If you ask more than one researcher about the climate episode known as the Little Ice Age, you are likely to get more than one answer. It's an odd fact of climate science, but some more recent events are harder to figure out than those in the more distant past.
Scientists have argued over the timing of the Little Ice Age, whether it kicked in around 1300 AD or closer to 1650. They have argued about its extent — whether it was global or really just a regional cold spell felt around the North Atlantic and hardly worthy of the name. What caused these windy, miserable conditions is another mystery.
But the fog is beginning to lift. New research, using new techniques, published in this week's issue of the journal Science establishes a strong connection in the timing of the advances of glaciers in the Scandinavian Alps and the growth of southern tropical glaciers of the Peruvian Andes.
The work of glacial geologist Joseph Licciardi of the University of New Hampshire and colleagues tells an interesting, complex story. Glaciers of the southern Tropics might have been expected to move in concert with New Zealand's Southern Alps, which reached their greatest extent 6,500 years ago, but that is not the case.
"If we compare records — New Zealand, Europe, Peru — we can say that the tropical Andes look like Europe but not New Zealand," said Licciardi. "What's emerging is a more complicated picture of recent glaciations."
The technique they use is called "surface exposure dating." When glaciers advance, they push along boulders and rubble that is left behind in ridges when they retreat. This moraine of rock, newly exposed, for the first time begins absorbing incoming cosmic rays from the atmosphere and forming a radioactive isotope of the element beryllium. The more beryllium-10, the older the moraine. Recent advances in nuclear chemistry allow researchers to detect such small traces of the beryllium that now they are able to precisely date these events right up to the fairly recent past. (The photograph, courtesy of Licciardi, shows moraines from two different advances at the foot of an Andean peak. The Little Ice Age moraine is in the background.)
What links the behavior of glaciers in these far flung regions during the past 1000 years? Earlier work by co-author David Lund of the University of Michigan detected a weaker Gulf Stream that delivered less tropical warmth into the North Atlantic during the Little Ice Age. A colder North Atlantic provoked the glacier expansion in Scandinavia, the thinking goes. This more southerly Gulf Stream track pushed the convergence zone of the easterly trade-winds farther south, bringing more snowfall to the Tropical Andes.