There is a mean correlation map between the AMOC and SST. The location of the sites is indicated by the number of stars. The image is from Lapointe et. al.
One of the persistent questions in historical climatology, environmental history and the earth sciences is what caused the Little Ice Age. Warming is the answer.
The Little Ice Age was one of the most cooling periods of the past 10,000 years and it was particularly pronounced in the North Atlantic region. The cold spell that seems to have set in around 600 years ago was responsible for crop failures, famines and epidemics throughout Europe, resulting in misery and death for millions. The mechanisms that led to this harsh climate state have not been fully understood. A new paper published in Science Advances gives an up-to-date picture of the events that brought about the Little Ice Age. The cooling appears to have been caused by a warm episode.
The results of their 3,000-year reconstruction of North Atlantic sea surface temperatures were published in the Proceedings of the National Academy.
Lapointe and Bradley discovered that there was a strong northward transfer of warm water in the late 1300s. The waters south of Greenland and the Nordic Seas became warmer than usual. "No one has ever seen this before," says Lapointe.
Warm water is usually transferred from the tropics to the north. The process called the Atlantic Meridional Overturning Circulation (AMOC) is similar to a planetary conveyor belt. Warm water from the tropics flows north along the coast of Northern Europe, and when it reaches higher latitudes and colder waters, it loses heat and becomes denser, causing the water to sink at the bottom of the ocean. The formation of this deep-water formation then travels south along the coast of North America and continues to circulate around the world.
The rapid loss of ice was caused by the warm water moving north because of the AMOC's strength. In the late 1300s and 1400s, huge amounts of ice were flushed out into the North Atlantic, which caused the AMOC to collapse. The collapse triggered a cooling.
Between the 1960s and 1980s, we have seen a rapid strengthening of AMOC, which has been linked with persistently high pressure in the atmosphere. Lapointe and Bradley think that the same atmospheric situation occurred just prior to the Little Ice Age.
Lapointe discovered that the answer was to be found in trees. When the researchers compared their findings to a new record of solar activity revealed by radiocarbon, they discovered that it was high in the late 1300s. High atmospheric pressure is usually caused by solar activity.
There was less ash in the air because there were fewer volcanic eruptions. The planet was more responsive to changes in solar output because of a cleaner atmosphere. Lapointe said that the effect of high solar activity on the atmospheric circulation in the North-Atlantic was particularly strong.
Bradley and Lapointe wondered if such an abrupt cooling event could happen again in our age of global climate change. They note that there is now less sea ice due to global warming, so an event like that in the early 1400s is unlikely. The build-up of freshwater in the north of Alaska has increased by 40% in the past two decades. Lapointe said that its export to the subpolar North Atlantic could have a strong impact on the ocean circulation. Over the past decade, there have been more frequent periods of high pressure over the island in the summer. 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 weakened or collapse of the AMOC. There is an urgent need to address uncertainties according to the authors.
Francois Lapointe, Little Ice Age was triggered by intrusion of Atlantic waters into the Nordic Seas. Science.org has a DOI of/10.1126/sciadv.abi8230.
The surprising cause of the Little Ice Age was discovered by researchers on December 15th, 2021.
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