The Little Ice Age Explained?

The painting “Washington Crossing the Delaware” is instantly familar to just about anyone who ever took a class in American History: it depicts the Revolutionary War commander and future first President crossing from Pennsylvania to New Jersey on Christmas of 1776 to surprise Hessian troops in Trenton and win a great victory. Anyone who has actually seen this part of the the Delaware in winter, though, might wonder why you pretty much never see it choked with ice.

It's not manmade global warming that's to blame: thick ice started to be rare on the lower Delaware in the 1800's, ages before the effects of heat-trapping carbon dioxide began to appear. The ice largely went away with the ending of the LIttle Ice Age (LIA), an all-natural episode of global cooling that lasted for hundreds of years around the middle of the last millennium. What caused the LIA, however, has never been fully explained. One theory involves the so-called Maunder Minimum, a period of reduced sunspot activity — which in turn is linked to a slightly dimmer, cooler sun. The problem there: the Minimum began in the mid-1600's and was over by the early 1700's, but the LIA started centuries before that.

Now comes a new study in Geophysical Research Letters, based on ancient lake sediments in Iceland and plant residues from Baffin Island, Canada, that claims to explain the LIA. The cause: a series of four massive volcanic explosions (think Krakatoa or Tambora), starting in 1275 A.D., which threw up massive amounts of aerosol particles that blocked sunlight and cooled the planet. It was an all-natural version of what some scientists now propose as a form of geoengineering. The aerosols eventually fell to Earth, but the repeated jolts of cooling then, say the authors, led to more ice cover in the North Atlantic, which reflected extra sunlight and altered ocean currents, keeping the planet chilled for centuries.

Mystery solved, then? Maybe. It's rare for a single study to accomplish such a feat, no matter how convincing it might seem. But if further research confirms the idea, it could turn out to be an important piece in the puzzle of how Earth's climate works.