by Brian Fagan
Tree-ring temperature reconstructions now span the entire Northern Hemisphere and come from over 380 locations. We have the first interannual and interdecadal temperature variability curves as far back as A.D. 1400 or earlier, with very reliable data for the years after 1600.5 Such temperature estimates, acquired by statistical regression analyses from modern instrument records or by proxies from historical records and other sources, are vital to establishing just how warm the late twentieth century has been in comparison with earlier times.
Major volcanic eruptions, like that which destroyed Roman Herculaneum and Pompeii in A.D. 79, are spectacular, often catastrophic events. The greatest of them can be detected in tree-ring sequences and through fine dust in ice cores. Eruptions have important climatic consequences because of the fine dust they throw out, which can linger in the atmosphere for years on end. Hypotheses linking eruptions and weather have been around a long time. Benjamin Franklin theorized that volcanic dust could lower temperatures on earth. In 1913, a U.S. Weather Bureau scientist named William Humphreys used data from the spectacular Krakatau eruption in Southeast Asia in 1883 to document the correlation between historic volcanic eruptions and worldwide temperature changes. Volcanic dust is some thirty times more effective in shielding the earth from solar radiation than it is in preventing the earth's heat from escaping. During the three years it may take for the dust from a large eruption to settle out, the average temperature of much of the globe may drop as much as a degree, perhaps even more. The effects tend to be most marked during the summer following a major volcanic event.
The provisional temperature curves for the Little Ice Age display some conspicuous downward spikes, when a single year was unusually cold. Almost invariably, these are associated with major eruptions, such as that of Mount Tambora in southeast Asia in 1815, the most spectacular eruption of the past 15,000 years. Over the next few years, Tambora's ash drifted through the atmosphere and dimmed the sun. The year 1816 appears as a sharp cold spike in the climatologists' temperature diagrams, the "year without a summer" when snow fell in New England in June and Europe shivered through a frigid September. Major volcanic eruptions almost invariably brought colder summers and bad harvests, natural phenomena unconnected with the endless perturbations of the Little Ice Age. During the seventeenth century, an unusual frequency of volcanic events contributed to the volatility of climate change.
What caused the Little Ice Age? Did small changes in the earth's axis affect global temperatures for five centuries? Or did cyclical fluctuations in solar radiation lead to greater cooling? The answer still eludes us, largely because we have barely begun to understand the global climatic system and the interactions between atmosphere and ocean that drive it. There are few certainties. One is that we still live in the Great Ice Age, somewhere near the midpoint of an interglacial, one of the many that have developed over the past three-quarters of a million years. In the fullness of time-according to some estimates, in the next 23,000 years-the world will most likely return to another glacial cycle, with temperatures as extreme as those of 18,000 years ago, when much of Europe really was in a deep freeze.
Slow, cyclical changes in the eccentricity of the earth's orbit and in the tilt and orientation of its spin axis have constantly changed patterns of evaporation and rainfall and the intensity of the passing seasons over the past 730,000 years. As a result, the world has shifted constantly between extreme cold and short warmer periods. The geochemist Wallace Broecker believes these changes caused the entire ocean-atmosphere system to flip suddenly from one mode during glacial episodes to an entirely different one during warmer periods. He argues that each flip of the "switch" changed ocean circulation profoundly, so that heat was carried around the world differently. In other words, Ice Age climatic patterns were very different from those of the past 10,000 years.
The Great Ocean Conveyor Belt, which circulates saltwater deep below the surface of the world's oceans. Salt downwelling in the North Atlantic Ocean plays a vital part in this circulation.
If Broecker is correct, then today's climatic mode results from what he calls the "Great Ocean Conveyor Belt." % Giant, conveyor-like cells circulate water through the world's oceans. In the Atlantic, warm, upper-level water flows northward until it reaches the vicinity of Greenland. Cooled by Arctic air, the surface waters sink and form a current that covers enormous distances at great depths, to the South Atlantic and Antarctica, and from there into the Pacific and Indian Oceans. A southward movement of surface waters in these oceans counters the northward flow of cold bottom water. In the Atlantic the northward counterflow is sucked along by the faster southward conveyor belt, which is fed by salt-dense water downwelling from the surface in northern seas. The Atlantic conveyor circulation has power equivalent to one hundred Amazon Rivers. Vast amounts of heat flow northward and rise into the Arctic air masses over the North Atlantic. This heat transfer accounts for Europe's relatively warm oceanic climate, which has persisted, with vicissitudes, through ten millennia of the Holocene.
We understand the Great Ocean Conveyor Belt in only the most general terms, but enough to know that circulation changes in the upper ocean have a profound effect on global climatic events like El Niflos. We know, also, that the chaotic equations of the atmosphere and ocean exercise a powerful influence on the swirling atmospheric streams, surface downwelling, and shifting currents of the North Atlantic. Broecker and others have recently turned their attention to the deep sea, to changes in the thermohaline circulation (marine circulation caused by differences in the temperature and salinity of sea water).?
Since it was discovered in the 1980s, we have assumed that the ocean conveyor belt has operated smoothly throughout the Holocene. Under this scenario, nearly equal amounts of deep ocean water originate in the North Atlantic and around the perimeter of the Antarctic and are thoroughly mixed during their northward and southward passages. This mixing occurs when surface water exposed to atmospheric gases descends into the deep ocean. This long-held assumption may be wrong, for we now know something is different in today's Southern Ocean. The Weddell Sea in Antarctica has produced much smaller quantities of mixed deep water than expected over the decades since scientific observations began. In contrast, the North Atlantic is now producing deep water at a rate consistent with that needed to maintain its natural carbon 14 (14C) level. Broecker theorizes that Southern Ocean deep water production was much greater during the Little Ice Age than today, just as it was during the last glacial maximum of the Ice Age and during the short-lived and cold Younger Dryas period of 11,500 years ago.
Antarctic deep-water production may have increased about eight hundred years ago during the fourteenth century, precisely when Europe experienced its first significant cooling, then slackened off as conditions warmed again after 1850. The end of the Little Ice Age saw two warming stages, one from the late nineteenth century to about 1945, the other from after 1975 to the present. Scientific observations over the past quarter-century display no signs of higher production, so most likely the deep-water production rate slowed well before the 1940s.
There are no easy answers to the conundrum of the Little Ice Age, but we can be certain that such minor "ice ages" occurred many times earlier in the Holocene, even if we still lack the tools to identify them. We would logically expect another such episode to descend on the earth in the natural, and cyclical, order of climate change, were it not for increasingly compelling evidence that humans have altered the climatic equation irrevocably through their promiscuous use of fossil fuels since the Industrial Revolution. We may be in the process of creating an entirely new era in global climate, which makes an understanding of the Little Ice Age a high scientific priority.
Climate has never been a fashionable topic in historical circles, largely because, until recently, paleoclimatology was a crude and infant science. Only a handful of scholars, among them the French scholar Le Roy Ladurie, Swiss climatic historian Christian Pfister, and the late British climatologist Hubert Lamb have pa
id careful attention to the social effects of volatile climatic shifts in the Little Ice Age. Most historians discount the importance of climate in shaping the events of the recent past, reacting, quite rightly, against the notion that climate change was a primary and simple trigger for major historical events. No one would argue today that climate change "caused" the French Revolution or that the bitter cold of twelfth century Greenland "caused" the medieval Norse to abandon their northernmost farms. This kind of environmental determinism, with its simple cause-and-effect arguments, is long vanished from serious discussion and with good reason.
The historian's caution was entirely appropriate in an era before treering networks, ice cores, and sophisticated statistical treatments of historical data. Today, a climatologist's portrait of the annual fluctuations of the Little Ice Age looks like a fine-toothed comb gone mad-a graph of sharp-edged spikes of cold and warm, of cycles of cooler and warmer conditions plotted above and below a benchmark for today's climate. The curve never stays still, but moves up and down, sometimes sharply and without warning, at other times trending more gently. It is as if we have set sail on a climatic sea, tossed to-and-fro from the moment we set out on our journey through time. We will rarely experience true stability, just the occasional calm spot: a decade of settled, warmer weather or more intense cold. The constant reality is unpredictable change fueled by complex interactions between atmosphere and ocean, by pressure swings on the other side of the world.
For the first time, we place a detailed graph of changing climate alongside the momentous historical events of the Little Ice Age, not as a remote backdrop but as a critical, and long neglected, factor in a complex equation of harvests, subsistence crises, and economic, political and social changes. We can see how cycles of cold or of excessive rainfall rippled across Europe, affecting monarch, noble, and commoner in different ways, changing the course of wars and the prosperity of fisheries, and fostering agricultural innovation. Climate change was a subtle catalyst, not a cause, of profound change in a European world where everyone lived at the complete mercy of a subsistence farming economy, where the ripple effects of poor wine harvests could affect the economic welfare of the Hapsburg empire. The Little Ice Age is the story of Europeans' struggle against the most fundamental of all human vulnerabilities.
As soon as the great ocean has been traversed there is such a great superfluity of ice on the sea that nothing like it is known anywhere else in the whole world and it lies so far out from the land that there is no less than four or more days journey thereunto on the ice.
-Konungsskuggsja (The King's Mirror), c. 1250
ailors on the northern reaches of the medieval world began to experience the effects of increasing cold a century before the great famine of 1315-21. The cooling first intensified in the northeast, around Franz Josef Land and Spitzbergen, and then spread westward across the Arctic. Many winters, thick pack ice moved further south than it had for centuries, then lingered in the stormy waters between Iceland and Greenland through early summer, hampering an already tricky ocean passage. Not every winter was exceptionally cold, but harsh conditions, freezing late spring temperatures and persistent pack ice more frequently compounded sailors' difficulties.
In Eirik the Red's day, Norse merchant ships (knarrs) had taken the most direct route from Iceland to East Greenland, along Latitude 65° north, then coasted south and west round Cape Farewell to the Eastern Settlement. Even in those warmer times, ships foundered in offshore gales, were dashed to pieces against the rugged Greenland and Icelandic coasts, capsized when overloaded, or were simply blown off course never to be seen again. Only a few of these maritime tragedies have left a record in history. In about 1190, a Norwegian knarr named Stangarf6li was set off course by a gale while bound from Bergen to Iceland and wrecked on the east coast of Greenland. A decade later, some hunters found the wreck, the skeletons of six men, and the perfectly preserved, frozen body of Icelandic priest Ingimund Thorgeirsson. Beside him lay wax tablets inscribed with crabbed runes that recorded his death from starvation.'
By 1250, many fewer ships made the crossing to the Norse colonies. Those that dared traveled a much more hazardous route, far from land in the open Atlantic. A skipper now sailed a day and a night due west from Iceland, then altered course southwestward to avoid the pack ice off southeastern Greenland. The new routing involved more time out of sight of land and a higher risk of foundering in the savage westerly gales that can blow in this part of the North Atlantic even in high summer. Inevitably, Greenland became more isolated from Norway and Iceland. Two and a half centuries later, in 1492, Pope Alexander VI remarked in a letter that "shipping to that country [Greenland] is very infrequent because of the extensive freezing of the waters-no ship having put into shore, it is believed, for eighty years."2 Alexander suggested that voyages might be possible in August. Political ties between the ancient Norse homeland and its western colonies loosened, and, in the case of Greenland, became almost nonexistent. However, the Pope was misinformed. Where the Norse no longer sailed, others now voyaged, among them Basque and English mariners in new designs of sailing vessels equipped to handle the roughest of winter weather. Ironically, they traveled to Iceland, Greenland, and beyond because the Church had declared fish a legal food on religious holidays.
In northwestern Europe at the end of the thirteenth century, few noticed that the spikes of colder, more stormy years were coming closer together. Those who suffered through cold winters and increasingly frequent sea floods perhaps thought of these occasional catastrophes of extreme cold or storm as manifestations of divine vengeance. At a distance of eight hundred years, we can discern a different pattern: the first signs of climatic deterioration. The winter of 1215 in particular was exceptionally cold in eastern Europe and caused widespread famine. Thousands of hungry Polish farmers headed in desperation for the Baltic coast, where they vainly believed fish were to be found. While hunger was rarer in the west, the swings of the North Atlantic Oscillation were growing wider and faster.
Making landfall on the eastern shores of the North Sea is always dangerous, especially in the narrows leading to the English Channel. Ever shifting sandbanks protect the low-lying shore with its endless beaches and narrow creeks. Even in good weather with a commanding breeze, the sailor approaches carefully, chart in hand, eyeing the featureless coast in search of rare conspicuous landmarks such as a tall church spire or a lighthouse. The shallows usually lie far offshore, ready to take off one's keel in a few catastrophic poundings from the short, steep seas. Heaven help the skipper who is caught close to land when a howling northwester makes the coast a dangerous lee shore. He tries to claw offshore, hard on the wind, pounding against steep seas that break over the bow as the stomach churns-partly from the motion, partly from fear. Thousands of sailors have perished in these violent waters over the centuries.
Ten thousand years ago, the southern North Sea was a marshy plain where elk and deer wandered and Stone Age foragers hunted and fished. England was part of the Continent until as recently as 6000 B.C., when rising sea levels caused by post-Ice Age warming filled the North Sea. By 3000 B.C., the ocean was at near-modern levels. Sea levels fluctuated continually through late prehistoric and Roman times but rose significantly after A.D. 1000. Over the next two centuries, the North Sea rose as much as forty to fifty centimeters above today's height in the Low Countries, then slowly retreated again as temperatures fell gradually in the north. The North Sea was brimful on the calmest days. When exceptionally high tides coincided with gale-force winds, raging waters surged over thousands of hectares of coastal farmland in a few short hours. The fourteenth and fifteenth centuries had more than their share of such catastrophes.
Seven centuries ago, the North Sea washed coastlines very different from those of today. For instance, a now-vanished shallow estuary extended deep into East Anglia, making Norwich and Ely important ports. Countless narrow creeks and channels penetrated into the farmlands, harboring thousands of small craft, traders, and fi
shermen. The lumbering grain ships and cargo vessels that plied these waters went well armed, for pirates would pounce on them from muddy hiding places deep in the coastal marshlands and sandy islands of the low shorelines between the Baltic and the English Channel. The same infiltrating creeks made the surrounding low-lying coastlines vulnerable to unpredictable storm surges, which would sweep up the narrow defiles and flood the land on either side, forcing entire villages to evacuate or drowning them almost without warning. We can imagine the scene, repeated so many times over the generations. Huge waves of muddy water attacked the shore, spray blowing horizontally in the dark mist masking the ground. The relentless ocean cascaded up beaches and into narrow inlets, devouring everything before it. Thatched farmhouses tumbled end-for-end in the waves; pigs and cattle rolled like dice across inundated fields. Bedraggled families clung to one another in trees or on rooftops until the boiling waters swept them away. The only sound was the shrieking wind, which drowned out everything-the growl of shifting gravel beaches, the desperate cries of drowning victims, the groaning branches of tree lashed by the gale. When the sky cleared, the sun shone on an enormous muddy lake as far as the eye could see, a desolate landscape devoid of human life.
No one could resist the onslaught of an angry North Sea, which contemptuously cast aside the crude earthen dikes of the day. The hydrological and technological know-how to erect truly permanent coastal fortifications did not yet exist. The first serious and lasting coast works date to after 1500, but even they were usually inadequate in the face of savage hundred-year storms. Small wonder the authorities often had trouble persuading peasants to settle on easily flooded lands.
