Between 950 and 1250, there was a major global climate change when

A new study questions the popular notion that 10th-century Norse people were able to colonize Greenland because of a period of unusually warm weather. Based upon signs left by old glaciers, researchers say the climate was already cold when the Norse arrived--and that climate thus probably played little role in their mysterious demise some 400 years later. On a larger scale, the study adds to building evidence that the so-called Medieval Warm Period, when Europe enjoyed exceptionally clement weather, did not necessarily extend to other parts of the world.

"It's becoming clearer that the Medieval Warm Period was patchy, not global," said lead author Nicolás Young, a glacial geologist at Columbia University's Lamont-Doherty Earth Observatory. "The concept is Eurocentric--that's where the best-known observations were made. Elsewhere, the climate might not have been the same." Climate scientists have cited the Medieval Warm Period to explain anomalies in rainfall and temperature in far-flung regions, from the U.S. Southwest to China. The study appears today in the journal Science Advances.

Norse, or Vikings, led by Erik the Red, first sailed from recently settled Iceland to southwestern Greenland around 985, according to Icelandic records. Some 3,000 to 5,000 settlers eventually lived in Greenland, harvesting walrus ivory and raising livestock. But the colonies disappeared between about 1360 and 1460, leaving only ruins, and a longstanding mystery as to what happened. The native Inuit remained, but Europeans did not re-inhabit Greenland until the 1700s.

The Greenlandic Vikings' apogee coincided with the Medieval Warm Period (also known as the Medieval Climate Anomaly), generally dated from about 950-1250; their disappearance followed the onset of the Little Ice Age, which ran from about 1300-1850. Both periods are firmly documented in European and Icelandic historical records. Thus, popular authors and some scientists have fixed on the idea that nice weather drew the settlers to Greenland, and bad weather froze and starved them. But there are no early historical climate records from Greenland. Recently, historians have proposed more complex factors in addition to, or instead of, climate: hostilities with the Inuit, a decline in ivory trade, soil erosion caused by the Vikings' imported cattle, or a migration back to Europe to farms depopulated by the Black Plague.

In the new study, the scientists sampled boulders left by advancing glaciers over the last 1,000-some years in southwest Greenland, and on neighboring Baffin Island, which the Norse may also have occupied, according to newly uncovered evidence. Glacial advances during the Little Ice Age have wiped out most evidence of where the glaciers were during the Norse settlement. But Young and his colleagues were able to find traces of a few moraines--heaps of debris left at glaciers' ends--that, by their layout, they could tell predated the Little Ice Age advances. Using newly precise methods of analyzing chemical isotopes in the rocks, they showed that these moraines had been deposited during the Viking occupation, and that the glaciers had neared or reached their later maximum Little Ice Age positions between 975 and 1275. The strong implication: it was at least as cold when the Vikings arrived as when they left. "If the Vikings traveled to Greenland when it was cool, it's a stretch to say deteriorating climate drove them out," said Young.

The findings fit with other recently developed evidence that the effects of the Medieval Warm Period were not uniform; some places, including parts of central Eurasia and northwestern North America, may actually have cooled off.

In the Atlantic region, the research includes a 2013 study of ocean-bottom sediments suggesting that temperatures in the western North Atlantic actually went down as the eastern North Atlantic warmed. Other studies of the region suggest a more complex picture. A 2011 study of a core from the Greenland ice sheet shows a strong cooling at the start of Norse occupation, and another in the middle, with interspersed warming. On the other hand, lake-bottom sediments from southwestern Greenland studied in 2011 by Lamont-Doherty paleoclimatologist William D'Andrea, suggest it might indeed have been warm when the Norse arrived, but that climate cooled starting in 1160, well before the Little Ice Age.

The new study may feed recent suggestions by other researchers that the Medieval Warm Period was in part just an extended phase of the North Atlantic Oscillation (NAO). Modern observations show that the NAO is a generally decadal-scale climate cycle, in which warm winds from the west strengthen and boost temperatures in Europe and Iceland, but simultaneously make southwest Greenland and Baffin Island colder, by sucking in more Arctic air. That makes the two regions seesaw in opposite directions.

Gifford Miller, a paleoclimatologist at the University of Colorado, called the paper "a coup de grace on the Medieval Warm Period." Miller said it shows "with great clarity of evidence" that "the idea of a consistently warm Medieval period is certainly an oversimplification and of little utility."

Astrid Ogilvie, a climate historian currently based at Iceland's Akureyri University, said the study "shows that the climate is clearly more complicated and variable than people earlier assumed." As for the Vikings, the climate story has been dimming for some time, she said. "I do not like the simplistic argument that the Greenland people went there when it was warm, and then 'it got cold and they died'," she said. "I think the Medieval Warm Period has been built on many false premises, but it still clings to the popular imagination."

The rocks were analyzed at the University of Buffalo, and at the Lamont-Doherty lab of geochemist and study coauthor Joerg Schaefer. The Lamont lab is among a handful that can precisely date such recent rock deposits. The analyses are done by measuring buildups of small amounts of Beryllium 10, an isotope created when cosmogenic rays strike rock surfaces newly exposed by melting ice.

In addition to Young and Schaefer, the paper was coauthored by Avriel Schweinsberg and Jason Briner of the University at Buffalo, who carried out the Greenland portion of the fieldwork.

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Materials provided by The Earth Institute at Columbia University. Note: Content may be edited for style and length.

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What was the Medieval warm period? What caused it, and did carbon dioxide play a role?

We are living in a world that is getting warmer year by year, threatening our environment and way of life.

But what if these climate conditions were not exceptional? What if it had already happened in the past when human influences were not part of the picture?

The often mentioned Medieval warm period seems to fit the bill. This evokes the idea that if natural global warming and all its effects occurred in the past without humans causing them, then perhaps we are not responsible for this one. And it does not really matter because if we survived one in the past, then we can surely survive one now.

But it’s just not that simple.

The Medieval climate anomaly

This Medieval period of warming, also known as the Medieval climate anomaly, was associated with an unusual temperature rise roughly between 750 and 1350 AD (the European Middle Ages). The available evidence suggests that at times, some regions experienced temperatures exceeding those recorded during the period between 1960 and 1990.

Despite being predominantly recorded in Europe, south-western North America and in some tropical regions, the Medieval warm period affected both the northern and southern hemispheres. But the temperature increase was not universal, varying across regions of the world, and did not happen simultaneously everywhere.

While the northern hemisphere, South America, China and Australasia, and even New Zealand, recorded temperatures of 0.3-1.0 ℃ higher than those of 1960-1990 between the early ninth and late 14th centuries, in other areas such as the eastern tropical Pacific Ocean, it was much cooler than today.

Mechanisms driving the Medieval warm period

The Medieval warm period was by and large a regional event. Its presence or absence reflects a redistribution of heat around the planet, and this suggests drivers other than a global increase in atmospheric greenhouse gases such as carbon dioxide.

The most likely cause of the regional changes in temperature was related to a modification of the El Niño–Southern Oscillation.

This recurring climate pattern of winds and sea-surface temperatures over the tropical eastern Pacific affects the climate and weather of much of the tropics and subtropics. It usually brings clouds and rain in the western tropical Pacific while making regions in the eastern tropical Pacific relatively drier and cooler.

During the Medieval warm period, an increase in solar radiation and decrease in volcanic eruptions created a La Niña-like event that changed the usual patterns. Stronger trade winds pushing more warm water towards Asia created wetter conditions in Australasia, droughts in the southern US and South and Central America, and heavy rains and flooding in the Pacific Northwest and Canada.

The increase in solar radiation also modified the atmospheric pressure system over the north Atlantic Ocean (North Atlantic Oscillation), which brought warmer winters and wetter conditions over northern Europe and most of north-eastern part of the North American continent. These conditions also affected the winter weather in Greenland, north Africa and northern Asia.

Unequal consequences for people and environments

For about 300 years, these new climate conditions changed ecosystems and radically altered human societies.

As northern Europe became warmer, agriculture spread and generated food surpluses. At the time, England was warm enough to support vineyards, centralised governments in Europe were becoming stronger, people no longer needed fortifications to protect their once limited arable lands, and many people left seeking new lands.

Similar agricultural expansion occurred in some parts of North America, but also in central Asia where farmers spread into the northern region of Russia, into Manchuria, the Amur Valley, and northern Japan. The early 13th Century marked the beginning of the conquests of Genghis Khan and his Mongol hordes.

With sea ice and land ice in the Arctic shrinking with the rising temperatures, new lands became accessible and Vikings travelled farther north than before. They eventually reached a “green” Greenland and Iceland where they (temporarily) settled.

Such long-distance voyages also happened in the southern hemisphere. The Medieval warm period coincided with the settlement of New Zealand and the development of new trade routes across the Pacific basin.

The warm conditions during this period brought many benefits to Earth’s plant and animal life, but in some other parts of the world, people’s lives were instead made worse by intense droughts. Parts of western America and the great Mayan cities of Central America were hit by mega droughts, and Andean civilisations wilted in the face of an emptied Lake Titicaca and faltering freshwater runoff in coastal river valleys.

Small, scattered communities of the Pacific basin were forced to gather into bigger and more complex societies, concentrated in coastal areas. They harvested seafood and complemented it with products from new types of agriculture (construction of canals and sunken food gardens, agricultural terraces in steep areas, and the irrigation of lowland crops).

In contrast, La Niña brought intense monsoonal flow into northern, central and western Australia’s arid lands, increasing floods and storms that likely disrupted hunter-gatherer settlement patterns in these regions.

What this means for the future

The fact that some areas of the world actually prospered during the Medieval warm period gives ammunition to the global warming sceptics’ position. But there are two fundamental differences that make the Medieval warm period different from what we are experiencing now.

1. The present-day baseline used for comparison to the temperatures in the Medieval warm period is 1960-1990. Although it is true that in some regions the temperatures equalled or exceeded this baseline, globally the planet was still cooler on average than today. Temperatures experienced since 2000 in the northern hemisphere are already hotter than any time during the Medieval warm period.

2. The Medieval warm period is an asynchronous regional warming caused by natural (not human-driven) climatic variation, whereas we are facing a homogeneous and global warming caused by human activity releasing too much greenhouse gas into the atmosphere.

Despite the uncertainties, the climate characteristics of the Medieval warm period make it an irrelevant analogue for the magnitude of climate change we are facing.

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