Long-range forecast: a prolonged interglacial

Provided the Milankovich theory of astronomical influences on insolation is indeed behind the pacing of glacial-interglacial episodes of the near past, it should be easier to forecast future change in overall climate than that of weather.  It turns out that the fluctuation of Earth’s orbital eccentricity (behind the roughly 100 ka periodicity of climate change for the past 1 Ma) is entering an historic low, due to the 400 ka period of one of its two cycles.  Modelling future insolation at high northern latitudes results in a damping of its fluctuations over the next 100 ka (Berger, A. and Loutre, M.F. 2002.  An exceptionally long interglacial ahead?  Science, v. 297, p. 1287-1288).  Left to climates own devices, the small changes in insolation may prolong the Holocene interglacial for as much as another 50 ka, instead of being now on the cusp of a descent into more frigid conditions.  Until recently, many climatologists looked to the last, Eemian interglacial as the model for the current one, and that lasted only 10 ka.

Of course, climate is no longer at the whim of astronomical forces and the Earth’s own circulation of energy, principally by the flow of energy in North Atlantic water, driven by deep water formed by sea-ice around Iceland.  Atmospheric CO2 stands about 30% higher than during previous interglacials, because of anthropogenic emissions.  Berger and Loutre factor in the “greenhouse” influence of the additional CO2, to find an ominous possibility that the Greenland ice sheet might well melt, with the climate entering an irreversible warming.  The climate, however, is not a model, and there is really no inkling of what surprises are in store from counter-intuitive behaviour of the many forces at work in it, under conditions that have no analogue during the whole of human evolutionary history.

Analogue of Archaean carbon cycle in Black Sea reefs

The Archaean world almost certainly had an atmosphere and oceans that were more or less free of oxygen.  Under such conditions the fate of dead organisms in the ocean, perhaps the remains of photosynthesizing cyanobacteria, would have been bacterial fermentation and the production of massive amounts of methane.  Along with volcanic emissions of carbon dioxide, methane in the atmosphere would have helped warm the planet at a time when the Sun emitted considerably less energy than it does now.  Methane is more strongly depleted in 13C than any organic or inorganic carbon compound.  So large falls in the d13C composition of organic carbon in Archaean rocks, around 2700 Ma have been taken by some palaeobiologists to signify methane metabolism.  Most methane-consuming bacteria today produce oxygen as a biproduct, so the negative excursions might indicate an early build up of more than a trace of oxygen in the Archaean atmosphere.  Discovery of bacterial communities on the floor of the Black Sea, which consume methane without oxygen production (Michaelis, W. and 16 others 2002.  Microbial reefs in the Black Sea fuelled by anaerobic oxidation of methane.  Science, v. 297, p. 1013-1015), suggest strongly that there may be little reason to suppose that Archaean conditions did involve free oxygen.

Off the coast of Crimea there are numerous sea-bed methane seeps in shallow water.  Surprisingly they are well-colonized by primitive bacteria, which produce thick mats held together by carbonate precipitates in completely anoxic conditions.  Laboratory cultures of the communities reveal that the consist of archaea and bacteria that respectively consume methane and reduce sulphate ions to sulphide.  The net result is that methane is oxidized by sulphate to produce calcium and magnesium carbonates, and lots of hydrogen sulphide (methane donates electrons for sulphate reduction, thereby becoming a source of carbon for cell metabolism).  Since much of the methane’s carbon ends up in stable carbonate – perhaps ten times more than in organic matter, such a process in the Archaean would have helped stabilize the “greenhouse effect” then.

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