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Whence the Little Ice Age?

In the second half of the past millennium, glaciers advanced and temperatures fell in many regions around the world, although the timings and durations of the cold spells differed widely between regions. Numerous causes for this Little Ice Age have been proposed, including cyclical reductions in solar irradiance, changes in ocean circulation, and increases in volcanic activity, but the direct radiative effects of these processes are either weak or short-lived and require substantial feedbacks within the climate system. Miller et al. now exploit the fact that small ice caps in Arctic Canada are retreating, exposing vegetation that was entombed when the ice caps were advancing. By radiocarbon-dating these plants, the authors can accurately determine the time of glacial advance. They identify abrupt summer temperature decreases in the late 13th century and between 1430 and 1455 CE; subsequently, most ice caps remained in an expanded state until the 20th century. Comparison with varved sediments recording ice-cap growth from Iceland provides support for these dates, which coincide with periods of enhanced volcanic activity. Climate model results suggest that short-lived volcanic eruptions, reinforced by weak solar irradiance changes, can cause a sea-ice/ocean feedback that yields a centuries-long reduction in summer air temperature across the Arctic.

Geophys. Res. Lett. 10.1029/2011GL050168 (2012).