New research is pointing instead at volcanic eruptions having been at the heart of the global cooling of the 17th century.
There has been some speculation that the so-called Little Ice Age, which hit the UK 400 years ago, a cold snap so severe that the river Thames was frozen enough for ice-fairs to be held on it for many years, might not have been the result of reduced solar activity after all.
New research is pointing instead at volcanic eruptions having been at the heart of the global cooling of the 17th century. Climatologists are aware that major glacial periods depend on the orbit of the planet, northern hemisphere summer temperatures on a slow decline for 8,000 years now as the earth reaches the point when it is farthest from the Sun.
This time could signal that we are about to enter the next glacial period in earth history, though an occasional blip like that of four centuries ago is not unknown. The Little Ice Age was a simple, synchronized global cooling, extent and timing varying from place to place, but generally happening from 1400 to 1800, North America and Europe experiencing much colder winters in this period.
The so-called Maunder Minimum was a period of low solar activity between 1650-1700, but this coinciding with the Little Ice Age seems not enough of a reason for such cold temperatures for so long. In an effort to get a clearer picture, researchers scoured glacial records, beginning with a small, northern Canadian ice cap.
This sort of glacier being frozen to the underlying ground means that it does not disturb the sediment below it. By dating organic matter within this, researchers could pin-point the onset of the Little Ice Age. Comparing this to records from Iceland ice cap in Langjökull, they worked out that the coldest times were the 15th and 16th centuries.
The cooling clearly started long before the Maunder Minimum, onset of the cold correlating to large volcanic eruptions, one in 1259 in the Mexico region that was perhaps the greatest single volcanic event of the last millennium, and then again in 1453, when Kuwae – South Pacific – erupted, spewing six times as much gas, ash, and rock as Pinatubo in 1991.
Scientists simulated these and more eruptions using a climate model, finding that the atmospheric pollution involved did indeed create a persistent cool period, possibly precipitating the Ice Age Little. Initial cooling caused arctic sea ice expansion, North Atlantic sea ice levels high both around 1200AD and again in 1400AD, through to the end of the Little Ice Age.
Melting sea ice then poured much more freshwater into the North Atlantic, weakening the northward heat transport and keeping sea ice levels higher than before the eruptions, so while the Maunder Minimum may have contributed to the Little Ice Age, it needed those several large volcanic eruptions to nudge the climate into such a cool state for several centuries. Thank goodness we are not experiencing these conditions now.