For more than 500 years, until the middle of the 19th century, much of the North Temperate Zone experienced the Little Ice Age (LIA)—the most extended period of anomalously cold summers since the end of the last real ice age 10 000 years ago. The LIA caused numerous famines and wiped out the Norse settlements in southern Greenland. Its cause and multicentury persistence have long been puzzling. Variations in solar irradiation and volcanic eruptions have been invoked as causes, but the one seems too weak and the other too ephemeral. Major volcanic eruptions can create worldwide sulfate aerosols that reflect enough solar radiation to create “volcanic winters.” Such aerosols, however, last only a few years. But now a team at the University of Colorado has used innovative radiocarbon dating of long-dead moss only recently uncovered as perennial icecaps recede in the Canadian Arctic (see photo) to chronicle with almost decadal resolution the LIA’s abrupt onset around 1300 AD. The team concludes that the LIA’s trigger was indeed a quick succession of four major volcanic eruptions in the second half of the 13th century. The team also carried out a long-term climate-model simulation to understand how 50 years of volcanic perturbations could set in train a chill that lasted more than 500 years. The simulation points to the southward expansion of Arctic sea ice, its reflectivity, and its disruption of warming currents in the North Atlantic as principal feedback mechanisms that kept the LIA going. (G. H. Miller et al., Geophys. Res. Lett. 39, L02708, 2012.)—Bertram Schwarzschild
