With Speed and Violence Fred Pearce - Global Commons Institute

Similar switchback temperature changes occurred regularly through the
last glaciation, and there were a number of other "flickers" as the planet
staggered toward a new postglacial world. Stone Age man, with only the
most rudimentary protection from a climatic switchback, must have found
that tough. Heaven knows how modern human society would respond to
such a change, whereby London would have a North African climate,
Mexican temperatures would be visited on New England, and India's
billion-plus population would be deprived of the monsoon rains that feed
them.
The exact cause of the rise and fall of the ice ages still excites disputes. But
it seems that the 100,000-year cycles of ice ages and interglacials that have
persisted for around a million years have coincided with a minor wobble in
Earth's orbit. Its effect on the solar radiation reaching the planet is minute,
and it happens only gradually. But somehow Earth's systems amplify its
impact, turning a minor cooling into an abrupt freeze or an equally minor
warming into a sudden defrost. The amplification certainly involves
greenhouse gases, as Arrhenius long ago surmised. The extraordinary way in
which temperatures and carbon dioxide levels have moved in lockstep
permits no other interpretation. It also probably involves changes to ocean
currents and the temperature feedbacks from growing and melting ice.
We will return to this conundrum later. What matters here is that a minor
change in the planet's heating—much less, indeed, than we are currently
inflicting through greenhouse gases—could cause such massive changes
worldwide. The planet seems primed to leap into and out of glaciations and,
perhaps, other states too.