By Liz Kalaugher
Once an ice sheet starts to melt, the surface of the ice gradually decreases in altitude and becomes warmer, leading to yet more melting in a positive feedback effect. According to Jonathan Bamber of the University of Bristol, UK, speaking at the Copenhagen session on tipping points, that makes the process pretty much irreversible once it’s started in earnest – you’d need a very substantial cooling for the ice sheet to return.
The complete collapse of the Greenland ice sheet would lead to around 6.5 m of sea level rise. So scientists are keen to know at what temperature melting of the ice sheet is likely to become irreversible. A few years ago Jonathan Gregory calculated this threshold at 3 degrees of temperature rise but Bamber says there are two lines of evidence that suggest
this is wrong – the past and the modelling future. “I think there are other processes in there that may be important,” he said. In the Eemian Greenland was about 5 degrees warmer than today, considerably above Gregory’s threshold, but there was still an ice sheet present (although probably about half its present volume) and it remained in place for 20,000 years.
Bamber has recalculated the critical threshold
temperature for ice sheet melting by forcing two surface mass balance models
with real future climate. The first model, a positive degree day (PDD) model,
which says that the ice sheet will melt if the temperature falls below zero,
gave a temperature threshold of 4 degrees. That figure is comparable to
Gregory’s threshold of a 3 degree average global temperature rise, which
corresponds to a temperature increase in Greenland of around 4.5 degrees.
But in Bamber’s second calculation the relatively
sophisticated energy balance model, which he believes better represents ice
sheet behaviour, gave a threshold of 8 degrees for irreversible melting of
Greenland – double the previously published threshold.