Book Summary: “The Long Summer: How Climate Changed Civilization” by Brian Fagan

The Long Summer

Title: The Long Summer: How Climate Changed Civilization
Author: Brian Fagan
Scope: 3 stars
Readability: 4 stars
My personal rating: 5 stars
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Topic of Book

Fagan reconstructs the climate record for the last 100,000 years and examines its impact on human history.

If you would like to learn more about long-term human history, read my book From Poverty to Progress: How Humans Invented Progress, and How We Can Keep It Going.

Key Take-aways

  • For most of human history, the world has been in an ice age or in between ice ages (which we may still be in today)
  • Before the last ice age, any permanent agriculture would have been impossible or at least very unlikely.
  • The last 15,000 years has seen a remarkably long and stable, warm period: “The Long Summer”.
  • It was during this time that humans invented agriculture and civilization.
  • In general, civilization has prospered during periods of warm and wet weather and declined during periods of cold and dry weather. The levels of precipitation probably had a stronger effect than the actual temperature.
  • The climate can change very quickly, often within one lifetime.

Important Quotes from Book

Today, we know that our ancestors lived through at least nine long glacial episodes over the past 780,000 years, separated by much shorter warm intervals. For three quarters of this time, the world’s climate was in transition from cold to warm and back again.

The Vostok core shows that the past 15,000 years, a time of prolonged global warming, have been the most climatically stable of the past four hundred.

In our efforts to cushion ourselves against smaller, more frequent climate stresses, we have consistently made ourselves more vulnerable to rarer but larger catastrophes. The whole course of civilization (while it is many other things, too, of course) may be seen as a process of trading up on the scale of vulnerability.

But Ur’s sudden collapse was a turning point in human history: the first time an entire city disintegrated in the face of environmental catastrophe.

Ice Ages

We know that the Ice Age began at least 1.5 million years ago with a gradual cooling of the global climate. Deep-sea cores from the Pacific document at least nine intense glacial periods over the past three-quarters-of-a-million years, each marked by gradual cooling, then rapid warming, only to be cut off again by renewed glaciation. For at least 500,000 of the past 780,000 years, the world’s climate has been in transition from warm to cold and back or vice versa. Glacial periods have lasted much longer than the warmer intervals.

The Vostok ice core takes us to about 420,000 years ago, through four transitions from glacial to warm periods. These shifts came at about 100,000-year intervals, the first about 335,000 years before the present, then at 245,000, 135,000, and 18,000 years ago-a cyclical rhythm.

There seem to be two periodicities involved, a primary one of about 100,000 years and another, weaker one of about 41,000 years. Together, they support the long-held theory that changes in the orbital parameters of the earth–eccentricity, obliquity, and precession of axis–cause variations in the intensity and distribution of solar radiation. These in turn trigger natural climatic changes on a grand scale. The global warming of 15,000 years ago is the most recent effect of these major gyrations, culminating in the Holocene, the millennia after the end of the Ice Age.

The Vostok core shows us that the world’s climate has almost always been in a state of change over these 420 millennia. But until the Holocene it has always oscillated. The Holocene climate breaks through these boundaries. In duration, stability, degree of warming, and concentration of greenhouse gases, the warming of the past fifteen millennia exceeds any in the Vostok record. Civilization arose during a remarkably long summer. We still have no idea when, or how, that summer will end.

Much of the vast landscape was a near-arid wasteland of unceasing winds. Yet a few thousand humans lived here, attracted by the herds of cold-loving mammals that flourished near the river valleys well south of the great ice sheets…. The most famous of these animals was the woolly mammoth… Steppe bison, wild horse, reindeer, musk ox, arctic foxes–the mammalian community of the steppe/tundra boasted twice as many species as that of the modern tundra.

Plant foods were so rare that almost all the diet came from meat, from quarry that was constantly on the move. Some groups in river valleys lived off fish and waterfowl. The hunting toolkits were light and portable, with lethal antler and bone spearheads that could inflict severe wounds at close quarters. These innovations would have been worthless, however, without a simple and little appreciated invention that is still in use today-the needle and thread.

The eyed needle allowed people to fashion garments that not only fitted the individual precisely but also combined fur from several animals, so that the user could benefit from the unique properties of each kind of skin.

The needle also brought another sartorial innovation – layered clothing.

The low plant productivity of the steppe/tundra meant that the late Ice Age humans relied almost entirely on meat, which in turn dictated a mobile life-way and large hunting territories. For much of the year, each group lived in isolation, perhaps having very sporadic contact with their closest neighbors, groups as small as themselves. But we know that these people were also part of much larger social networks.

The keys to survival were efficient technology, both to allow life outside in subzero temperatures and to kill animals large and small, and social arrangements that allowed for both mobility and catastrophe. In a world of small hunting bands always on the move, there was always the danger that all the men in a band might perish in a hunting accident, or that the one childbearing woman in a group would die in childbirth. There was constant social tension, owing to an environment where hunger often threatened.

Late Ice Age 18,000-13,000 BC (cold)

Cro-Magnons in Europe subsisted on reindeer hunting with spears and atatl on the tundra.

After about 18,000 years ago, the rhythm of the pumps became more changeable, the climatic shifts sometimes startlingly abrupt.

Then, suddenly, around 15,000 years ago, the warming accelerated dramatically and the ceremonies petered out. The ancient Ice Age bestiary of mammoth and bison, arctic fox and reindeer, migrated northward with the retreating tundra. Birch and deciduous forests spread rapidly into the deep river valleys. Some of the bands moved northward, following their quarry. Others abandoned the great rock shelters and dispersed into much smaller bands, living off solitary deer and other forest animals and, more and more, off plant foods.

The dancing bison and reindeer on the walls faded behind slowly forming stalagmites. By 12,000 years ago, the last late Ice Age Cro-Magnon hunting societies had vanished in the face of natural global warming, to be rediscovered by archaeologists only in the 1860s.

Ice grew slowly but dissipated rapidly, which may account for the trend toward slow cooling and rapid warming characteristic of much Ice Age climate change.

Heinrich events marked the coldest point of the cycle.

The intrusion of millions of gallons of glacial freshwater into the northern Atlantic Ocean had the effect of shutting down the circulation of warmer water in the Gulf Stream, which depends on the downwelling of salt water in the Labrador Sea. The inevitable result: a deep freeze in Europe as the prevailing warm westerly winds faltered. Cold, dry, and windy conditions settled over a broad region across North America and Europe, and extended as far south as subtropical Asia and Africa. Much of the world became drier, because the cooling reduced the amount of water vapor as storm tracks moved southward. A Heinrich event, then, is a feedback loop–a quick warming that causes its own end in a quick cooling.

The youngest major cold snap was the so-called Younger Dryas event of 11,000 BC, which endured for ten centuries. The Holocene has recorded one of the longest periods of stable climate on record. Yet subtle warmings and coolings of the North Atlantic have occurred about every fifteen hundred years, of which the Little Ice Age of AD. 1300 to 1860 is the most recent example.

The Great Warming 15,000-11,000 BC

The Gulf Stream is part of a vast global conveyor belt of moving water that has the

power to change climate and alter human lives…  The Atlantic conveyor system has power equivalent to one hundred Amazon Rivers and is one of the great drivers of global climate.1 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 with its moist westerly winds, which has persisted, with vicissitudes, through the Holocene.

The Cro-Magnons were experts at assessing the condition of their prey, especially the fatness of the animals. 4This is why the main reindeer drives were probably in autumn, after the animals had gorged on rich plant food during the warm months. Many historic hunter-gatherer societies were selective in seeking out fatter animals and marrow. The meat from fatter animals tastes better and provides a feeling of satiety that leaner flesh does not. Fat is a major source of energy, is more efficiently metabolized than protein, and stores important vitamins and essential acids.  These hunting societies relied heavily on large and medium-sized mammals-aurochs, bison, mammoth, reindeer, wild horses, and other prey.

By 12,700 B.C., summer temperatures at some locations were warmer than today… the steppe vegetation that covered much of the European landscape during the late Ice Age slowly became denser and more productive, with invasions by junipers, willows, and other shrubs. Then the tree cover thickened.

By 12,000 B.C., birch forests covered much of England and many parts of western and northern Europe. The only check for trees spreading across Europe was their rate of natural dispersal. Some trees, like birch and elm, disperse their seeds by wind. These clearly advanced more rapidly than oaks, whose seeds are dispersed by birds and other agencies such as streams and are also much slower growing. Experts believe that trees such as birch, pine, alder, and hazel could advance at a rate of 1-2 kilometers a year over periods of five hundred to two thousand years.

After two turbulent millennia of rapid vegetational change, Europe looked fundamentally different. The birch forest that had spread first across the north was now pushed far northward into Scandinavia and northern Russia. Tundra and steppe effectively vanished. These environmental changes created unique challenges for people adapted to a deep-frozen world.

For a start, big-game hunting became problematic. Between 14,000 and 9,500 B.C., a wave of extinctions affected the Cro-Magnon’s favorite quarry, especially animals with body weights of over 44 kilograms..

What role did human hunters play in this extinction? Almost certainly a very minor one, for their ancestors had lived alongside and preyed on the same large mammals for tens of thousands of years. For this reason, it seems unlikely that they overkilled large mammal populations, even if they eventually contributed.

As the game dispersed and grew scarcer, plant foods became more abundant and the obvious key to survival. The mixed deciduous woodland that colonized much of western Europe was a highly productive, if seasonal, environment in terms of edible plants, especially in spring and fall. In fall, there were rich nut harvests from hazel and other trees. There were fruit and fungi, grass seeds and edible tubers, as well as the ubiquitous bracken rhyme, easy pickings for people who knew their environments intimately. With much longer growing seasons, even a child could collect enough food to satisfy its hunger for much of the year. In southern Europe, for example, Mediterranean stone pine forests yielded kernels that had a protein value two-thirds that of lean steak and could feed entire families for months on end.

The changeover to plant foods required no technological innovations, for the artifacts used to gather and process wild grasses, nuts, or tubers were simplicity itself-wooden digging sticks, hides, trays or baskets made from vegetable fibers, and a variety of stone grinders and pestles carefully shaped from convenient boulders.

Inevitably, then, the Europeans of the great warming turned to plant foods, especially starchy seeds and nuts, which could usually be stored for years and provided a much more reliable staple than grease or small mammals.

The Cro-Magnons had always hunted with the spear and spear-thrower, which are excellent weapons when used at close quarters to harvest migrating reindeer. Such implements can inflict fatal wounds when wielded by an expert stalker but are cumbersome in thick forest, where the long shaft catches on branches and undergrowth. At some point either very late in the Ice Age or during the early stages of the great warming, when forests were beginning to displace hitherto scrub-covered tundra, some European hunters developed a new and much more lethal hunting weapon-the bow and arrow.

The bow was a tremendous advance over the spear and spear-thrower. It allowed the shooting of a projectile at a speed of 100 kilometers an hour, far faster than the most aggressive of spear casts. Furthermore, one could shoot up to 200 meters and achieve a remarkable accuracy between 20 and 50 meters. This is the optimum range, for beyond this distance the force of penetration diminishes rapidly.

Bows and arrows had other important advantages. The hunter was no longer dependent on just one missile but carried an entire quiver of arrows that weighed less than a single spear and spear-thrower. A bow and arrow was effective against a wide range of animals and was a far more versatile weapon. Spears and spear-throwers are highly effective for close range hunting, for stabbing reindeer or wild horses during mass drives. They are far less efficient against solitary animals and smaller creatures, many of which are fast-moving targets that offer the hunter only a fraction of a second to aim and fire.

Drought 11,000-10,000 BC

With the warming in the Levant plant foods-the wild grasses of spring and early summer, and the acorn and pistachio nuts of fall-were now so abundant that many groups lived not in temporary camps but in much larger permanent communities, where they built substantial round dwellings with thatched roofs.

Every fall, the Natufians harvested millions of acorns and pistachios. Both nuts have the advantage that they are easily stored and can keep for two years or more if secured away from insects and rodents. Harvesting is straightforward-a matter of shaking the branches or climbing into trees to collect the ripe fruit.

The productivity of oak groves can be astounding, although the yield of individual

trees varies sharply from year to year and from one species to another. Acorn meal was an important staple in many parts of the world in ancient times, and was still important in nineteenth-century Europe. Unfortunately, data on harvest yields is hard to come by, but in California’s North Coast ranges, yields as high as 590 to 800 kilograms per hectare

were not uncommon. Such yields could have supported 50 to 60 times more people than were in the area at European contact. Acorns are nutritious, containing as much as 70 percent carbohydrates, about 5 percent protein, and between 4.5 and 18 percent fat. They have one major drawback: they are a labor-intensive crop to process. Shelling and pounding them takes hours, far longer than milling grass seeds. Even then, the meat is inedible, for acorns contain bitter-tasting tannic acid, which has to be leached away by soaking them with time-consuming cate before cooking.

Acorns and pistachios produced food surpluses more than large enough to permit Natufian communities to remain for long periods at one location.

But the surplus came at a price-a vast expenditure of daily labor. In California, the anthropologist Walter Goldschmidt once observed a woman pound three kilograms of acorns in three hours. It took her another four hours to leach the meal by flushing it through water. After seven hours, she ended up with 2.6 kilograms of edible meal, enough to feed her family for several days. A hunter can skin and butcher a deer, on the other hand, in a few minutes. The hunt may take longer than acorn harvesting, but food preparation is much simpler and more cost effective.

When acorns became a staple, life in a community changed profoundly… To pound and leach acorns for regular daily consumption involved a quantum jump in women’s work, to the point that they were tied to their mortars and pestles, as well as their storage bins.

After tens of thousands of years of unfettered mobility, the Natufians were now anchored by their acorn harvests to long-term base camps. But with relatively predictable harvests and good storage bins, such more-or- less permanent settlement was entirely feasible.

At first, the experiment was successful. The new, larger settlements flourished and expanded over many generations. Populations throughout the oak and pistachio belt grew rapidly. Soon neighbors hedged in each group’s territory as the landscape filled in, creating potential for conflict over nut groves and other foods, especially in dry years.

Inevitably, a rapidly growing population overexploited what was, ecologically speaking, still a marginal environment exceptionally vulnerable to even minor climatic shifts. Some bands expanded into drier, even more marginal lands. The stage was set for a serious crisis. In about 11,000 BC the crisis came in a series of intense droughts that endured for many generations.

The Younger Dryas was triggered by the sudden collapse of an ice dam that released the fresh water from Lake Agassiz in Winnipeg. The flow into the Atlantic kept warm sea water from cooling and sinking. The Atlantic conveyor belt shut down and air temperatures plummeted and rainfall declined.

Wild plant species ancestral to some of the world’s most useful crops flourished within the Fertile Crescent and still do today. So did aurochs and boar, wild goats and sheep. Once domesticated, this remarkable diversity of useful plants and animals provided foragers-turned-farmers with a balanced source of raw materials like vegetable and animal fiber, oil, and milk, and, ultimately, with the means to build transportation.

A computer model found that if the crop was harvested in a near-ripe state with stone-bladed sickles (widely found in early farming sites) or by simply uprooting the stalks, then full domestication would have been achieved within just twenty to thirty years. But if the crops were reaped when less ripe, the process would have taken longer, perhaps two or three centuries.

Within a few generations, the habit of repeated planting and harvesting changed the genetic makeup of wild grasses and altered the course of history. The savage droughts of the Younger Dryas were almost certainly the trigger for the change.

Centuries of Summer

One of the greatest natural disasters to affect humanity came in about 5600 BC., when the rising waters of the Mediterranean flooded the deep basin of the Euxine Lake, 150 meters below the Sea of Marmara, to form the Black Sea.

Starting around 5600 BC in one of the most significant population movements of human history, farmers jumped first to the upper Danube, next to the upper Rhine and Nekkar rivers, then down the Rhine into Poland and finally to southern Belgium and northern France. Within a few centuries, clusters of farming villages had settled a band of easily cultivable loess soils and river valleys from western Hungary to the Low Countries.

The pioneers entered a world where dense forest stood like a dark green phalanx mantling hillside and valley alike.

Only a few thousand forest hunters dwelt among the trees. They were elusive, cautious people, armed with bows and arrows and an intimate knowledge of myriad woodland plants-bog cranberries, mushrooms, wild garlic.

The agriculturalists settled the new lands just as their ancestors had done, carefully selecting easily cultivable soils, leapfrogging over neighboring village communities to find unoccupied fertile land. Within a few generations, a lattice of isolated homesteads, hamlets, and villages extended up the river valleys and along the forest edge. Each community took up land that was essentially empty. No one had cultivated these soils before; crop yields were high, and harvests were easily amplified with game and plant foods like acorns.

Almost invariably, the earliest Linearbandkeramik settlements lie at the edges of river valleys and on fertile, well-drained loess soils. The natural humidity of river valley gardens, their high productivity, and the ease of working them by hand without heavy tools meant that one could use the same plot of land many times without even fertilizing it. But eventually the soil would become exhausted and the village would move to a new site, again contributing to the spread of the new economies.  As each community cleared land and established itself, so the next generation moved off, along river valleys and through more open country, to establish another village some distance away in virgin land.

But inevitably, populations grew on the fertile soils, the larger communities swelled, and the best arable land was taken up. During the fifth millennium BC., the process of filling in began. People moved onto drier, heavier soils and adopted heavier implements of tillage, such as the simple scratch plow, to turn over thick sod so that it could aerate and drain.

The edible landscape had been eaten; people would now have to work harder for their bread.

But by 3500 BC., Europe was a changed landscape. It was a land of timber architecture, of free-standing dwellings clustered in hamlets and villages– still a forested continent, but one gradually being altered to accommodate new ways of living off the land.

By 5000 BC., the major climatic shifts that affected humanity were largely over. Sea levels had stabilized at near-modern levels, the great ice sheets were almost gone, and global vegetation was effectively that of today except when modified by human activities. The Holocene is the longest period of stable, warmer climate to have descended on earth since 15,000 years ago.

Droughts and Cities 6200-1900 BC

The Mini Ice Age of 6200 to 5800 BC. was a catastrophe for many farming communities between the Euxine Lake and the Euphrates River.

Then, in 5800 BC., the good times returned. The Atlantic circulation switched on; the moisture-laden Mediterranean westerlies abruptly resumed. Within a few generations the farmers expanded from their places of refuge into a warmer and better-watered landscape throughout the Fertile Crescent, to the banks of the Tigris and Euphrates rivers.

Here was abundant water, easily diverted into storage basins and onto fields. All the farmers needed to do was to build simple levees and canals. By 5800 BC., small agricultural communities dotted the landscape of southern Mesopotamia.

Within three thousand years, the tiny hamlets of 5800 B . had become some of the earliest cities on earth. Urban centers like Eridu, Nippur, Ur, and Uruk were surrounded by green patchworks of heavily irrigated fields and labyrinths of narrow canals. Cities arose here because farmers were tied to places where they could water their lands, and unfettered movement was impossible because so much of the landscape was completely dry. A city was a different entity from a village, not just larger in size but requiring both economic specialization and much more centralized social organization than smaller-scale societies. This scale of operation led almost inevitably to still larger political entities, to city-states, and eventually empires, loose alliances that linked cities and their rulers over large areas.

Mesopotamian rainfall may have been as much as 25 to 30 percent higher than today, much of it derived from summer monsoons, which resulted in a sevenfold increase in overall moisture because of a higher ratio of rainfall to evaporation.

For the next thousand years, life was good. Everyone lived in small, dispersed communities, close to strategic watercourses or natural storage basins, where they could fish as well as farm, and could irrigate the land without excessive labor. Then, about 3800 BC., the climate became suddenly drier, a trend that affected southwestern Asia and the eastern Mediterranean region for well over a thousand years.

At the same time, landowners began double-cropping, using plows and draught animals, and shortening fallow periods, while making a much greater labor investment in canals.

In these early days, water supplies were a household and community concern, rather than that of the central government, but this would change as the city became ever more powerful. When harvest came, every able-bodied person worked in the fields from dawn to dusk until the crop was gathered. This “family farm” model persisted for centuries, but eventually gave way to far more centralized irrigation works that were part and parcel of city government. Ubiquitous officials collected most of the harvest as taxation for the state’s granaries. More and more, people depended on the state for food, on rations paid out for services rendered.

By 3100 B.C., the southern cities had become the world’s first civilization. Sumerian civilization was a mosaic of intensely competitive citystates, each presiding over a highly organized hinterland, ruling over territories that butted up against those of their equally competitive neighbors. Each city-state had its own secular and religious leaders, its own patron deity, and thousands of people under its sway.

By 2800 BC., over 80 percent of the Sumerian population lived in settlements covering at least ten hectares, a form of “hyperurbanism” that lasted only a few centuries. By 2000 BC, the figure had declined to under 50 percent, as people moved away from cities that were again suffering through catastrophic drought.

Birth of Cattle Herding in Sahara

The father of the Saharan cattle herds was the primordial wild ox, the aurochs.

A modern biologist found the buffalo wary of carnivores and other possible threats obscured by trees or tall grass. The herds were much more relaxed when a potential predator was out in the open, walking slowly among them. Presumably, ancient game herds, even aurochs, by all accounts as unpredictable an animal as the notoriously ill-tempered buffalo, may have acted in the same way, allowing hunters to move freely among them as long as they remained in full view. Such free movement was vital to people who had but the simplest of bow-and-arrow technologies to pursue large animals.

When conditions worsened, Smith believes, the small aurochs herds of the desert became even smaller, closer-knit breeding units. The beasts were reluctant to abandon water sources, making it easier for hunters to move among them and cull them at will. Inevitably, cattle and humans came into close contact. The hunters gained such a complete familiarity with aurochs behavior that they began to control the movements of individual herds, preventing them from moving from one place to another and thus assuring continuance of their meat supply. By culling the more intemperate beasts, they soon gained genetic control over the herd, which led to rapid physiological and behavioral changes in the animals. The newly domesticated cattle were easier to control and may have enjoyed a higher calving rate, which would have yielded greater milk supplies.


Compared with turbulent rivers like the Tigris in Mesopotamia or the Indus in Pakistan, the Nile was relatively predictable. A normal flood allowed a good crop season over about two-thirds of the floodplain. A rise that peaked two meters below average could leave up to three-quarters of some Upper Egyptian provinces totally unirrigated. For all these uncertainties, the Nile was a huge oasis in 4500 BC., with plenty of fertile soil, ample pasturage, and many hectares of ponds, swamps, and marshes where fish teemed and edible foods abounded. Compared with their contemporaries in southern Mesopotamia, who spent months every year laboring on the simple irrigation canals on which their survival depended, the Egyptians had it easy.

El Nino

El Nino causes the Southeast Asian Monsoons to fail.

With almost mind-numbing regularity, subsistence farmers died by the millions. The historian Mike Davis has estimated that between 30 and 50 million tropical villagers between the Sudan and northern China perished of drought, famine, and disease during the nineteenth century, more than in all that century’s wars put together. Twenty-one out of twenty-six droughts since 1877 have been attributed to El Nifios, the most severe also coinciding with heavy snow cover in Eurasia.

Celts and Romans 1200 BC to 900 AD

The frontier between the continental and Mediterranean zones currently lies on the southern edge of France’s Massif Central, where the vegetation changes within a few meters from temperate to Mediterranean. The archaeologist Carole Crumley has tracked the movements of this ecotone over the past three thousand years. Her findings show that the boundary lay as far south as latitude 36.N, along the North African coast, in colder centuries. Warmer times would shift the frontier as far northward as the North Sea and Baltic Sea coasts, a distance of some 880 kilometers-no less than 12 degrees of latitude. Crumley believes that these north-south shifts in climatic zones had a dramatic, and hitherto unsuspected, effect on European history.

Fortunately for the Celts, their highly flexible farming strategies and cattle-herding practices were well suited to an uncertain climate.

In most places, cooler temperatures and ample rainfall had the effect of fostering agricultural productivity resulting from new farming methods, especially the use of the plow and iron tools. The carrying capacity of good soils rose sharply as the land was enclosed and kept permanently cleared of regenerated woodland. Much of what the Romans called Gaul, as well as southern Britain, now came under cultivation.

The Romans did not tamper much with the basic pattern of indigenous land occupation.

With iron came a new social order, no longer egalitarian but more hierarchical, even tribal in its loyalties. As the landscape became more crowded and boundaries more clearly drawn, we find more and more weapons appearing- slashing swords and shields, bronze helmets, even pieces of armor. Raiding and warfare were now an integral part of daily life. In some places, war became endemic, so much so that chiefs built strongly fortified settlements on hilltops, on lake promontories, and even on islands in lakes. By 600 B.C., temperate Europe was a landscape of hill forts, many occupied by substantial communities.

What do people do when confronted with food shortages within confined territorial boundaries? If they are subsistence farmers, they fall back on game and edible plants within their territories. If those are in short supply, as they are bound to be when more and more land is farmed or grazed, they try to move. By 400 B.C., moving was not an option in many parts of the world, so the alternative to hunger was to help yourself to your neighbor’s grain and herds. From casual raids to endemic war is but a short step when populations keep rising and food shortages become more commonplace. Inevitably, social values changed profoundly. Now doctrines of war and individual bravery came to the forefront.

The Celts appeared to be in Italy to stay, but the inexorable forces of climate change were against them. By 300 B.C., the ecotone between the continental and Mediterranean zones had moved north, at least as far as modern-day Burgundy. The shift brought a much more Mediterranean climate, with warm, dry summers and wet winters, to the more southerly Celtic domains. Roman agriculture, which was based on extensive production of a few crops such as wheat and millet for large urban populations, was much better suited to the semiarid southern European environment.

As the ecotone moved sharply north, Rome gained power rapidly. By the second century,

The conquest of Gaul and its aftermath caused such profound disruptions in Celtic life that generations passed before the Romans felt a need to reorganize their new province. When they did, the much warmer, Mediterranean-like conditions made their agriculture highly suitable for provinces remodeled in their imperial image.

The Roman empire was an order of magnitude more complex an enterprise than its predecessors. As an economic entity, it was a far more powerful and integrated way of creating wealth. But underneath the panoply of state and its far-flung possessions lay a startling vulnerability to climate. Political stability and control of outlying areas ultimately depended on the length of cereal-growing seasons in the Mediterranean zone. As long as this climatic regimen extended far to the north, food supplies were reasonably secure and Roman rule based on a sound economic foundation. The empire could survive climatic stresses that would have taxed less closely organized civilizations. Ordinary cold and drought cycles had little effect. Nor did major ENSO events. But major shifts in European climatic zones, with their attendant temperature and rainfall changes, affected

Roman rule profoundly. If the growing season shortened in the north and there were long cycles of poor harvests, the security of Gaul and the west was in question.

By 500 BC conditions were cooler and wetter throughout the west, making any form of large-scale cereal production very much harder over much of Gaul. The frontier between the continental and Mediterranean zones once again lay across North Africa. Ice even formed on the Nile River during the winter of A.D. 829.

The ecotone shift of the sixth century coincided with a major natural catastrophe. What may have been a huge volcanic eruption in 535 AD brought the densest, most persistent dry fog in recorded history to Europe, southwestern Asia, and China. Widespread famine, hunger, and bubonic plague followed once the abundant surpluses of the harvest ofthe year before were consumed.

The event of 535/6 was the single most abrupt climatic occurrence of the past two thousand years, perhaps due to a volcanic outburst exceeding even the Mount Tambora eruption of 1816 in its intensity.

By 900 AD, the Mediterranean ecotone once again shifted further north at a time when the wars and political chaos of earlier centuries were stabilizing somewhat and monasteries were introducing more sophisticated forms of agriculture to feed towns and their own communities… For the four centuries of this aptly named Medieval Warm Period, average summer temperatures in the west were between 0.7 and 1.0 degrees Centigrade above twentieth-century averages, even warmer in central Europe. Growing seasons lengthened; vineyards flourished across southern and central England.

Great Droughts 1 to 1200AD

For the five centuries of the Medieval Warm Period, from 900 AD to 1300, Europe basked in warm, settled weather, with only occasional bitter winters, cool summers, and memorable storms. Summer after summer passed with dreamy days, golden sunlight, and abundant harvests.

In the Americas, the same five centuries witnessed severe drought, hunger, warfare in the north, and the collapse of two major civilizations to the south.

Short-term climatic events like droughts do not often leave a clear footprint… From the California coast to the Maya lowlands to Lake Titicaca, five centuries of sudden aridity wrought havoc on human societies already living close to the environmental edge.

Coastal upwelling regions are a mere 1 percent of the ocean surface, but collectively they account for about 50 percent of today’s global fish landings.

By 800 AD, immediately before the collapse, perhaps eight to ten million Maya lived in the lowlands, a staggeringly high density for a tropical environment with low natural carrying capacity.

But the most severe drought of all, from 750 to 1025, coincides with the great Maya collapse of the southern lowlands.

The fundamental cause of the Maya collapse, then, was at least three major droughts that brought hunger and catastrophic social change.

Seven weeks after Easter in 1315, sheets of rain spread across a sodden Europe, turning freshly plowed fields into lakes and quagmires. The deluge continued through June and July, and then August and September. Hay lay flat in the fields; wheat and barley rotted unharvested … By the time the barrage of rains subsided in 1321, over a million-and- a-half people, villagers and city folk alike, had perished from hunger and famine-related epidemics.

The great rains of 1315 marked the beginning of what climatologists call the Little Ice Age, a period of six centuries of constant climatic shifts (that may or may not be still in progress).

But the exigencies of the Little Ice Age helped bring about an agricultural revolution, which began during the fifteenth and sixteenth centuries in the Low Countries, then spread to England a hundred years later.

In western Europe, only France remained agriculturally backward in an era when deteriorating climate made bad harvests more frequent.

If you would like to learn more about long-term human history, read my book From Poverty to Progress: How Humans Invented Progress, and How We Can Keep It Going.

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