Warming may have triggered Northern Hemisphere glaciation

While I write this issue of EPN it is supposed to be early spring outside, and that is clearly what the ducks reckon as well – they are beginning to, er um, frolic.  But there has been two weeks of snow and frost.  Britain and the rest of Europe owe the frigid snap to cold air spilling westwards from northern Asia; the influence of the Siberian winter high-pressure area.  Although somewhat lost in the recent kerfuffles about whether or not global warming is a fact or a misreading of data, the inevitable build up of mid-continental cold dense air in winter might have interesting consequences, should climate warm.  Normally, areas far from the oceans remain dry as well as getting very cold through radiative heat loss in winter.  When spring comes, such snow as there is soon disappears and the extremes of cold are replaced by surprisingly high summer temperatures, as anyone who has visited Siberia or Northern Canada will know.  Should moist air find its way into such areas during winter, vastly more snow would fall.  Its melting would take longer, and more solar radiation would be reflected back to space in spring.  Such an albedo feedback could induce generalised cooling.  Now evidence has emerged that the earliest known growth of land ice in North America was linked to warming of the ocean from which winds blew over it (Haug G.H. et al. 2005.  North Pacific seasonality and the glaciation of North America 2.7 million years ago.  Nature, v. 433, p. 821-825).  In fact it is axiomatic that growth of continental ice sheets requires a supply of moisture and snow that exceeds the rate of summer melting and ablation, as well as cold winters.

Most theorising about the onset of Northern Hemisphere glaciation has centred on changes in North Atlantic circulation due to closures of the straits where the Isthmus of  Panama now links North and South America, and the start of southward deep-water circulation from the latitude of Iceland.  In fact both are known to have preceded the last Ice Age by a good 2 Ma.  The actual start around 2.7 Ma coincided with an increase in obliquity of the Earth’s orbit that would have led to periods with cold northern summers.  Without abundant mid-continent snowfall, that in itself would not have set ice sheets forming in earnest.  The multinational team of oceanographers studied sea-floor sediment cores from the sub-Arctic Pacific.  To their surprise, sea-surface temperatures provided by evidence from planktonic organisms show evidence at 2.7 Ma for on the one hand cooling of the sea surface (from foraminifer oxygen isotopes) yet considerable warming on the other (from organic chemicals secreted by coccolithophores).  Resolving this paradox requires a careful assessment of the ecological behaviour of the two groups of organisms.  The authors’ explanation involves the onset of density stratification in the North Pacific, so that the surface warmed quickly in summer, retaining warmth during autumn, and warmed slowly in spring from its minimum temperature.  Both result from the high thermal inertia of water.  The productivity of silica-secreting diatoms plummeted to a fifth of its earlier levels at 2.7 Ma as well, explained by ocean stratification reducing the supply of nutrients from deep water upwellings.  Intuitively, a warm sea upwind of the North American continental interior should have generated high snowfall in late autumn and winter.  Haug and colleagues modelled the contrasting effects of an ocean with water overturn and mixing with one that tends to become stratified, to simulate snowfall over the North American Arctic.  From a situation in the Pliocene with snowfall over Greenland and the Arctic islands, the scenario shifts to heavy snow over the whole Arctic in the earliest Pleistocene.  It seems that the trigger for the Great Ice Age was a hemisphere away from the “usual culprit”, the North Atlantic, although its vagaries, once glacial cycles were underway, probably controlled the details thereafter.

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