Scientists discover ‘surprising’ cause of Europe’s little ice age in late medieval era

Change in ocean currents – similar to phenomena seen today – likely cause behind substantial cooling, US scientists say

Following an era known as the medieval warm period, temperatures in Europe in the early 15th century fell sharply in what has become known as the little ice age.

This remarkable cold period brought increased glaciation in mountains, expansion of some areas of sea ice, crop failures, famines and disease across Europe.

Undependable summers were followed by harsh winters, during which rivers and canals routinely froze over. In the UK, the first River Thames “frost fair” was held in 1608, and was an almost annual occurrence until the last one in 1814.

The little ice age was not believed to have been due to a global period of glaciation, but its exact cause has remained uncertain.

There are numerous explanations, including heightened levels of volcanic activity, reduced solar activity, the impact of the black death reducing the human population, and the impact of European diseases on South American populations, which in turn affected deforestation and reforestation rates.

But scientists at the University of Massachusetts now believe they have found a new key factor in why temperatures plunged to their coldest in 10,000 years.

“Surprisingly, the cooling appears to have been triggered by an unusually warm episode,” the researchers said.

The discovery came after Lead author Francois Lapointe, a postdoctoral researcher and lecturer in geosciences at the University of Massachusetts, and Raymond Bradley, distinguished professor in geosciences, also at the University of Massachusetts, came across new data suggesting a rapid change in sea temperatures.

Their previous work, which built a 3,000-year reconstruction of North Atlantic sea surface temperatures, revealed a sudden change from very warm conditions in the late 1300s to unprecedented cold conditions in the early 1400s, only 20 years later.

Using various sources to obtain detailed marine records, Dr Lapointe and Professor Bradley discovered there had been an abnormally strong northward transfer of warm water in the late 1300s which peaked around 1380.

As a result, the waters south of Greenland and the Nordic Seas became much warmer than usual.

“No one has recognised this before,” said Dr Lapointe.

The researchers said that usually there is always a transfer of warm water from the tropics to the arctic.

It’s a well-recognised process called the Atlantic Meridional Overturning Circulation (AMOC), which is like a planetary conveyor belt.

When it is functioning normally, warm water from the tropics flows north along the coast of Northern Europe, and when it reaches higher latitudes and meets colder arctic waters, it loses heat and becomes denser, causing the water to sink at the bottom of the ocean.

This deep-water formation then flows south along the coast of North America and continues on to circulate around the world.

But in the late 1300s, the AMOC strengthened significantly, which meant that far more warm water than usual was moving north, which in turn caused rapid arctic ice loss.

Over the course of a few decades in the late 1300s and 1400s, vast amounts of ice were flushed out into the North Atlantic. This additional ice not only cooled the North Atlantic waters, but also diluted their saltiness, ultimately causing AMOC to collapse. It is this collapse of the conveyor belt which triggered substantial cooling, the researchers said.

Furthermore, a similar process could now be underway.

Between the 1960s and 1980s, we have also seen a rapid strengthening of AMOC, which has been linked with persistently high pressure in the atmosphere over Greenland.

Dr Lapointe and Professor Bradley believe the same atmospheric situation occurred just prior to the little ice age – but what could have set off that persistent high-pressure event in the 1380s? The answer, Dr Lapointe suggested, can be found in trees.

The researchers compared their findings to a new record of solar activity revealed by radiocarbon isotopes preserved in tree rings, and discovered that unusually high solar activity was recorded in the late 1300s.

They said such solar activity tends to lead to high atmospheric pressure over Greenland.

At the same time, fewer volcanic eruptions were happening on earth, which means that there was less ash in the air. A “cleaner” atmosphere meant that the planet was more responsive to changes in solar output.

“Hence the effect of high solar activity on the atmospheric circulation in the North-Atlantic was particularly strong,” said Dr Lapointe.

On the question of whether a second little ice age could now emerge, Dr Lapointe and Professor Bradley said there is now much less arctic sea ice due to the climate crisis, so an event like that in the early 1400s, involving sea ice transport, is unlikely.

“However, we do have to keep an eye on the build-up of freshwater in the Beaufort Sea (north of Alaska) which has increased by 40 per cent in the past two decades,“ said Dr Lapointe.

”Its export to the subpolar North Atlantic could have a strong impact on oceanic circulation. Also, 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.

“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 authors said there is now ”an urgent need“ for further research to address these uncertainties.

The research is published in the journal Science Advances.