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Tuesday, 11 January 2011

The Sensitivity of Little Ice Caps


In trying to learn some lessons about how sensitive small ice caps may be on undulating plateau surfaces, we are lucky to have such good data from NW Iceland.  Historical records and maps go back a long way, and on the above map I have marked four different maps of ice edges, the earliest of which is from Knopf in 1734.  That was between two of the coldest phases of the Little Ice Age.  Note that between 1734 and 1844 the ice caps of Drangajokull and Glamajokull both expanded.  They probably reached their greatest Little Ice Age extent around 1850 -- at a time of great hardship (and glacier advances) across Europe.  But 50 years later there had been a catastrophic glacier retreat, which continued through to 1913-14.  By that time Glamajokull had all but disappeared.  A century later, there is still nothing there but some permanent snow-patches.

Drangajokull has survived.  Why?  That's an interesting question, since the two plateaux of Glama and Dranga are at more or less the same altitude -- with extensive areas over 700m and summits at around c 920m.  But Icelandic glaciologists have pointed out that the "glaciation limit" -- which roughly equates to the firn line or ELA (equilibrium line altitude) -- is at around 1000m on Glama but at around 800m on Dranga.  So on Glama, with a slightly warmer and more oceanic climate, ablation exceeds accumulation, and glaciers can no longer survive.  On Dranga, in contrast, there is (or was until recently) a rough state of balance.

There are some interesting lessons here if we want to understand what happened on the hill masses of SW England around the peak of the Devensian and other glaciations.

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