Ten years ago the records of climate proxies from the Greenland ice sheet set new benchmarks for understanding how climate has varied over the last 100 thousand years – annual ice layers allowed division of that data to as fine as decades. Variations in the ice cores helped explain many of the variations found in more blurred data from sea-floor sediment cores in the Northern Hemisphere. Variations could be correlated with changes in the formation of North Atlantic deep water at high latitudes and the destabilisation of North American and Scandinavian glaciers. The whole hemisphere behaved in concert, through long-distance connections in climatic processes, but high-latitude processes seemed to dominate. Development of 234U/230Th dating extended high precision to carbonates that have been precipitated from groundwater to form stalagmites or speleothem. The latest results from speleothem, collected on the Indian Ocean island of Socotra, cover 14 thousand years between 56 and 42 ka, and resolve down to only 8 year intervals (Burns, S.J. et al. 2003. Indian Ocean climate and an absolute chronology over Dansgaard/Oeschger events 9 to 13. Science, v. 301, p. 1365-1367). They show variations in rainfall on the island, though the d18O proxy, and thus changes in the strength of the Indian Ocean monsoon. In terms of shape, the stalagmite record closely resembles d18O changes in the Greenland ice cores, although the two have opposite senses, because the Greenland proxy is for air temperature above the ice cap. During the frigid Heinrich events that saw massive southward waves of icebergs, rainfall over Socotra was low. It became higher as high-latitude conditions warmed in Dansgaard-Oeschger events. The fine speleothem resolution shows a dramatic change-over that took only 25 years or so. The explanation is that warmer conditions increased equatorial evaporation from the oceans. But water vapour is the dominant “greenhouse” gas, and a wetter atmosphere would become warmer. So the question of whether low- or high latitudes drove the changes is still an open one. If North Atlantic events were the driver, then the tropical processes would greatly amplify their effects. One big problem emerges from the joint research by US, Swiss and Yemeni scientists. The highly reliable U/Th dating gives ages for each event that are about 3000 years older than those interpreted from the ice cores. The authors are convinced that the ice-core ages need revision, yet there are discrepancies with the event-ages from other similarly dated speleothems. Commenting on the paper, Frank Sirocko of Johannes Gutenberg University of Mainz in Germany (Sirocko, F. 2003. What drove past teleconnections. Science, v. 301, p. 1336-1337) makes the point that maybe the quality and age of ice core records lie behind the widely accepted view that high-latitude process drive climate. He presents an excellent global image of modern sea-surface temperatures that show the main oceanic shifts of energy – the leakage of cold circum-Antarctic waters northwards, the westward movement of equatorial warm waters to which the El Niño – Southern Oscillation (ENSO) is due, and the unique movement of warm water to Arctic regions in the North Atlantic that is connected to deep water formation. To that he adds the major effect of continental winter snow cover in central Eurasia, that affects albedo and the size of the winter high-pressure zone there. Is there a teleconnection between that and events in the North Atlantic? Nobody knows, because there are no data to compare, yet. Another uncharted but likely linkage is between the ENSO and processes in the circum-Antarctic current. Using currently accepted dating of ice cores, records from those in the Antarctic show air temperature changes that precede those from Greenland by several thousand years. In that respect, the Socotra record possibly has a link with the South Polar climate. Until the issue of dating is sorted out, it will always be difficult to make concrete statements about global climate change.
Interestingly, in the same issue of Science, sea-floor data (between 9 and 16 ka) from the Cariaco Basin off Venezuela, at about the same latitude as Socotra, mimic the Greenland records to within 30 to 90 years (Lea, D.W. et al, 2003. Synchroneity of tropical and high-latitude Atlantic temperatures over the last glacial termination. Science, v. 301, p. 1361-1364).
“Greenhouse” controls challenged
There’s data gathering and there’s theorising. In palaeoclimate studies the two come into conflict. Theory suggests that CO2 is likely to be the principal driver for climatic ups and downs, probably on all time scales. Atmospheric CO2 estimates from the past are based on proxies of different kind, and the various models that they support do not tally vary well. Worst of all they do not fit climate records through the Phanerozoic at all well, except in the crudest possible way. Only the long-lived Carboniferous to Permian “icehouse” and Tertiary cooling tally, and then only in Berner’s GeocarbIII model. One of the best records of major climate shifts, aside from continental tillites, are marine sediments that contain ice-rafted debris, in particular the palaeolatitudes to which they extend. They record four major cooling episodes: Late Ordovician; Devonian to Late Permian; Late Jurassic to Mid Cretaceous; and those since about 35 Ma ago. The oxygen isotope record from Phanerozoic fossils, partly correlated with ocean temperatures also suggest 4 global coolings in the last 545 Ma. Either the CO2 modelling needs more detail, or the whole issue of the “greenhouse” effect is under question. That is the conclusion of a study by Nir Shaviv of the Hebrew University of Jerusalem, and Ján Veiser of the Ruhr University and The University of Ottawa (Shaviv, N.J. & Veiser, J. 2003. Celestial driver of Phanerozoic climate? GSA Today, Huly 2003, p. 4-10). Veiser has been analysing the chemistry of carbonates, especially their oxygen isotopes, for his 30 year career, and has amassed more data than any other geochemist on carbonate-related issues. The two have worked together because their interests fit together extremely well. Shaviv has reconstructed the variation of cosmic ray flux from studies of the exposure of iron meteorites to them, blended with analysis of how the Solar System moves through the spiral arms of our galaxy. Cosmic rays are known to affect the Earth’s cloudiness and therefore albedo. Greater cosmic ray flux should increase the amount of solar energy reflected away by the Earth, thereby causing global cooling. The degree of fit between the cosmic ray flux and palaeoclimatic records is so good that up to 2/3 of climate variation may be connected with the Earth’s celestial position. That is, as it passes through the star-rich spiral arms cosmic rays intensities go up. This happens every 140 Ma or so, which fits very well with the 4 icehouse periods during the Phanerozoic. They even suggest that the climate-CO2 relationship may be the opposite of that generally agreed; climate might drive carbon dioxide levels. A secondary role for “greenhouse” gases wreaks havoc on attempts at modelling climate change feared to result from increasing anthropogenic releases. Shaviv and Veiser’s work comes at a particularly awkward time for climate modellers, who have just initiated a programme for running huge simulations by corralling the combined computing power of millions of home PC users, similar to the approach pioneered by the SETI Institute (Allen, M.R. Possible or probable. Nature, v. 425, p. 242). Perhaps the view of Phillip Stott, that climate modelling is a complete waste of time (Stott, P. 2003. You can’t control the climate. New Scientist, 20 September 2003, p. 25) might sink in as a result of the possible link between cosmic ray flux and climates of the past. Stott believes that acting on the output of such models might perhaps even be dangerous, since we clearly do not understand short-term climate change well enough.
Precambrian CO2 levels
Whether or not fluctuations in the “greenhouse” effect drive climate change, the fact remains that CO2, methane and water vapour all act to retain solar heat in the Earth system. Were it nor for their presence in the atmosphere, the Earth would be about 33 degrees colder than it is. It would be covered by ice. Theoretical modelling of how stars evolve suggests that the Sun had progressive less energy output going back in Earth’s history. Only gaseous heat retention could have prevented a sterile, frigid planet. Yet periods of cooling sufficient to hold large amounts of water in surface ice have occurred only a few times, 4 in the Phanerozoic, a flurry of so-called “Snowball” epochs in the Neoproterozoic and the earliest known glaciation around 2200 Ma ago. The earliest coincided with the first evidence for free oxygen in the atmosphere, and may have been caused by that. Methane, a more powerful “greenhouse” gas than water or carbon dioxide and abundantly produced by anaerobic decay, is easily oxidised. In later time, it has been ephemeral in the atmosphere, unless continuously released, for instance by destabilisation of gas hydrate in sea-floor sediments. Warming by CO2 has undoubtedly kept total frigidity at bay since then. The problem is charting just how much was in the air, because most estimates have been based on studies of palaeosols that give odd and very imprecise results for the early Palaeozoic (see Shaviv and Veiser, 2003; previous item).
Photosynthetic organisms derived their carbon from CO2, either in the air or dissolved in water through equilibration with the atmosphere. The extraction favours lighter 12C, so biological activity results in their products being depleted in the heavier 13C by about 25 parts per thousand (‰) relative to carbon in air and water. If organic carbon becomes buried, the remaining carbon in the surface environment gets richer in 13C, and that signature becomes fixed in contemporaneous carbonates, both organic and inorganic. It is therefore possible to use the two carbon-isotope signatures to estimate the reservoir of CO2; its proportion in contemporary air. However, the degree of fractionation depends on the specific carbon metabolism of different organisms, yet most organic carbon in sediments is a mixed product of widely differing life styles. That severely blurs estimates of atmospheric carbon dioxide content. What is needed are data from a single source with known metabolism. Acritarchs are fossil remains of single-celled marine eukaryotes that were, and still are, marine photosynthesisers. They are made of degraded hydrocarbons. Advanced ion-microprobe resolution is now sufficient to produce carbon-isotope measurements of individual fossils (about 200 micrometres across). Sediments from northern China, roughly 1400 Ma old, contain abundant little-altered acritarchs and carbon isotope data from them give good estimates of atmospheric CO2 levels, that are independent of other methods (Kauffman, A.J. & Xiao, S. 2003. High CO2 levels in the Proterozoic atmosphere estimated from analyses of individual microfossils. Nature, v. 425, p. 279-282). The estimates suggest between 10 to 200 times higher contents than today, but just about sufficient to keep the Earth above the limit of glacial temperatures when solar luminosity was about 88% of the present. Acritarchs are present throughout the Neoproterozoic, and it should prove possible to examine the critical periods of “Snowball” conditions using this method.