Into an Ice Age Kurt M

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news and views Into an ice age Kurt M. Cuffey

Analyses of a new ice core from Greenland yield the first high-resolution picture of the start of the last ice age in the Northern Hemisphere, and of the onset of climate instability as our planet cooled.

he relatively warm and stable climate that humanity has enjoyed for the Tpast 10,000 years will inevitably give way to a new ice age — a tremendous en- & C. ALEXANDER

vironmental transformation that is destined B. to bury the sites of Boston, Edinburgh and Stockholm under glacial ice. In The Day After Tomorrow, the Hollywood movie most notable for its public abuse of thermo- dynamics, a new ice age starts in only one week. What does such a transition look like in reality? A new ice core1 that samples the entire 3-km thickness of the north-central Green- land ice sheet (Fig. 1) provides us with an unprecedentedly rich and precise view of the onset of the most recent ice age, some 120,000 years ago. Figure 1 Core territory — the Greenland ice cap, which holds a climate record going And it is from the loca- back to the last interglacial, the Eemian. Inset, cross-section of an ice core that tion of greatest interest resulted from drilling slightly offset from the main bore hole; the half-moon shape — the North Atlantic is composed of Eemian ice. region, where rapid climate changes have been reports to the contrary5,high- something like a volcanic centre — a highly

most dramatic in the past. lighting apparent climate in- unusual, localized zone of high heat flow WWW.GLACIOLOGY.GFY.KU.DK/NGRIP This achievement, reported stabilities within the Eemian, from the underlying crust6.This heat is melt- on page 147 of this issue1,is were clearly mistaken, as the new ing the basal ice, eliminating the oldest ice the result of the efforts of the report demonstrates1.Yet the degree of layers,and creating a relatively warm and wet multinational North Greenland Ice Core climate stability during the Eemian is of environment — which, as the authors point Project (NGRIP). Remarkably, individual intense interest: the Eemian was slightly out, merits further investigation for signs of years of snow deposition are distinguishable warmer than the world is now, providing microbial life. This melt also significantly for events as far back as 123,000 years in the an analogue for a possible future climate reduces the thinning of deep ice layers,allow- past — a few years ago, such a resolution was warmed by atmospheric pollution. ing the extraordinary annual resolution of thought to be unattainable. Further to the north of central Greenland the glacial-onset record reported by the The story of how the impossible became (see the map on page 147), the climate is NGRIP team. possible would itself make a fine movie, drier. It should therefore be possible to find The melt has eliminated much of the complete with dramatic scenes of enlighten- ice from this last interglacial period safely Eemian ice. Fortunately, however, enough ment in the mass spectrometry lab as isotope above the disturbed basal zone. Thus the remains to provide a clear record of its latter analyses reveal past climatic changes. In the NGRIP project was born. It seemed likely few millennia, in which the climate was early 1990s, two deep ice cores recovered that this new core would recover a record of rather stable but warmer than the present by from central Greenland yielded a detailed the entire last interglacial climate and the at least 5 C.The persistence of stable climate environmental history extending 100,000 start of the ice age. There was little hope for under such warm conditions is a key obser- years back in time, through the entirety high temporal resolution,however;ice layers vation. But the loss of the older ice precludes of the last glacial climate2.These showed are cumulatively thinned as the regional flow an answer to the even more important that climate was frightfully unstable. Within compresses them downwards against the question of whether climate was stable in the space of a few decades, the North subglacial bedrock topography and stretches the five or so millennia following the onset of Atlantic region could evidently warm by them horizontally. The vertical compression warmth,from about 130,000 years ago,when 10 C, while smaller changes of tempera- causing thinning is strongest if there is no meltwater runoff from Greenland would ture and moisture occurred over wide areas melt at the ice sheet bed. And this was pre- probably have been greatest7.It has been of the planet3. cisely the situation expected for the NGRIP suggested that increased freshwater flux to Unfortunately, structural disturbance of site, given that basal temperatures are gener- the North Atlantic might have the capacity the deepest ices4 prevented these cores from ally well below the melting point throughout to induce climate change by affecting the revealing either the onset of the glacial northern and central Greenland. ocean’s density structure and circulation8. period or events during the preceding warm But the beds of ice sheets,hidden beneath The start of the glacial period was charac- interglacial period, known as the Eemian, kilometres of opaque ice, contain surprises. terized by a mostly gradual cooling, lasting about 130,000 to 120,000 years ago. Initial The new NGRIP core proved to be located on about five millennia1.The growth of ice

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sheets is necessarily a slow process,limited by ice sheet.Analysis of basal ices gives direct Prokaryotes Cells lacking a true nucleus. the transfer of moisture through the atmos- and compelling evidence that the ice sheet Gene transcription occurs in the cytoplasm. phere, and it appears likely that this process retreated significantly during this period9. initially limited the rate of climatic cooling. There is only one way to fill this gap. A Then, approximately 114,000 years ago, new ice core will have to be extracted from Archaebacteria Prokaryotes with a plasma membrane of isoprene ether lipids. Protein with temperatures having dropped less than the dry regions of north-central Greenland, synthesis occurs on distinctive, halfway to typical full glacial values, the first but at a safe distance from the heat-flow archaebacterial-type ribosomes. Synonymous rapid climate changes began — as docu- anomaly discovered at the NGRIP site. The with Archaea. mented here for the first time. The timing cost and effort of such a project are trivial and characteristics of these events offer an compared with the possible impact of a Eubacteria Prokaryotes with a plasma invaluable subject for climate modellers; the rise in sea level, and maybe even rapid membrane of fatty acid ester lipids. Protein mechanisms underlying rapid climate change climate change, induced by warming of the synthesis occurs on distinctive, eubacterial- type ribosomes. Synonymous with Bacteria. are still being debated, and climate models Arctic region. ■ have not yet convincingly predicted them. Kurt M. Cuffey is in the Department of Geography, There is much work yet to be done on 507 McCone Hall, University of California, Eukaryotes Cells possessing a true nucleus the NGRIP core, especially examining the Berkeley, California 94720-4740, USA. (lacking in prokaryotes), separated from the cytoplasm by a membrane contiguous with high-resolution characteristics of the record, e-mail: [email protected] the endoplasmic reticulum (also lacking in quantifying the temperature history, and 1. North Greenland Ice Core Project members Nature 431, prokaryotes). Include double-membrane- investigating the biogeochemical changes 147–151 (2004). bounded cytoplasmic organelles derived from eubacterial endosymbionts11–13. The plasma that accompanied the transition to glacial 2. Hammer, C., Mayewski, P.A., Peel, D. & Stuiver, M. (eds) J. Geophys. Res. 102 (C12), 26317–26886 (1997). membrane consists of fatty acid ester lipids. climate. The overview presented in this 3. Severinghaus, J. P. & Brook, E. J. Science 386, 930–934 (1999). Protein synthesis occurs on ribosomes related 1 issue is sufficient to demonstrate that it is a 4. Chappellaz, J., Brook, E., Blunier, T. & Malaize, B. J. Geophys. to the archaebacterial type. Synonymous with Eucarya. valuable and remarkable core. Yet the Res. 102, 26547–26557 (1997). NGRIP project has not achieved its primary 5. Greenland Ice-Core Project members Nature 364, 203–208 (1993). Proteobacteria A name introduced for the goal:a reasonably complete record of climate 6. Fahnestock, M., Abdalati, W., Joughin, I., Brozena, J. & during the last interglacial. How warm did group that includes the purple bacteria and Cogineni, P. Science 294, 2338–2342 (2001). relatives18. The endosymbiotic ancestor this period get? Were any parts of it climati- 7. Marshall, S. J. & Cuffey, K. M. Earth Planet. Sci. Lett. 179, 73–90 of mitochondria was a member of the cally unstable? Such information is crucial (2000). proteobacteria as they existed more than for evaluating climate models of a warmer 8. Committee on Abrupt Climate Change Abrupt Climate Change: 1.4 billion years ago. Inevitable Surprises (National Academies Press, Washington world, and for understanding sea-level DC, 2002). changes induced by melting of the Greenland 9. Koerner, R. M. Science 244, 964–968 (1989). Figure 1 Who’s who among microbes. In 1938, Edouard Chatton coined the terms prokaryotes and eukaryotes for the organisms that biologists Evolutionary biology still recognize as such3.In 1977 came the report of a deep dichotomy among prokaryotes19 and designation of the newly discovered groups as Early evolution comes full circle eubacteria and archaebacteria. In 1990, it was William Martin and T. Martin Embley proposed2 to rename the eukaryotes, eubacteria and archaebacteria as eucarya, bacteria and Biologists use phylogenetic trees to depict the history of life. But archaea. Although widely used, the latter according to a new and roundabout view, such trees are not the best names left the memberships of these groups way to summarize life’s deepest evolutionary relationships. unchanged, so the older terms have priority.

harles Darwin described the evolu- eukaryotic cells: mitochondria (the power- and organelles) and prokaryotes (cells that tionary process in terms of trees,with houses of the cell) and chloroplasts (of no lack both). For a decade, the dominant Cnatural variation producing diversity further importance here). At the endosym- approach has involved another intracellular among progeny and natural selection shap- biotic origin of mitochondria, a free-living structure called the ribosome,which consists ing that diversity along a series of branches proteobacterium came to reside within an of complexes of RNA and protein,and is pres- over time. But in the microbial world things archaebacterially related host — see Fig.1 for ent in all living organisms. The genes encod- are different, and various schemes have been terminology. This event involved the genetic ing an organism’s ribosomal RNA (rRNA) devised to take both traditional and molecu- union of two highly divergent cell lineages, are sequenced,and the results compared with lar approaches to microbial evolution into causing two deep branches in the tree of life those for rRNAs from other organisms. The account. Rivera and Lake (page 152 of this to merge outright. To this day, biologists ensuing tree2 divides life into three groups issue1) provide the latest such scheme, based cannot agree on how often lateral gene trans- called domains (Fig. 2a). The usefulness of on analysing whole-genome sequences, and fer and endosymbiosis have occurred in life’s rRNA in exploring biodiversity within the they call for a radical departure from conven- history; how significant either is for genome three domains is unparalleled, but the pro- tional thinking. evolution; or how to deal with them math- posal for a natural system of all life based on Unknown to Darwin, microbes use two ematically in the process of reconstructing rRNA alone has come increasingly under fire. mechanisms of natural variation that dis- evolutionary trees. The report by Rivera and Ernst Mayr3,for example, argued force- obey the rules of tree-like evolution: lateral Lake1 bears on all three issues.And instead of fully that the rRNA tree errs by showing gene transfer and endosymbiosis. Lateral a tree linking life’s three deepest branches eukaryotes as sisters to archaebacteria, there- gene transfer involves the passage of genes (eubacteria,archaebacteria and eukaryotes), by obscuring the obvious natural division among distantly related groups, causing they uncover a ring. between eukaryotes and prokaryotes at the branches in the tree of life to exchange bits The ring comes to rest on evolution’s level of cell organization (Fig. 2b). A central of their fabric. Endosymbiosis — one cell sorest spot — the origin of eukaryotes.Biolo- concept here is that of a tree’s ‘root’, which living within another — gave rise to the gists fiercely debate the relationships between defines its most ancient branch and hence the double-membrane-bounded organelles of eukaryotes (complex cells that have a nucleus relationships among the deepest-diverging