The Sudden Onset of the Little Ice Age

The earth's climate does not change gradually. It lurches from one state to another in sudden, dramatic shifts; this year you're in a warm period, next year a cold one. I think one of the scariest findings of science is the graph above, based on climate proxy data from the Greenland ice core. Look at those dramatic rises and fall -- all the fluctuations of the past 4000 years, which have had dramatic effects on our civilization, are blips compared to the extreme changes that took place around the end of the last Ice Age.

Look farther back and you see the same dramatic shifts when that Ice Age began, around 120,000 years ago. (Like the way the people who made this graph reinforced the height of the curve in the Eemian, the last warm period, by putting a mountain behind it?)

Anyway, a new look at the onset of the Little Ice Age, which lasted from about 1300 to 1850 AD, suggests that it also began suddenly:

It’s long been known that much of the globe became chillier during the Renaissance. By the 17th century, temperatures in the Northern Hemisphere had fallen by half a degree Celsius compared with medieval times. Ice skating on London’s frozen River Thames became popular.

To pin down when this climate change began, Miller’s team traveled to Baffin Island on the northern fringes of Canada. Small glaciers in this region tend to respond quickly to temperature changes. Carbon dating of moss entombed in Baffin’s ice revealed two sudden advances of the snow line that killed off the vegetation: a sudden cold spell between 1275 to 1300, followed by intensifying cold between 1430 and 1455.

Testing whether this chill extended beyond Canada took the researchers to the Langjökull glacier, the second largest ice cap in Iceland. Layered sediments from a nearby lake appeared progressively thicker in the 14th century — exactly what would be expected if the glacier expanded and ground away the landscape.

These chillier conditions began during an especially active time for volcanoes. “The second half of the 13th century had the most volcanism of any period of the past 1,500 years,” says Alan Robock, an atmospheric scientist at Rutgers University in New Brunswick, N.J. 

Normally the earth recovers rapidly from episodes of volcano-induced cooling, but these researchers suggest that a series of cooling events in a short time led to such a big expansion of sea ice, which reflects sunlight and cools the earth, that the cooling became self-sustaining. Then again, maybe it was something else. But whatever it was, it happened quickly.

And while I'm producing climate graphs, let me post this one again, which explains why so many scientists are freaked about the climate future. You can see that by and large the temperature and the amount of carbon dioxide in the atmosphere are closely related. And right now the level of atmospheric CO2 is soaring. The temperature, so far, is not, but if the CO2-temperature relationship holds, we are in for a warming the likes of which the planet has not seen in 10 million years. The last time atmospheric CO2 was as high as it is now, back in the Miocene, global sea level was 50 meters (165 feet) higher than today.