so must quickly find and enteranother erythrocyte. This recognition andinvasion of host cells is executed through a‘sensor’ of polypeptide components, which

are largely located in a complex of organelles(termed rhoptries and micronemes) at thedistinctive apical pole of the merozoite2.

Because the merozoite components thatmount the invasion represent antigens thatcould potentially be used for developing avaccine against malaria, it is important tounderstand the mechanisms that haveevolved in malaria parasites to avoid a hostblockade against invasion. In theory, malariaparasites should be vulnerable to neutraliz-ing antibody responses mounted by the hostagainst any of the critical merozoite proteinsthat enable them to invade the host cell. Also,any alteration in a complementary receptoron the target host cell could disrupt theinteraction and prevent the merozoitefrom entering it.

Like many other pathogens, malarial par-asites have evolved mechanisms to overcomesuch tactics. One used by P. falciparum,which causes a severe malaria in humans,and by P. knowlesi, a monkey parasite, is toexpress up to three alternative ‘sticky’adhesin molecules in merozoites. Theseadhesins all help the merozoite to invade itstarget host cell, but they each engage differ-ent receptors on the erythrocyte3,4.

Preiser et al.1 have found evidence for an

unusual and fascinating mode of clonal phe-notypic variation used by another Plasmodi-um species, P. yoelii to avoid the barricades ofits host, a small African rodent (Thamnomysrutilans). Clonal antigenic variation of para-site-derived proteins presented at the surfaceof occupied erythrocytes is a knownphenomenon in malaria5–7. The uniqueform of molecular variation described byPreiser et al.1 takes place not in the intracel-lular vegetative trophozoite, however, butduring the development of the invasivemerozoite parasite form (see their Fig. 3 onpage 620).

Preiser et al. have investigated the patternof transcription for a large multigene familythat expresses antigenically variable proteins,known collectively as p235, which localize inthe rhoptry organelles at the apical pole of P.yoelii merozoites. There are at least 11, andprobably up to 50, genes encoding discreteand variable members in the family8. Activeimmunization experiments, passive transferof antibody and binding of at least one p235protein to mature erythrocytes have indicat-ed that these proteins play a role in selectingerythrocytes for merozoite invasion9.

Based upon their analysis of p235 tran-scripts in individual parasites at differentstages of development, Preiser et al.1 proposethat a single p235 gene is transcribed in eachnucleus that is replicated during schizogony.This by itself is not unusual. But in this case,what is unusual is that the particular p235gene transcribed can be different for eachnucleus present in a single developing sch-izont. The authors surmise from this thateach of the individual merozoite progenyderived from a parent schizont could expressa different p235 protein.

Why would the parasite use this methodof phenotypic variation? It is presumed (butnot yet confirmed) that the specificity ofp235 proteins for their target receptors isvariable, so a single mature schizont couldgive rise to a mosaic of merozoites, each pos-sessing unique receptor specificity, or at leastantigenicity. Accordingly, this could affordthe parasite some form of insurance againstthe possibility of all the progeny (mero-zoites) of a schizont being destroyed byimmune responses specific for some p235proteins but not others. Survival advantagecould also be accrued if this pattern ofreceptor expression maximized the benefitsof differences in the specificity of host-cellrecognition. Expansion into new host nicheswould be facilitated, and parasitaemiascould be maintained in the face of a changinghaematological landscape where erythro-cytes of a certain maturity were eitherincreasing or decreasing in number.

Although it is not the natural host of P.yoelii, the white laboratory mouse can beinfected by this parasite, making it a popularmodel for investigating malaria. In this con-text, there are two relevant questions. First, is

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562 NATURE | VOL 398 | 15 APRIL 1999 | www.nature.com

Malaria

A new escape and evasion tacticJohn W. Barnwell

in their ability to represent many keyprocesses involving physical, chemical andbiological interactions of the coupledocean–atmosphere–land system15,19. Soalthough we might expect global warmingto affect the ENSO cycle, at the moment nofirm conclusions can be drawn.

In short, the strength of the 1997–98 ElNiño may be traceable to interactions ofthe ENSO cycle with some combination ofglobal warming trends, the Pacific DecadalOscillation, the seasonal cycle and theMadden–Julian Oscillation. Complicatingthe picture is a general background ofweather noise and chaotic interactions inthe climate system. Yet unconsideredprocesses may also be involved.

This complexity may be daunting, butthere is a bright side. The 1997–98 El Niñowas not only among the strongest onrecord, it was also the most comprehensive-ly observed20 (Fig. 1). The ‘climate event ofthe century’ may be over, but measure-ments from the recently completed ENSOobserving system will stimulate researchinto the causes and consequences of El Niñofor years to come. The hope is that we willultimately be able to unravel the variousthreads that were woven together to pro-duce the 1997–98 El Niño, and to translatethat knowledge of interactions spanning

intraseasonal to centennial timescales intobetter models for climate forecasting.Michael J. McPhaden is in the NOAA/PacificMarine Environmental Laboratory, 7600 Sand PointWay NE, Seattle, Washington 98115, USA.e-mail: mcphaden@pmel.noaa.gov1. Kerr, R. Science 283, 1108–1109 (1999).

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5. Rodbell, D. T. et al. Science 283, 516–520 (1999).

6. Webster, P. J. & Palmer, T. N. Nature 390, 562–564 (1997).

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17. Timmerman, A. et al. Nature (in the press).

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