The gas-hydrate “gun”
As fears of anthropogenic climate warming have risen, so more geoscientists have looked in detail at the stratigraphic record for signs of past warming, and funds have become more targeted towards palaeoclimatology. One of the most important discoveries was that the end of the Palaeocene, about 55 Ma ago, was a time of sudden global warming during the overall cooling that has characterised the Cenozoic. The first sign that something strange had happened then came from using the oxygen isotope geothermometer on plankton tests from marine drill core that passed through the boundary. There seemed to have been a 7º C jump in surface seawater temperature. An explanation for the thermal spike arose after carbon isotopes revealed a coincident spike in the lighter 12C. Periods of low primary biological production can impose such anomalies, because photosynthesis selectively binds light carbon in carbohydrate. However, some of that light carbon ends up buried in sea-floor sediments, so another explanation for a negative excursion in d13C is that organic carbon has somehow been released from sedimentary storage to the atmosphere. So, either there was a sterile ocean or a massive release of organic carbon at the Palaeocene/Eocene boundary. Some kind of erosion to achieve the second possibility could not have led to such a speedy shift in carbon isotopes. The accepted explanation, suggested in 1995, stemmed from organic carbon that had been metabolised by methanogen bacteria in anaerobic sea-floor sediments to form methane. Given low enough sea-bottom temperatures and sufficient pressure, methane can crystallise with water to form an icy substance, known as gas-hydrate or clathrate, in sea-floor sediments. Being an unstable compound, gas hydrate can break down rapidly if seafloor temperature rises or sea-level falls. And, of course, the methane can rush to the surface as bubbles. Being 4 times more efficient than carbon dioxide at trapping thermal radiation emitted by the Earth’s surface, methane releases are excellent explanations for sudden warmings in the stratigraphic record. And there is a great deal of methane locked as gas hydrate beneath the sea floor, about 2 teratonnes (2 x 1012 t). Quirin Schiermeier reviews the basic concept (Scheiermeier, Q. 2003. Nature, v. 423, p. 681-682), but poses the question of how methane-induced warming is reversed. Methane is quickly oxidised to CO2 in the atmosphere, so lessening its warming effect. So a “spike” that lasts thousands of years has to be fed by continual releases. Since warming drives gas hydrate breakdown, something must intervene to stop the releases before the warming becomes a “runaway greenhouse”. One view, and probably the correct one, is that warmth and more CO2 drives up biological activity so that the increased atmospheric carbon is “pumped” down by living processes, back to sedimentary burial. If sufficient nutrients are available, there is no way of stopping this negative feedback until a balance is restored. Schiermeier reports that new ocean drilling plans to test the hypothesis that the Palaeocene/Eocene warming accelerated continental erosion, which was able to wash the crucial nutrients phosphorus and iron into the oceans. Experiments have shown that increased iron in ocean-surface water far from land – now pretty sterile because it is iron-deficient – sparks up photosynthetic plankton. That is one possible way of artificially drawing down anthropogenic CO2. The problem is, if such a process was involved in cooling the Eocene Earth, it took about 100 thousand years.
Red Sea record links to northern hemisphere climate
In his forthcoming book, Out of Eden: the Peopling of the World (Constable and Robinson, July 2003), Stephen Oppenheimer offers the novel suggestion that fully modern humans left Africa by island hopping on log rafts across the Straits of Bab el Mandab, which connects the Red Sea to the Indian Ocean. The rationale to his suggestion is that sea-level falls during major glaciations would have partially exposed the shelf that lies beneath the Straits, presenting a route to SW Arabia across only 18 km of island-dotted sea. As today, it would have been impossible to trek across the deserts of the Middle East after a northward African migration along the Nile, without chains of wells. His thesis then sees humans migrating along coasts eventually to reach east Asia at about 70 ka. Precisely when the Straits of Bab el Mandab became shallow enough would have been determined by global climatic conditions, for only glacial maxima result in sufficient sea-level falls for such island hopping to be possible.
The shallowing of the shelf across the southern outlet of the Red Sea would have had a profound impact on seawater circulation. Already having restricted connection to the world’s oceans, Red Sea water has elevated 18O levels, because evaporation from it favours loss of lighter 16O. With more restricted circulation, evaporation would have driven this up further. Geoscientists from the Universities of Southampton, Tuebingen and Göttingen, and the Geological Survey of Israel have analysed the variation in oxygen isotopes of foraminifera from a Red Sea core to quantify ups and downs in sea level in more detail than possible from open-ocean cores, which have uncertainties of about ±30m) (Siddall, M. and 6 others 2003. Sea-level fluctuations during the last glacial cycle. Nature, v. 423, p. 853-858). The method that they used models the effects on Red Sea oxygen isotopes of evaporation and changed circulation to estimate how the depth of the Straits of Bab el Mandab changed. They claim a precision of ±12m. Through the period from 70 to 20 ka, leading up to the last glacial maximum, their sea-level record tallies nicely with climate records from both Antarctic and Greenland ice cores, including shifts linked to the short-lived Heinrich and Dansgaard-Oeschger cycles. During the last glacial maximum(18-20 ka), sea-level fell by almost 120 m, so that the Straits of Bab el Mandab were on average only 15 m deep. The first human Exodus out of Africa to populate Eurasia would have been between 120 to 130 ka, as suggested by Oppenheimer, when sea level probably fell a little further. However, at about 65 ka, sea level dropped to about 100 m below modern levels, perhaps presenting another window of opportunity.
Broecker reviews climate triggers
Wallace Broecker, of the Lamont-Doherty Earth Observatory at Columbia University, was the first to quantify in 1975 the 19th century prediction of Svante Arrhenius that increasing atmospheric carbon dioxide would drive up global temperatures. Broecker’s early work lies at the centre of concern about global warming, and his subsequent contributions are enmeshed with the entire study of past climate change. A review by him of current ideas on palaeoclimates of the recent past is therefore compulsory reading, for all geoscientists (Broecker, W.S. 2003. Does the trigger for abrupt climate change reside in the ocean or in the atmosphere? Science, v. 300, p. 1519-1522. As well as the astronomically connected cyclicity that is apparent in all kinds of climate record through the Pleistocene, those records are punctuated by sudden, short-lived phenomena, whose magnitudes and pace are sufficiently dramatic to focus attention on processes that are probably entirely terrestrial. Foremost among these during the last glacial interglacial cycle are the astonishing coolings of Heinrich’s iceberg armada events and the possibly catastrophic (in a human as well as an ecological sense) Younger Dryas, which reversed warming from the last Glacial Maximum, and the equally sudden warmings associated with Dansgaard-Oeschger events. Broecker’s review focuses on the two mechanisms that have been suggested to underlie these overturns. One links such changes to shifts in whole-ocean water circulation, especially the ons and offs of deep-water circulation beneath the North Atlantic, the other to perturbations of the way in which atmosphere and ocean interact in the tropics.
An entirely plausible scenario for climate-driving changes in North Atlantic water circulation is flushes of freshwater from the surrounding continents, so that formation of sea ice leaves residual water that is not saline or dense enough to sink and drag in water from lower latitudes. The problem is that the complete thermohaline cycle, which impacts on global atmospheric circulation, has a period longer than the changes that might be induced by its perturbation in the North Atlantic. Tropical atmosphere-ocean dynamics are the largest elements in global climate, in terms of the energy and mass that are shifted, so they are a natural candidate for a driving mechanism. Tropical climate shifts abruptly today in well-known ways, most important being the El Niño-La Niña cycle. There is no ponderous underlying dynamic that would damp down connections between cause and global effect, and prevent sudden climate change. Yet, some kind of “flywheel” is essential to keep long-term cyclicity going and lock sudden changes into century to millennium-long climate “states”, which should rapidly decay if effect rebounded on cause, as it does in the case of El Niño-La Niña. Broecker covers all the critical evidence that has borne on both hypotheses up to now. His conclusion is interesting. Both hypotheses are very much model led, and in need of as much empirical support as can be had. Yet, and here is the nub, the crucial data are those bearing on correlating times of events that are recognised all over the place. Time resolution is of the greatest importance, since climate transitions are fast; faster in fact than we can presently resolve before historical times. It is entirely likely that suitable resolution of times past may be absolutely impossible. Both hypotheses have a lot of empirical and theoretical support. So, what is the problem of combining them in a cunning way? Partly, that may be because reductionism (controlling a few variables and looking for developments in another simple set) still plagues science. That is odd in climatology, where all motions and energy changes palpably relate to one another, with no control of a rational kind. Reductionism demands ever more staggering computing power and speed, to “keep all the eggs in the air”. There is always the feeling, as Jimmy “Shnozzle” Durante observed in his musical monologue, The Man Who Found The Lost Chord, that if you find a hitherto overlooked connection, then everything goes well; if you can remember it! Broecker suggests that the missing connection must “transmit” from deep ocean water to tropical atmosphere.