The Little Ice Age (LIA) was a recent and significant climate perturbation that may still be affecting the Earth’s climate, but nobody knows what caused it. In this post I look into the question of why it ended when it did, concentrating on the European Alps, without greatly advancing the state of knowledge. I find that the LIA didn’t end because of increasing temperatures, decreasing precipitation or fewer volcanic eruptions. One possible contributor is a trend reversal in the Atlantic Multidecadal Oscillation; another is an increase in solar radiation, but in neither case is the evidence compelling. There is evidence to suggest that the ongoing phase of glacier retreat and sea level rise is largely a result of a “natural recovery” from the LIA, but no causative mechanism for this has been identified either.
The Little Ice Age (LIA) was a period of lower global temperatures defined by temperature reconstructions based mostly on tree ring proxies. Figure 1 shows the results of fifteen such reconstructions for the Northern Hemisphere with three instrumental records added after 1900 (data from NOAA/NCDC). The period of lower temperatures between about 1450 and 1900 roughly defines the LIA, but the high level of scatter (cunningly muted by plotting the more erratic reconstructions in lighter shades) makes it impossible to pick exact start and stop dates:
Figure 1: Northern Hemisphere temperature reconstructions over the last 2,000 years
Because of the problems with temperature reconstructions this post concentrates on the European Alps, where long-term instrumental records – some going back to the early 1700s – provide information on temperature and precipitation changes around the time the LIA came to an end. Another reason for concentrating on the Alps is that almost half of the world’s glaciers that have long-term monitoring data are located there.
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Bond cycles are defined by petrological tracers from core samples in the N Atlantic that link to the pattern of drift ice distribution. They provide a record of shifting ocean currents and winds, in particular periodic weakening of the North Atlantic current and strengthening of the Labrador current. These cycles shape what we perceive as climate change in the circum North Atlantic realm, for example the Little Ice Age and Medieval Warm Period. They leave a small mark on global average temperatures that are difficult to resolve in the proxy temperature records
