Medieval Warm Period

Based on Wikipedia: Medieval Warm Period

When the Vikings Sailed to Vinland

Around the year 1000, a Norse explorer named Leif Erikson sailed west from Greenland and landed on the shores of what is now Newfoundland. His people established a small outpost there, the first European settlement in North America, nearly five centuries before Columbus. What made this remarkable voyage possible wasn't just Viking courage or shipbuilding skill. The climate itself had opened a door.

The Medieval Warm Period had arrived.

For roughly three centuries, from about 950 to 1250 of the Common Era, temperatures in the North Atlantic region climbed high enough to transform the possibilities of human civilization. Ice retreated. Seas became navigable. Lands that had been marginal for farming suddenly became productive. And people, being people, rushed in to take advantage.

A Period With Many Names

Scientists call this era by several names, each revealing something about how we understand it. The Medieval Warm Period is the most common label. Some prefer Medieval Climate Optimum, which carries an implicit judgment that warmth was beneficial, at least for some. Others use Medieval Climatic Anomaly, a more neutral term that acknowledges the climate wasn't just warm—it was strange in various ways, bringing droughts to some regions and floods to others.

The naming matters because it shapes how we think about the phenomenon. Was it a golden age? A disruption? Simply a variation? The answer, it turns out, depends entirely on where you stood.

The Discovery of Deep Time

We know about the Medieval Warm Period not from written records alone—though medieval Europeans did write about their weather—but from the Earth itself. Scientists read the planet's climate history the way historians read documents, but their archives are stranger and more various.

Tree rings tell one story. In warm, wet years, trees grow faster and lay down thicker rings. In cold or dry years, growth slows. A cross-section of an ancient bristlecone pine or an oak beam from a medieval cathedral becomes a barcode of climate stretching back centuries or millennia.

Ice cores tell another. In Greenland and Antarctica, snow falls year after year, compressing into ice that traps tiny bubbles of ancient atmosphere. Scientists drill down through these frozen archives, extracting cylinders of ice that preserve the composition of air from the time of the pharaohs, the Roman emperors, the Viking explorers. The ratio of different oxygen isotopes in this ice reveals the temperature when the snow fell.

Lake sediments tell yet another story. Pollen settles to the bottom of lakes, and the types of plants represented reveal what was growing nearby, which in turn reveals the climate. Sediments also preserve the shells of tiny organisms whose chemistry reflects the temperature of the water they lived in.

By weaving together these different threads, paleoclimatologists—scientists who study ancient climates—have built a picture of temperature fluctuations stretching back thousands of years. And one of the most prominent features of that picture, at least in the North Atlantic region, is a warm bump centered on the eleventh and twelfth centuries.

Not a Global Phenomenon

Here is where the story gets complicated. When researchers first identified the Medieval Warm Period in the mid-twentieth century, they assumed it was a worldwide event. If Europe was warm, surely everywhere else was too?

Not quite.

In 2019, a major international collaboration called the Pages-2k Consortium examined climate records from around the world and found something striking. The Medieval Warm Period was real, but it wasn't synchronized. Different regions experienced their warmest conditions at different times. The warmest fifty-year stretch in one part of the world might overlap with a cooler period somewhere else.

This matters enormously for understanding what caused the warming and for comparing it to today's climate change. Modern global warming, driven by greenhouse gases that spread through the entire atmosphere, is remarkably synchronized across the planet. The Medieval Warm Period was something different—a regional phenomenon, or perhaps several regional phenomena happening to overlap in time.

In fact, when scientists averaged temperatures across the entire globe for the medieval period, they found something surprising. The Earth as a whole may have been slightly cooler than in the early twentieth century, by about 0.03 degrees Celsius. The North Atlantic was warm, but other regions were not.

What Caused the Warmth?

If greenhouse gases weren't the culprit, what was? Scientists have identified several likely factors, though the precise combination remains debated.

The sun may have burned a bit brighter. Our star's output isn't perfectly constant—it varies slightly over decades and centuries. Evidence suggests solar activity was relatively high during the Medieval Warm Period, pumping a bit more energy into Earth's climate system.

Volcanoes may have cooperated by staying quiet. Major volcanic eruptions blast sulfur compounds into the upper atmosphere, where they form tiny particles that reflect sunlight back into space and cool the planet for a year or two. A string of quiet centuries, without major eruptions, would remove this cooling influence and allow temperatures to rise.

Ocean circulation patterns may have shifted. The Atlantic Ocean has a vast conveyor belt of currents that carries warm water northward from the tropics and returns cold water southward at depth. Changes in this circulation can dramatically affect temperatures around the North Atlantic rim. Some evidence suggests this system was running particularly strong during the Medieval Warm Period.

Computer models suggest that natural variability alone can't fully explain the warming. Something had to push the climate—probably some combination of the factors above, acting together.

The Greening of Greenland

The impact of the Medieval Warm Period is written most dramatically in the history of Greenland. The island got its name around 985, when a Norse explorer named Erik the Red, banished from Iceland for manslaughter, sailed west and found a land of fjords and mountains. The story goes that he called it Greenland to attract settlers, a bit of marketing genius that has puzzled people ever since, given how white with ice the place appears today.

But Erik wasn't lying, exactly. The southern tips of Greenland, where the Norse established their settlements, really were green. The climate was warm enough for cattle, sheep, and goats. The settlers built farms and churches. For several centuries, Greenland was a functioning outpost of European civilization, connected by ship to Norway and Iceland, exporting walrus ivory and furs.

The relationship between climate and colonization was more complex than simple warmth enabling settlement. Some research suggests the window of optimal conditions was actually quite brief. But there's no question that the Norse settlements depended on a climate favorable enough for farming and navigation.

As the Medieval Warm Period waned, the Norse adapted. Their diet shifted dramatically. In the early years, about three-quarters of their food came from farming, with seafood making up the rest. By 1300, those proportions had reversed. Seal hunting provided over three-quarters of their calories. The settlers were responding to a cooling, stormier climate that made farming increasingly difficult.

The end came gradually. The last written record from the Greenland settlements dates to 1412. Over the following decades, the remaining colonists seem to have simply left, returning to Iceland and Scandinavia where farms were becoming more available as populations there recovered from the Black Death. The climate hadn't killed the settlements directly, but it had made them economically unviable.

The Long Summer of Medieval Europe

In Europe, the Medieval Warm Period brought an almost unbroken stretch of warm summers lasting from about 1170 to 1310—nearly a century and a half. This warmth transformed agriculture and, through agriculture, transformed society.

Vineyards spread northward. England, which today struggles to ripen grapes, produced wine. Grain yields increased. Populations grew. The High Middle Ages, with its cathedral building, its universities, its expanding commerce, unfolded against a backdrop of agricultural abundance.

Glaciers retreated. In the Alps, passes that had been blocked by ice opened up, facilitating trade between northern and southern Europe. Some smaller glaciers disappeared entirely, though the larger ones survived and later advanced again, preserving in their ice the record of these warmer centuries.

But warmth didn't mean uniform benefits. Eastern Europe experienced increased flooding as precipitation patterns shifted. In Sardinia, a coastal region that had been abandoned during Roman times saw the sea level rise during the Medieval Warm Period, reshaping the landscape. When people returned, they found a different coastline and had to establish new ports.

The winters, interestingly, remained cold. Temperature reconstructions for Central Europe show that winter cold persisted more or less continuously from 1000 through the late 1800s. It was the summers that made the Medieval Warm Period distinctive.

Drought in the American West

Cross the Atlantic to what would later become the United States, and the Medieval Warm Period looks quite different. Here, the signature wasn't warmth alone—it was drought.

Tree ring records from California, the Great Basin, and the Colorado Plateau reveal prolonged dry spells that make modern droughts look brief by comparison. Lake levels dropped. Forests retreated. The landscape we think of as the American West—already arid—became even drier.

These droughts shaped human history. Native American settlements responded to the changed conditions. At Cahokia, near present-day St. Louis, one of the largest pre-Columbian cities north of Mexico, the droughts may have contributed to social disruption. Archaeological evidence shows patterns of violence, population movement, and the breakdown of long-distance trade networks during this period.

The relationship between climate and social change was complex, of course. Climate didn't simply cause these disruptions—people responded to changing conditions in various ways, and those responses interacted with political, economic, and cultural factors. But climate set the stage.

Alaska, meanwhile, experienced three intervals of comparable warmth over the past two thousand years: one from roughly 1 to 300 of the Common Era, another from about 850 to 1200 (overlapping with the conventional Medieval Warm Period), and the third beginning around 1800 and continuing to the present.

The Southern Hemisphere's Delay

One of the most intriguing findings about the Medieval Warm Period is that it arrived late in the Southern Hemisphere. The warming that began around 950 in the North Atlantic didn't show up in the Southern Ocean until about 150 years later.

This lag time tells us something important about how climate works. The two hemispheres are connected, but not rigidly so. Changes in one region take time to propagate to others. The oceans, which store and transport enormous amounts of heat, act as both connectors and buffers.

In Antarctica, the pattern was especially complex. Some regions were unusually cold during the Medieval Warm Period while others were unusually warm. The continent's climate was responding to different forces than the North Atlantic's, or responding to the same forces in opposite ways.

In tropical South America, ice cores reveal a distinct Medieval Warm Period from about 1050 to 1300, followed by the Little Ice Age in the fifteenth century. But the tropical warming didn't reach the levels that would later be seen in the late twentieth century—temperatures then were unprecedented in at least 1,600 years of the ice core record.

China's Complex Pattern

Chinese climate records, among the longest and most detailed in the world, show that China experienced warming during the Medieval Warm Period, but the pattern was far from uniform across that vast country.

The northeast and central-east regions showed significant temperature changes, warming during the Medieval Warm Period and cooling during the subsequent Little Ice Age. But northwest China and the Tibetan Plateau barely registered the change. The warming was real but patchy, concentrated in some areas and absent from others.

Monsoon patterns shifted dramatically. The East Asian Summer Monsoon, which brings rain to much of eastern China, reached its strongest intensity of the past thousand years during the Medieval Warm Period. This intensified monsoon was highly sensitive to the El Niño Southern Oscillation, the pattern of warming and cooling in the tropical Pacific that affects weather worldwide.

Some regions became wetter. The Mu Us Desert, on the border between Inner Mongolia and northern China, experienced increased moisture. Peatlands in southeast China expanded. But other sites in southern China actually became drier during the same period. The monsoon was stronger overall, but its effects varied enormously from place to place.

This spatial complexity—wet here, dry there, warm in this valley, cool on that plateau—is characteristic of the Medieval Warm Period everywhere. It wasn't a simple global thermostat turned up a notch. It was a reshuffling of climate patterns, with winners and losers in every region.

The Pacific's Contrary Evidence

The Pacific Ocean adds another layer of complexity. Coral records from the tropical Pacific suggest that conditions during the early Medieval Warm Period were actually cool and dry, consistent with a persistent La Niña pattern—the opposite of El Niño.

La Niña brings cooler sea surface temperatures to the central and eastern tropical Pacific, along with different rainfall patterns around the Pacific rim. If La Niña conditions dominated the early Medieval Warm Period, it would help explain some of the drought patterns in the American West while also reminding us that "warm period" is a potentially misleading name for what was really a reorganization of the entire climate system.

A 2013 study found that Pacific Ocean temperatures were indeed about 0.9 degrees Celsius warmer during the Medieval Warm Period than during the Little Ice Age, and about 0.65 degrees warmer than in the decades before the study. But even within the Pacific, patterns varied. The northeastern Pacific was actually cooler during the Medieval Warm Period than during the Little Ice Age—the opposite of the general trend.

The Little Ice Age Follows

The Medieval Warm Period didn't last. By the mid-thirteenth century, temperatures in the North Atlantic region were declining. What followed has come to be known as the Little Ice Age, a period of cooling that lasted, depending on how you define it, from roughly 1400 to somewhere between 1700 and the mid-nineteenth century.

The cooling was dramatic. Glaciers advanced down Alpine valleys, swallowing farms and villages. The Thames River in London froze solid enough to hold frost fairs. Crop failures and famines became more common. The Norse settlements in Greenland, already struggling, faded into abandonment.

But like the Medieval Warm Period, the Little Ice Age wasn't globally synchronized. Different regions cooled at different times. Some areas that had been warm during the Medieval Warm Period became cold; others didn't change much at all.

The causes of the Little Ice Age are debated, but they probably include decreased solar activity (a period called the Maunder Minimum saw almost no sunspots for decades), increased volcanic activity, and changes in ocean circulation. In some sense, the Little Ice Age was the Medieval Warm Period in reverse—the same factors that had pushed the climate warmer now pushed it cooler.

What It Means for Today

The Medieval Warm Period matters for understanding our current climate predicament, but not in the simple way it's sometimes invoked.

Some have argued that the Medieval Warm Period proves current warming is natural and nothing to worry about. After all, if it was warm a thousand years ago without industrial greenhouse gases, why should we blame today's warmth on human activities?

This argument misunderstands the science in several ways. First, the Medieval Warm Period was regional, not global. Second, it had identifiable natural causes—solar variability, volcanic quiescence, ocean circulation changes—that don't explain today's warming. Third, today's warming is global, rapid, and continuing, driven by greenhouse gas concentrations that are higher than at any point in at least 800,000 years.

The Medieval Warm Period does teach us that climate can change significantly even without human influence, and that those changes can have profound effects on human societies. The Norse expansion and retreat, the droughts of the American West, the flooding of Eastern Europe—all of these show how sensitive human civilization is to climate shifts.

But they also show something else: that past climate changes were regional and relatively slow, allowing societies time to adapt. Today's changes are global and rapid, affecting everyone at once and outpacing many natural systems' ability to adjust.

The Pioneers of Paleoclimatology

Our understanding of the Medieval Warm Period began with a British scientist named Hubert Lamb, one of the founders of paleoclimatology. In 1965, Lamb published research combining evidence from botany, historical documents, and meteorological records to reconstruct past climates.

Lamb concluded that evidence was "accumulating in many fields of investigation pointing to a notably warm climate in many parts of the world, that lasted a few centuries around 1000–1200 CE, and was followed by a decline of temperature levels till between around 1500–1700 CE the coldest phase since the last ice age occurred."

This framing—a warm medieval period followed by a Little Ice Age—became the standard picture for decades. But as more data accumulated from more regions, the picture grew more complicated. The Intergovernmental Panel on Climate Change noted as early as 1990 that the Medieval Warm Period "may not have been global." By 2001, their assessment concluded that evidence "does not support globally synchronous periods of anomalous cold or warmth over this time frame."

The science had evolved. What seemed at first like a simple global pattern revealed itself to be a complex mosaic of regional variations.

Reading the Earth's Memory

The study of past climates is, in some ways, a form of planetary archaeology. Scientists dig into ice and mud and wood, extracting evidence of conditions that no human witnessed or recorded. They reconstruct temperatures from the chemistry of ancient shells, rainfall from the width of tree rings, vegetation from the pollen trapped in lake sediments.

This work requires patience, precision, and a willingness to accept uncertainty. No single record tells the whole story. Tree rings might be influenced by local conditions that don't reflect the broader climate. Ice cores can be disturbed by ancient storms or modern drilling. Lake sediments can be mixed by burrowing organisms or turbulent water.

The power of paleoclimatology lies in combining multiple lines of evidence. When tree rings, ice cores, lake sediments, and historical documents all tell the same story, confidence grows. When they disagree, scientists must figure out why—and in the process, often learn something new about how the climate system works.

The Medieval Warm Period emerged from this careful accumulation of evidence. It remains a subject of active research, with new data continuously refining our understanding of when it occurred, where it was felt, and what caused it.

A Lesson in Complexity

Perhaps the most important lesson of the Medieval Warm Period is that climate is complex. It doesn't change uniformly everywhere. Regional variations matter enormously. What counts as warm or cold depends on where you stand and what you're measuring.

The Vikings who sailed to Greenland experienced a genuine climate opportunity—warmer seas, longer growing seasons, reduced ice. The Native Americans of the Southwest experienced something quite different—devastating droughts that disrupted settlements and trade networks. Both were manifestations of the same global climate reorganization, but their effects were nearly opposite.

This complexity doesn't make climate change any less real or important. It makes it more so. Climate affects every aspect of human life—where we can grow food, how we build our homes, whether our cities flood or our forests burn. Understanding how climate varied in the past helps us prepare for how it might vary in the future.

The Medieval Warm Period was a time of change, opportunity, and disruption. It reminds us that the stable climate we've enjoyed for most of recorded history isn't guaranteed. The Earth's climate has always varied, and human societies have always had to adapt. The question now is whether we can adapt fast enough to changes that are happening faster than ever before.