The Little Ice Age, which set about 600 years ago, was one of the coldest periods during the past 10,000 years. It was responsible for crop failures, famines, and pandemics throughout Europe, resulting in the misery and death of millions of people. Until now, the causes of this cooling remained mysterious.
New research led by the University of Massachusetts Amherst has found a rather paradoxical answer to the puzzle of the Little Ice Age: the cooling appeared to have been triggered by an unusually warm episode.
By analyzing a 3,000-year reconstruction of North Atlantic sea surface temperatures, the researchers noticed a sudden change from very warm temperatures in the late 1300s to unprecedented cold conditions only 20 years later, in the early 1400s. Apparently, there was an abnormally strong northward transfer of warm water in the late 1300s, which peaked around 1380, and resulted in the unusual warming of the waters south of Greenland and the Nordic Seas.
Usually, there is always a transfer of warm water from the tropics to the Arctic through a process called the Atlantic Meridional Overturning Circulation (AMOC). However, in the late 1300s, AMOC strengthened significantly, moving north far more water than before, and causing rapid arctic ice loss. From the late 1300s to early 1400s, vast amounts of ice were flushed out into the North Atlantic, cooling its waters and diluting their saltiness, ultimately causing AMOC to collapse. According to the scientists, this collapse triggered a substantial global cooling.
One of the causes for the changes in atmospheric circulation affecting the AMOC has been the unusually high solar activity recorded in the late 1300s, which led to higher atmospheric pressure over Greenland. At the same time, fewer volcanic eruptions provided a cleaner atmosphere that was more responsive to changes in solar output.
In the wake of these discoveries, scientists are wondering whether out current climate change could cause such an abrupt cooling event. While the diminishing of artic ice due to global warming makes such a scenario rather unlikely, other climatic processes currently at play could have quite unpredictable results.
“Persistent periods of high pressure over Greenland in summer have been much more frequent over the past decade and are linked with record-breaking ice melt,” said study lead author François LaPointe, a postdoctoral fellow in Geosciences at UMass Amherst.
“Climate models do not capture these events reliably and so we may be underestimating future ice loss from the ice sheet, with more freshwater entering the North Atlantic, potentially leading to a weakening or collapse of the AMOC.”
The study is published in the journal Science Advances.