Ice extracted from ice sheets by core drilling has provided the most detailed historical information on climate variation at high latitudes and about the varying gas and dust content of the atmosphere. It provides the best time-resolution currently available, sometimes of the order of 50 years. Cores from the Greenland ice sheet revolutionised ideas about the controls over short-term climate shifts in the northern hemisphere – the millennial-scale Heinrich and Dansgaard-Oeschger events. It is from those revelations that fears have arisen about the consequences of deep-ocean circulation shut-downs that might arise from current global warming. The Greenland ice goes back only to cover the last glaciation and part of the interglacial period the preceded it. Until recently, the Vostok ice core from Antarctica gave the greatest penetration into past climatic events, to around 430 ka that covers the last four glacial epochs. Again, Vostok revolutionised our understanding of past climate change, principally the differences between climate behaviour in interglacials, and those between the records from northern and southern hemispheres. North and south have not been in exact harmony, at least as far as high latitudes are concerned. Ocean-floor sediment cores and those from mid-latitude glaciers do give hints of a global harmonisation of events though. Since we live in an interglacial period, for the last three of which the previous ice-core records suggest a span around 10 ka, it has seemed likely that ours wouldn’t have lasted much longer than it already has under purely “natural” conditions. Modelling the possible effects of anthropogenic warming on climate that may be about to change anyway within this millennium, has left climatologists undecided about the future. That blurring is as much to do with the unknown direction that an unstable climate might take and the limitations of modelling, as with knowledge of past events. So, the more information on past interglacials, the better the chance of getting a “handle” on the climatic frying pan out of which humanity seems to be on the point of jumping. The European Project for Ice Coring in Antarctica (EPICA), which involves 57 scientists from 10 European countries, has dramatically expanded the scope for comparison with the past by a 3 kilometre core from one of the deepest parts of the Antarctic ice (EPICA, 2004. Eight glacial cycles from an Antarctic ice core. Nature, v. 429, p. 623-628). The potential information that eventually will flow from the core will dwarf that from any previous climatic research project. It covers the period when climate settled into a roughly 100 ka rhythm, probably linked with the weakest of the astronomical controls of solar heating, that of orbital eccentricity, and thereby a bit of a mystery even if it twangs the harmonics of purely terrestrial climatic processes.
The first focus, naturally enough, is on the fourth interglacial epoch before the present one, which ended about 400 ka ago. In terms of overall astronomical forcing, that is the time when insolation patterns were most similar to those during the Holocene. Vostok only covered the latter stages, but now its entire span is covered. All the preliminary time-series for it indicate that it was considerably longer than the last three interglacials, around 25 ka rather than 10. Its initiation following the waning of the preceding full glacial period follows a similar patter to the early Holocene; the warming was interrupted by a sudden, one-off cooling, somewhat like the Younger Dryas around 12 ka ago. Although the first EPICA report contains preliminary ideas on several important topics, the one that has caused a stir is that duration of the 5th interglacial. Maybe out own warm times will be naturally prolonged for several more millennia, in which case fears of instability and a plunge to full glaciation soon could be set aside with some relief. However, the abstract to the article, concludes ny saying, “…our results may imply that without human intervention, a climate similar to the present one would extend well into the future” [my italics]. But we do intervene, and nobody knows the outcome of that on a climatic pace of change that follows the almost infinitesimally small orbital-obliquity forcing of probable oceanic process that really call the tune.
Smoking gun for end-Palaeocene global warming: an igneous connection
The sudden warming of the Earth at the start of the Eocene 55 Ma ago has been a topic touched on several times in EPN. It is widely regarded as a consequence of rapid release of methane from sea-floor gas hydrate, a risk that modern anthropogenic warming presents if deep-water temperatures rise much above their present near-freezing temperatures. However, no evidence gives a direct connection to the “clathrate gun”. The disturbance in carbon isotopes of marine sediments at the P-E boundary is most easily linked to a massive methane release at the time, but precisely where it began has been unknown. Many shallow marine basins, such as the North Sea, have a pockmarked modern floor attributed to minor gas release in much more recent times. The phenomenon can destabilise the sea bed, so more recent releases have been carefully documented where oil-production platforms are situated. A clue to the much larger release at 55 Ma stems from detailed seismic exploration of western Norway that involved over 150 thousand kilometres of profiling (Svenson, H. et al. 2004. Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, v. 429, p. 542-545). The surveys revealed that beds immediately beneath the base of Eocene sediments are riddled with hydrothermal vents complexes, which take the form of mounds, craters and eye-shaped structures. Some are huge, extending to 5 km across. The profiles also show that beneath the vents are pipes of disrupted strata which extend to the depth of a complex of igneous sills of the North Atlantic large igneous complex, itself emplaced at about 55 Ma. The sills underlie about 80 thousand square kilometres and most of the vents occur within this area. Biostratigraphic dating of the youngest sediments disrupted by the vents gives ages between 55.0 and 55.8 Ma. Intrusion of magma into a deep sedimentary sequence unsurprisingly would set hydrothermal circulation going. If, as they did, the hot fluids reached the sea bed, they would pass through a zone of gas hydrate, destabilise it and release massive amounts of methane to the atmosphere. In the case of the Norwegian shelf, the intrusions were into deeply buried organic rich rocks, further encouraging methane formation; probably a great deal more than from gas hydrate. An estimate of 1012 tonnes of methane generated thermally off Norway is enough to result in a change in carbon isotopes as large as that known from the P-E boundary. In fact, similar sediments throughout the end-Palaeocene North Atlantic large igneous province are likely to have been “over matured” in this way, and no other explanation for the increase in “greenhouse” gases seems necessary. The clear connection with large scale magmatism in thick sedimentary basins may help focus ideas about similar methane-related episodes of global warming, such as the C-isotope excursions at the Permian-Triassic and Triassic-Jurassic boundaries, and within Jurassic and Cretaceous sequences.
Earth’s early climate and methane
At the time the Earth accreted, some 4.6 billion years ago, the Sun was less bright than it is now, so that its warming effect was 30% less. Without some means of retaining in the ancient atmosphere what heat was available, the Earth would have been frigid. This “faint, young Sun” problem would have persisted into the time when the geological record begins, around 4 billion years ago, slowly increasing in its energy output to its modern level. Even in the oldest rocks, there is abundant evidence for the dominance of liquid water at the surface in the form of oceans and river transport across continents. Low solar warming would have made that impossible, and pole-to-pole ice would have made the Earth a highly reflective planet that could never escape glacial condition. That is, unless the atmosphere contained sufficient “greenhouse” gases to retain far more solar energy than now. The favoured gas, until recently, has been the same one that dominates fears of global warming today – carbon dioxide – that volcanoes probably emitted throughout Earth’s history. However, estimates of how much would have been needed to keep the surface free of sea ice and land glaciers, for which there is no evidence until about 2.3 billion years, are extremely high (hundreds of time greater than now). Levels greater than 8 times present levels encourage the precipitation of iron carbonates in soils, yet soils from the late Archaean and Palaeoproterozoic contain none. At those times, CO2 concentrations less than 8 times present ones would not have prevented runaway “ice-house” conditions, so some other gas had to be involved in atmospheric warming. James Kasting of the University of Michigan, who has been involved in studies of ancient atmosphere and climate for 25 years, summarises the case for methane being the means of keeping Earth free of ice while the sun was fainter in a recent article (Kasting, J.F. 2004. When methane made climate. Scientific American, v. 291(1), p. 52-59). Only about 1000 parts per million of atmospheric methane would have been needed to keep the early Earth ice-free, because its “greenhouse” effect is extremely efficient. After oxygen rose to become a major atmospheric gas (since 2.2 billion years), heating induced by methane releases has been tempered by its rapid oxidation to CO2. At several times in the past, when there were massive methane releases from sea-floor sediments, such as the end of the Palaeocene, that oxidation prevented the opposite problem, a runaway “greenhouse”. That is “another story”, involving the rise of photosynthesising organisms. Kasting’s main theme is the role of methane-generating Archaea (once known as archaebacteria) soon after the origin of life. In the absence of oxygen, rising methane from thriving methanogen communities could itself have produced irreversible heating, were it not for methane’s ability to polymerise to heavier hydrocarbons through photochemical reactions. That would have produced a “smog” that not only would have acted as a reflector for solar radiation, but would have added chemical “feedstock” to early life. Kasting gives a fascinating, all-sided summary, but misses what seems to be an obvious point. Without atmospheric methane, any water on Earth would have frozen soon after it appeared, however that happened, perhaps by outgassing, perhaps delivered by comets. Without liquid water, life processes cannot develop. That opens the possibility for a much earlier origin of life, of the methane generating variety, than anyone has dared to speculate on. Many methanogens metabolise hydrogen and CO2. Volcanoes emit small amounts of hydrogen gas, but an even larger source is from sea-floor hydration of ultramafic lavas, common in early times. Almost certainly the very earliest times would have provided a suitable environment for methanogens to emerge.