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Saturday, 22 January 2011

Snowblitz theory revisited

My thoughts about (a) the extent of snowcover over Southern England during glacial phases, and (b) the existence of small thin ice caps over the uplands of Cornwall, Devon and Somerset reminded me that back in the 1970's there was a lot in the literature about the "snowblitz" theory.  The leading proponent of this was Nigel Calder, who made a popular BBC TV programme in 1974 and wrote an associated book with the title "The Weather Machine."

To put it in context, at that time a number of climatologists were pointing out that there had been three decades of global COOLING (look at the above graph) -- triggering speculation that the current interglacial was coming to an end, having lasted for about 10,000 years (That would be par for the course.)  At the time, I subscribed to that idea myself, because sea ice extent was increasing, and many glaciers worldwide had started to advance after decades of retreat.  So, we thought, we can now expect a gradual slide towards the next glacial episode.  Indeed, some politicians were warning that governments in the most vulnerable parts of the world should immediately start laying up food stocks in anticipation of coming food shortages!!  Others were arguing that we would have to spread soot onto the polar ice sheets and sea ice in order to encourage melting and counteract the effect of global temperature decline.

Another part of the debate revolved around the idea that global temperature could "flip" with great speed from a warm phase to a cold phase or vice versa.  Data from many sources were showing a number of episodes in the past when temperatures had rocketed up or down in the space of just a few centuries.  How on earth could this happen, given the massive inertia of the global climate?  Were global catastrophes (like volcanic eruptions or meteorite strikes)  to blame?  There was a lot of talk about catastrophism at the time -- and people like Hubert Lamb and George Kukla became involved in intense debates about what the MECHANISMS for these rapid changes might be.  And then Nigel Calder came along with his nightmare scenario involving the snowblitz theory.  It wasn't taken all that seriously by the experts, but at the time it was a seductive idea because we were all into systems analysis, and we all talked about positive feedback mechanisms etc.  In essence, Calder argued that we could be plunged into a new Ice Age (by which he meant a new glacial episode) within a couple of hundred years by an inexorable process which started with a dusty or cloudy episode shutting off solar radiation from extensive polar and mid-latitude areas during a sequence of winters and summers.  Heavy snowfalls would thus continue to lie on the ground from one winter to the next, with the high albedo of the ground surface radiating back solar radiation and thus cutting off summer warming of the ground surface.  Heat loss would be exacerbated by the establishment of a high-pressure block accompanied by clear skies for many months at a time.  According to Calder, winter snowfalls would then simply accumulate, with inadequate summer heat to melt them off -- and the ice age would be triggered off by positive feedback.  Hubert Lamb contributed to the debate by calculating that the thickness of the snowcover over the land surface might accumulate at the rate of 50 cm per year -- enough to give us 100m of accumulated snow over 200 years.  This in turn would be sufficient to create firn and then glacier ice at depth.  So hey presto!  You have an ice-covered landscape, about which mere mortals would be able to do nothing at all........  remember that disaster movie from 2004 called "The Day After Tomorrow"?  You get the general idea.....

 The idea of the imminent new ice age has gone -- in fact it did not survive beyond about 1978, when clear signs began to emerge of the "cool decades" coming to an end -- leading, as we all know, to an inexorable and accelerating period of climate warming, linked to man-made CO2 increases in the atmosphere.  Look at the above graph, which shows the cool phase in a longer context.

Since the 1970's there has been a huge amount of research on the topic of rapid climate change, much of it centred on the Younger Dryas cooling episode.  The main question has been "Can GLOBAL climate (as distinct from regional climate or weather patterns) change as rapidly as 6 or 7 deg C in the course of a single millennium, or is there a theoretical limit to the rate of change at about one deg C per millennium?"  Evidence from ice sheet cores, sea-floor sediments and elsewhere has flooded into the public domain, some of it contradictory but generally leading to this conclusion: cold episodes (glacial phases) can end extremely rapidly, as a result of catastrophic ice sheet and glacier melting, with positive feedback mechanisms coming into play.  So a global warming of more than 5 deg C in a millennium does seem to be possible -- and is supported by field evidence.   On the other hand global cooling is a much slower process, and an equivalent cooling of 5 deg C is now thought to take at least 5,000 years.

Maybe there are more surprises out there, waiting to be uncovered, but for the moment the snowblitz theory is very much out of fashion.  There is something to be said for a high-latitude snow accumulation scenario during the growth of the Laurentide Ice Sheet in Northern Canada (proposed by Jack Ives) and during the growth of the Greenland Ice Sheet (Fristrup and others) -- but the survival of an extensive lowland cover in high latitudes is favoured by the rhythm of the seasons, with very short summers and very long winters.  In mid-latitudes, in contrast, it would be difficult to make the heat flux equations stand up to scrutiny -- and the feeling is that even during a glacial episode, except maybe with an ice sheet edge quite close, winter precipitation would almost always be approximately balanced by summer melting.

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