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THE PROSPECT of the greenhouse effect warming the globe slightly over the next few decades is sometimes dismissed as unimportant compared with natural fluctuations of temperature from season to season, and even from day to day. But changes in temperature of one or two degrees, no bigger than the ones we are likely to experience over the next 20 years, have had profound influences on human cultures over the past two thousand years. Although we may think that our modern technology will help us to ride out such variations better than our ancestors did, there are still lessons to be drawn from looking at the winners and losers in the climatic stakes of the recent past.

Historical records and data such as the remains of pollen from the variety of plant species trapped in layers of mud in beds of lakes, tell us, for example, that the great spread of Roman culture after about 500 BC coincided with a warming of the European part of the world, out of a period of severe cold. The slight improvement and warmth persisted until about AD 400, giving the Roman Empire a climate that was distinctly milder than western Europe enjoys today.

The subsequent deterioration in climate was followed, roughly a thousand years ago, by a period of warmth sufficiently pronounced that it has become known to historians of climate as the ‘little climatic optimum’. The name reflects the fact that the warmth lasted for only a few centuries, and that European climatologists often regard a slightly warmer world than we have been used to as a good thing. The greenhouse effect may yet prove them wrong, but the name has stuck.

During the peak of the little optimum in Europe, people grew vines up to 5 degrees farther north and 200 metres higher above sea level than in the 1960s, so perhaps there could have been vineyards around the River Tyne, as well as the Moselle. This suggests that average temperatures were a little more than 1 Degree C higher than today in those days. The little optimum came too late to benefit the Roman Empire. But it played a major part in encouraging the spread of another European culture, the Norse seafarers. These settlers farmed for 500 years in Greenland, only to die out when their climate changed for the worse in the 16th century.

Climatologists and historians are no longer wholly dependent on proxy records of climatic change, such as the distribution of particular species of plants. The ice sheets over Greenland, deposited as layers of snow falling each year and accumulating on top of each other, contain frozen in their heart a direct record of the temperature of the North Atlantic region each year. Climatologists can read this record, by drilling into the ice and extracting a long core of these layered ice sediments, which can be dated by counting downwards from the surface rather like dating a piece of wood from its rings . It is a frozen thermometer that reveals how temperatures have changed from decade to decade.

The record in and around Greenland shows that the little optimum was in full swing by the early part of the 8th century, and that the warmth, with average temperatures between0 and -1 Degree C, persisted with only minor fluctuations until well into the 11th century. One of the most fascinating features of the temperature pattern revealed by the ice measurements (carried out by Willi Dansgaard and colleagues in Copenhagen) is the way that it meshes with and enhances the limited accounts from history of the spread of Norse culture around the northern rim of the Atlantic. The story of the Viking voyagers has been embroidered down the centuries, but large chunks of what seem to be genuine history are to be found in the sagas, the oral histories of those days, written down and preserved centuries later. The Landnam Saga tells of the first settlements in Iceland, and the Greenlander Saga tells of voyages and settlements farther west.

Historians sometimes try to distinguish between the terms ‘Viking’ and ‘Norse’. The Vikings were seafaring adventurers, tough and hardy. Some of them were pirates and robbers, who looted, raped and pillaged on the coastal lands of Europe. They were the more disreputable representatives of the Norse culture, which back home in Scandinavia ran to respectability in the form of ‘proper’ nations, kings and the rule of law, albeit a somewhat harsher form of law than we are used to today. But the distinction quickly becomes blurred. The Normans who invaded England in 1066 were descended from Vikings who had settled in France. Similarly, the people who spread the Norse culture westward across the Atlantic were certainly Vikings, though in a generation or two their descendants became respectable Norse men and women, the Westvikings, offering allegiance to the homeland and the Christian religion. But these colonies flourished only as long as the climate permitted.

The sagas tell that Norse voyagers were blown off course and reached Iceland by mistake twice in the 850s. The first attempt at a Norse settlement there, led by a farmer, Floki Vilgerdason, took place in the 860s. He could scarcely have picked a worse time. As we now know from the record in the ice cores, Floki’s band of adventurous farmers arrived in the lands of the North Atlantic at the end of a run of cold decades in the middle of the little optimum. He lost his cattle in a severe winter, and came home to Scandinavia with tales of ‘a fjord filled up by ice’, as the Landnam Saga records. And so, the saga continues, ‘he called the country Iceland’. Ironically, this is just about the last mention of sea ice near Iceland for 300 years. In the 870s, the North Atlantic was warming up, and other settlers following in Floki’s wake found Iceland much more hospitable. They established a thriving colony, in spite of the island’s name.

Over the next couple of centuries, in the warmth of the little optimum, some Norse travellers voyaged to the Mediterranean, trading with Italy and the Arab world. Others, following the great river systems, moved far into continental Europe, where they helped to found the state that became Russia. Some even followed the rivers south and east as far as Byzantium. From 900 to 1100, if Europe belonged to anyone, it belonged to the Norse. They very nearly established permanent colonies in America, as well.

Floki and the farmers who followed him had not been Vikings in the true sense of the word, although they must have been rather tougher than the average farmer in Europe today. But the next stage in the sagas of the Westvikings fully lives up to their bloodthirsty image.

In 960, back in Norway, a rather nasty character called Thorvald Asvaldsson killed a man and was forced to flee to Iceland, taking his family with him. A hundred years after Floki’s ill-fated voyage, the settlement was well established, and the good land in the south of the island was all occupied. Thorvald had to make do with poor land in the north. But his son, Erik, married into a good family and set himself up on a better farm. He seemed set for a secure life in Iceland when a violent streak to match that of his father surfaced.

Outdoing Thorvald, Erik killed two men. In 982, he was banished from Iceland for three years, to give him time to cool down. The sagas refer to him as ‘Erik the Red’, and it is tempting to see this as an indication of his violent temper: it may just be that he had red hair.

Erik, with a shipload of followers, headed west. He had decided to use his period of exile to explore a region that he had heard of in vague stories: islands to the west of Iceland that had only been seen by lost voyagers, more anxious to return home than to explore. The land he reached was mostly rough and rugged. But there was a deep fjord on the southwestern coast, well protected from the sea, warmed by the Gulf Stream, and with adequate land for farming nearby. Conditions were rather like those he had left behind in Iceland, and Erik called the new country Greenland.

Historians may never know just why he chose this name. According to one version of the story, it was because the Vikings could see sunlight glinting green on the glaciers of the mountains. A more plausible version, recorded in some of the sagas and certainly fitting Erik’s character, is that he chose the name in a deliberate confidence trick, as part of a swindle to lure Icelanders west to a new colony where Erik would be in charge and there would be no threat of further exile.

But those written versions of the sagas were recorded centuries later, when conditions in Greenland were even harsher than they are today. Researchers now know, from the ice thermometer, that Erik arrived in Greenland near the end of a particularly warm period of the little optimum, and that the coastal region where he landed must have been green and fertile, by the standards of Icelanders at that time. There may well have been an element of exaggeration in Erik’s tales to the folks back home, but probably no out-and-out lies.

If Greenland and Iceland had been discovered in the same year by the same explorer, their names might, quite logically, be reversed. One of the islands is covered by ice, and the other is more green and fertile. But their names are the wrong way round largely because of minor fluctuations in climate. Iceland was settled at the end of a cold spell, and Greenland near the end of a warm spell. Since then, things have changed, so that Greenland is not green in the agricultural sense today.

Once they had settled in Greenland, it was probably inevitable that the Norse would also reach the mainland of North America. In fact, it was Leif Eriksson, the son of Erik the Red, who led the first expeditions to explore the new land. They established at least two settlements, and, for a time, provided valuable timber for the settlers in Greenland. Leif became both rich and famous from his travels, earning the name ‘Leif the Lucky’. But the luck of the Westvikings was about to change.

By the end of the 12th century, the little climatic optimum was past its peak. Temperatures declined by about two degrees from the relative warmth of the past few centuries to, typically, two or three degrees below freezing. When the ice came back to Greenland in full force, the Norse colonists were doomed, but not because life in that part of the world became impossible. They failed to survive because they did not adapt to the changing conditions around them.

In round terms, the Greenland colonies survived for 500 years, from 1000 to 1500, so they were far from being complete failures: the United States of America have been independent for less than half that time. Erik’s original colony at the southern tip of Greenland was known as the Eastern Settlement; another colony, further north but on the western side, was known as the Western Settlement. Over a large part of that 500 years, the Norse in the Western Settlement, in particular, were in contact with the Arctic hunters generally referred to today as ‘Eskimos’. In this case, they were members of the Inuit people. The lifestyles of the two cultures could hardly have been more different. The Inuit were nomadic, and followed their food across the ice. They could live anywhere they found that food.

The Norse, in contrast, were settled people, with farms and cattle, but they also used the nearby sea as a resource. Their colonies were small, with perhaps 5000 in the Eastern Settlement and 1500 in the Western Settlement, but they did well as long as the weather remained benevolent. Migrating harp seals, which arrived on the west coast of Greenland each spring, were an important source of food, and the summers were warm and long enough for grass to provide pasture and a supply of hay for the winter.

The colonists traded polar bear skins and walrus tusks eastward, to Iceland and Scandinavia, in exchange for items they could not produce for themselves. The most dramatic example of this, and of the temporary wealth of the colony, came in 1125, when the Greenlanders traded a live polar bear for a bishop from Norway, whom they ensconced in grand style in a cathedral built of stone, with its own home farm. Two hundred years later, when the colony was already a hundred years older than the US is now, the church owned about two-thirds of the best grazing land on the island. But this very European structure to the Greenlanders’ society was probably their undoing.

When the North Atlantic region cooled by about 2 Degree C in the 13th and 14th centuries, the colonies in Greenland were affected in many adverse ways. Sea ice spread southward, making voyages to Iceland more difficult and dangerous. With the ice, the Inuit came south, into more direct conflict with the Norse. And on the farms, summer was now too short and wet to provide enough hay to see the cattle through the winter. Even the seals seemed to have changed their migratory habits, perhaps because of changes in ocean currents, removing another essential resource.

In the face of all this, the Norse carried on their traditional way of life as best they could, for as long as they could. There is a sense of epic tragedy in the decline of the colonies. The Greenlanders’ last bishop died in 1378, and was never replaced; there was no deliberate contact with the colonies at all after 1408, although occasional ships would put in to trade or seek shelter. Archaeological studies have now shown how the surviving members of the shrinking community carried on farming and raising cattle. The eloquent testimony of skeletons from the graveyard shows that as conditions became harsher and food more scarce, the average height of the Greenlanders declined from about 177 centimetres in Erik’s day to about 164 centimetres by the 1400s.

Europeans maintained intermittent contact with the colonies during the 15th century. The last bodies laid to rest in the graveyard, preserved by the even more severe weather that followed, were dressed in styles from Europe from about 1500. Early in the 16th century, the last colonist died. In 1540, ships driven to Greenland by severe weather found no one left alive, and one dead man frozen where he had fallen. The last of the Norse Greenlanders may have missed rescue by only a short time. To put all this in perspective, that last Norseman was closer to us in time than he was to Erik the Red, the founder of the colonies.

But the tragedy need never have happened. The very conditions that killed off the Norse colonists allowed the Inuit to thrive. Thomas McGovern, of Columbia University, New York, has shown how the Norse could have survived if they had abandoned their cattle and concentrated on developing resources from the sea. In effect, this would have meant adopting and adapting the lifestyle of the Inuit people. Farmers who struggled to the bitter end to keep their cattle alive could have done better by changing over to goats and sheep, creatures better able to fend for themselves, and using the extra time this gave them for fishing. A more radical possibility, McGovern suggests, is that even in the 12th and 13th centuries, the Greenlanders could have developed the kind of whaling, fishing and sealing villages that are characteristic of modern Greenland.

One reason why the Norse followed none of these options is that they had no wood to build boats in the European style. Yet the Inuit used boats built by a completely different technique, out of skin. With skin boats, the Norse in Greenland could have spread themselves out in villages and homesteads along the coast, trading with each other by sea.

The Greenlanders did not even have enough sense to adopt the warm clothing that the Inuit wore. The stories of frozen bodies dressed in the European style of 1500, and of the last lonely Greenlander, may bring a tear to the eye, but it ought to be one of exasperation. European fashions of the year 1500 were not suitable clothes to wear in the winters of that time in Greenland. Single-minded conservatism, according to McGovern, may have been the single most important factor in the extinction of the Norse in Greenland. You can almost see that last Norseman muttering to himself as he trudged through the snow, ‘If cattle farming was good enough for Erik, then it’s certainly good enough for me.’

So does that absolve the vagaries of the climate from blame for the extinction of these colonies? The answer is both yes and no; conservatism would not have led to disaster if the climate had not shifted. The lesson is that human society has to be flexible enough to adapt to changing climatic conditions. That is an important lesson, since the climate did not stop changing in the 16th century: climatic fluctuations are happening today, and include some that result from human activities.

In many parts of the world, people are even now faced, for different reasons, with the same choice that confronted the Norse in Greenland 700 years ago: adapt, or die.

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Drilling for history by degrees

ICE CORES give researchers a record of the temperature when snow formed and fell thousands of years ago. To read this record, they have to overcome many practical problems, not least of which is working in the Arctic or Antarctic. These include the little difficulties inherent in drilling a core of ice 404 metres long, extracting it from the ice sheet, and counting the layers in the ice to provide a calendar going back for 1420 years. On top of this, the Danish researchers who first obtained such a record of variations in the climate of Greenland since the 6th century had to measure the proportions of different kinds of oxygen atoms in the ice layers, in order to find the temperature of the region when that ice was falling as snow.

The trick is now standard. It depends upon the fact that there are two forms of oxygen atoms, called oxygen-18 and oxygen-16, in the air and water of our planet. Oxygen-18 is heavier than oxygen-16, so some molecules of water are heavier than others. Lighter molecules evaporate more easily from the ocean to form clouds, so the water vapour in the clouds is made up, proportionately, of more oxygen-16, and less oxygen-18.

In the far north, those clouds produce snow. The exact proportion of oxygen-18 that gets into the clouds (and therefore into the snow) depends on the average temperature over the ocean surface in a particular year. So measuring the ratio of oxygen-18 to oxygen-16 in ice at a particular level in the core reveals, directly, the temperature in the year that ice formed from freshly fallen snow.

Temperatures measured in this way can be checked against historical records for the past few decades, corresponding to the top layers of ice. This calibrates the frozen thermometer and confirms that it is indeed a good guide to past temperatures.

Mary Gribbin is a teacher and author of science books for children. With John Gribbin, she has written Children of the Ice, to be published in 1990 by Basil Blackwell.