In early February 1814, an elephant walked across the surface of the Thames near Blackfriars Bridge in London. The stunt was performed during the frost fair, when temperatures were so cold that for four days the top layers of the river froze solid. Londoners promptly held a festival, complete with what might now be called pop-up shops and a lot of unlicensed alcohol.
Nobody could have known it at the time, but this was the last of the Thames frost fairs. They had taken place every few decades, at wildly irregular intervals, for several centuries. One of the most celebrated fairs took place during the Great Frost of 1683-1684 and saw the birth of Chipperfield’s Circus, but the river in central London has not frozen over since 1814.
The frost fairs are perhaps the most emblematic consequences of the “little ice age,” a period of chilly weather that lasted for several centuries.
Illustration: Mountain People
However, while Londoners partied on the ice, other communities faced crop failures and other threats. The story of the little ice age is one of societies forced to adapt to changing conditions or perish.
It is also a long-standing mystery. Why did the climate cool and why did it stay that way for centuries? Thanks to decades of studies, an explanation is finally being closed in on. The emerging story involves volcanoes, the oceans, possibly the sun and possibly also genocide.
Like most things in science, the little ice age was discovered slowly and piecemeal.
“This all came about because there were lots of documentary records from around Europe, stating that there were some really cold winters,” said Paola Moffa-Sanchez, a climatologist and assistant professor at Durham University.
These were reflected in records of grain prices, which rose because of crop failures and ships’ logs saying that Greenland was surrounded by sea ice and unreachable.
The term “little ice age” was coined by a Dutch-born geologist named Francois Matthes, who in a 1939 report said that glaciers in the Sierra Nevada in California had regrown at some point within the past few millennia.
The term stuck, but it took decades to narrow down the timeframe.
This was done by British climatologist Hubert Lamb, who went on to found the Climatic Research Unit at the University of East Anglia.
In a study published in 1965, Lamb used European temperature records going back many centuries to identify a “notably warm climate around AD1000-1200” that was “followed by a decline of temperature levels till between 1500 and 1700 the coldest phase since the last ice age occurred.”
This chilly period was “undoubtedly upsetting for the human economies of those times (and perhaps of any time),” he wrote.
Since then, climatologists have tried to specify the duration and extent of the little ice age, but this has proved difficult. Most of the records showing the cooling are from Europe, and records from elsewhere do not always show it.
“It’s not a global phenomenon, in that it wasn’t cool everywhere,” said Alexander Koch, a researcher at Simon Fraser University in Burnaby, Canada.
In some places, such as China, the period was marked by a wetter, but not cooler climate.
What is more, “the cooling was not continuous,” said Dagomar Degroot, an environmental historian and associate professor at Georgetown University in Washington. “It came in waves that reached different places at different times at different magnitudes.”
For this reason, the start and end of the little ice age are a little vague.
“The classical definition is between 1400 and 1850,” Moffa-Sanchez said.
However, “some people say 1300 to 1850,” pushing the start date back significantly, she said.
There is broad agreement that the 1400 to 1800 period is within the little ice age, but outside that it is fuzzy, Moffa-Sanchez said.
Either way, the 1814 frost fair was near the end of the period.
Furthermore, the little ice age was not technically an ice age. These are periods in which the Earth has permanent ice at both poles, and we have been in one for more than 2.5 million years. Within that period, the ice sheets have repeatedly waxed and waned.
The most recent ice advance, which is colloquially called the ice age, but is really a glacial period, occurred from about 115,000 to 11,700 years ago.
Since then, it has been a relatively warm period called the Holocene.
In that context, the little ice age is minor.
“We’re talking about several 10ths of a degree centigrade,” Degroot said.
In contrast, the coldest portion of the last glacial period is thought to have been about 6°C cooler than the past 4,000 years.
However, even this small change mattered, Degroot said.
“On a regional or local scale, the anomalies could be really quite extreme,” he said, adding that documentary evidence shows that “at least some people were aware that they were living in an anomalous kind of climate.”
What was going on? The story is not entirely settled, but researchers are increasingly confident about the initial trigger: volcanoes.
“You have these eruptions that are happening in clusters,” Degroot said.
A 2015 study used data from ice cores to identify 25 major eruptions from the past 2,500 years. Between 1200 and 1400 there were huge eruptions of the Samalas volcano in Indonesia, the Quilotoa in Ecuador and El Chichon in Mexico.
Big eruptions blast sulphate aerosols into the stratosphere, the layer of the atmosphere above the weather. These aerosols reflect some of the sun’s rays back into space, cooling the Earth.
In recent times, the 1991 eruption of Mount Pinatubo in the Philippines triggered up to 0.5°C of surface cooling.
Usually, an eruption only cools the climate for a few years.
“But if you have big eruptions in clusters, that can set off positive feedbacks in the climate system,” Degroot said.
For example, several years of cooler temperatures cause sea ice to expand. Sea ice is white, so compared with dark blue water it reflects more radiation back into space.
“That can prolong and exacerbate the cooling,” Degroot said.
There can also be knock-on effects in the ocean, Moffa-Sanchez said.
If the winds change, huge rafts of sea ice can be carried south from Greenland into the Labrador Sea. There, they interfere with the huge Atlantic currents that carry warm water from the tropics toward Europe.
This is a complex story in which the slight initial cooling from the volcanoes triggers changes in the Earth system that lead to more and lasting cooling, but it seems to hold true.
A 2018 modeling study found that it was not possible to explain the little ice age without invoking volcanic eruptions, although that did not mean other factors were not also at work.
Another possible factor is the sun. The amount of energy it pumps out varies ever so slightly, most famously over the 11-year solar cycle during which activity varies from a maximum to a minimum and back again.
The effects on Earth are so small that they are hard to detect, but the sun sometimes has more of an effect.
Several times in the past 1,000 years, Earth’s star has entered a “ grand minimum,” in which it spends several decades being less active.
The most recent was the Dalton Minimum between 1790 and 1820. This was preceded by the Maunder Minimum of 1645 to 1715. Before that, there is thought to have been the Sporer Minimum between about 1460 and 1550 and the Wolf Minimum around 1280 to 1350.
Such grand minima can cool the planet, although by no more than 0.3°C, and probably less.
Grand minima probably played a role in some of the chilliest episodes, Moffa-Sanchez said.
In her studies of the period, “cold centuries coincided with these really famous solar minima,” she said.
She has found evidence that grand minima affect wind patterns, with knock-on effects for ocean currents and heat distribution.
However, it seems unlikely that grand minima alone caused the little ice age. The timings do not fit and, in any case, the climate effects of grand minima are much smaller than those of massive eruptions.
It could be that a solar maximum in the late 1300s also played a role. A study published in December last year found that this shifted wind patterns, interfering with the crucial Atlantic warm water current.
The key point is that this is not an either-or debate, Moffa-Sanchez said, adding: “It’s a likely combination of all of them.”
The idea of numerous causes helps to explain why the climate shifts came in waves, rather than there being one abrupt change to a lasting cooler state.
“It was not that you had this little ice age period where it was always cold,” Moffa-Sanchez said. “You just had several cold centuries throughout this four-century-long period.”
However, one big puzzle remains. The coldest period of the little ice age was around 1610, and it does not coincide with a grand minimum. Nor was there a particularly big eruption: Peru’s Huaynaputina went off in 1600, but while the blast was large it was not exceptional.
Instead, the suggestion is that this cold spell was caused by humanity — in a truly horrible way.
In 1492, Christopher Columbus reached the Americas. Over the following decades, Europeans began colonizing them. In the process, they fought with indigenous Americans, often killing them, but even more lethally, they brought diseases. One of the worst was smallpox, which killed millions.
As well as being a genocide and a tragedy, this might have had an effect on the climate.
Many indigenous Americans were farmers who had cleared forests for their crops, and when they died the trees grew back, drawing carbon dioxide out of the air and cooling the planet.
This scenario was first outlined by climatologist William Ruddiman in 2003 as part of his “early Anthropocene” hypothesis that humans have been affecting Earth’s climate for millennia, albeit less than we are today.
The idea that mass deaths among indigenous Americans led to climate cooling has received tentative support from modeling studies. Still, it has been controversial, because there are so many uncertainties about the key numbers.
However, Koch and his colleagues published an updated analysis in 2019.
They went through the argument step by step and tried to quantify everything, from the number of people who died to the extent of reforestation. They concluded that the European arrival led to 56 million deaths by 1600.
This dreadful toll meant trees grew again on 56 million hectares of land, removing 27.4 billion kilograms of carbon dioxide from the air.
“It’s a really interesting theory,” Degroot said.
However, he said he remains skeptical because it is not known how land use was changing in other parts of the world, especially Africa.
While the question of whether human society contributed to the little ice age remains up for discussion, what is clear is that the little ice age affected human society.
For example, Norse settlers made their home in Greenland in 985 and stayed for centuries, only abandoning it in the early 1400s. It has often been suggested that the little ice age played a role in that.
However, a study published in March looked at sediments from the region and found no sign of cooling, but it did find a drying trend, which would have meant less grass to feed livestock.
Such stories might be true, but it is crucial to remember that people were not passive victims, Degroot said.
“You don’t just see people who are completely at the whim of changing climatic conditions, you see people changing adaptively,” he said.
Degroot said there was often remarkable activity in the Arctic, despite the increase in sea ice.
Between 1611 and 1619, European whalers operated off the shores of Svalbard, because there was a shortage of vegetable oil and whale oil was a substitute.
For the Dutch Republic, the period between 1560 and 1720 was something of a golden age, despite or even because of the cold winters. It did not depend on home-grown agriculture, so local crop failures were less of a problem, Degroot said.
Instead, the republic’s economy was driven by merchant ships, whose operators devised ingenious ways to cope with the cold — and the Dutch thrived while their neighbors struggled.
“It so rarely is what you’d expect,” Degroot said.
Studying how past societies responded to climatic shifts such as the little ice age is likely to help us all as the climate crisis becomes ever more intense.
“Hopefully, we can learn from them, figure out what they did right, what they did wrong,” Degroot said.
We had better learn fast, because the little ice age was just a taster.
Back then, the average global temperature cooled by a fraction of a degree, but we have already warmed it by 1.1°C, and are set to blow past 1.5°C in the next few decades.
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