More elderly readers of these pages may remember having heard in their school days back (circa 1970s) that scientists then thought an ice age would be coming soon. I certainly do – even though the alleged ‘global cooling consensus’ in the scientific literature of the time has recently been disproved as a myth.
Now an interesting new paper in Nature [subscription] suggests that a rapid natural transition towards a stable glacial climate, with permanent ice sheets covering large parts of North America and Eurasia, could indeed be ahead.
Thomas Crowley and William Hyde ran a coupled energy-balance/ice-sheet model to test their hypothesis. When forced with long-term variations in daily solar radiation which result from small cyclical changes in the Earth’s orbit, the best-fit model run (that is the one which most accurately reproduced ice sheet variations during the last 3 million years) predicted a rapid transition towards a cold climate regime to occur merely some 10,000 to 100,000 years from now.
Models and theory do indeed suggest that at critical points (namely when climate variability is at a maximum) large ice sheets can rapidly develop from very small perturbations in solar forcing.
The Nature authors hypothesize that the increasing climate variability within the past million years may indicate that the climate system is approaching a ‘bifurcation’ point at which it will undergo a transition to a new stable state with vastly expanded ice sheets. In the new regime, most of Canada, Britain, Russia and Germany would be permanently covered by thick ice.
Should it be attained, the new state “would be more ‘symmetric’ than the present climate, with comparable areas of ice/sea-ice cover in each hemisphere, and would represent the culmination of 50 million years of evolution from bipolar nonglacial climates to bipolar glacial climates”.
But will this ominous bifurcation point really be reached at any time soon? Well, we know from sediment and ice cores that the increase in climate variability during the last one million years, compared with early Pleistocene climate fluctuations, is real. (The Holocene climate of the last 10,000 years was exceptionally stable, but large glacial-interglacial climate oscillations occur on much longer time periods.)
Now enter humankind, however. Crowley and William’s study assumes a planetary climate system undisturbed by anthropogenic activity, such as fossil fuel burning. But at the current level of greenhouse gas emissions it seems clear that man-made global warming will easily eclipse any naturally occurring transition towards a colder climate regime.
“Presumably,” the authors write, “future society could prevent this transition indefinitely with very modest adjustments to the atmospheric CO2 level.”
For now, it’s everybody’s guess whether this is good or bad news.