The impact of the changing climate was not just felt on land either. At the time of the Little Ice Age, cod was an important source of protein throughout the North Atlantic region, especially for coastal populations. Abundant, large, and nutritious, it was in great demand, in part because the Catholic Church allowed its adherents to eat fish, but not red meat, on Fridays. It was also a permitted food throughout Lent. It was easily preserved; salted cod is light, has a high food value, and can be kept for long periods without refrigeration. Long before the Little Ice Age, Norse explorers had carried salt cod as their principal food on sea voyages. It became an essential and very valuable commodity, and as a result the cod fishing industry was large and extremely competitive. As the Little Ice Age began, fish seemed to be there for the taking, not subject to the same season-to-season vagaries of the weather that affected crops and livestock. But that optimistic outlook was soon to change.
Cod have a limited tolerance for temperatures outside a narrow range of about four to seven degrees Celsius. Below two degrees they suffer kidney failure. Because of this, their geographical range, especially at the cold northern extremes, is particularly sensitive to changes in water temperature. Unfortunately, existing records are not detailed enough to trace exactly how cod populations shifted during the Little Ice Age. However, several general trends are clear. As the northernmost Atlantic progressively became ice-filled due to the colder weather, the cod moved south. Catches around Greenland decreased, as did those near Iceland. Fishing off the coast of Norway also became more difficult. The fish didn’t completely disappear from any of these localities, but their numbers dwindled, and it became harder and harder for the Basque, English, and Dutch fishing boats that regularly visited these waters to fill their holds with cod. As a result, and in spite of the generally stormier weather, they began to search farther afield. When their explorations first led them to the coast of North America is not known—then as now, fishermen didn’t want to give away their secrets. But in 1497, the Italian explorer Giovanni Caboto, known to the English as John Cabot, sailed along the eastern coast of Canada and reported waters teeming with cod. In places, they were abundant enough to literally scoop out of the sea with baskets. Cabot noted that the sea was also full of Basque fishing boats. This was only a few years after Christopher Columbus “discovered” North America; the fishermen had certainly been there long before. Just as the warmth of the Medieval Warm Period had allowed the Vikings to sail westward to North America, so the cod populations, responding to the cold of the Little Ice Age, led Europeans across the Atlantic to the New World.
Although the Little Ice Age was, on average, significantly colder than the immediately preceding and following periods, it was by no means a single, uninterrupted interval of cold. There were reprieves, sometimes fairly long ones. There seem to have been significant spikes upward in temperature in the early 1500s and again in the early 1700s, each followed by a return to much colder times. There are hints that cod migrations, political crises, and the frequency of famines were all affected by these fluctuations. In an especially cold period through the mid and late 1600s, cod essentially disappeared from the water around the Faeroe Islands, where they had previously been abundant. About the same time, residents of the Orkney Islands off the north coast of Scotland were several times startled by the improbable appearance on their shores of Inuit people in kayaks, presumably forced south and east by enlarging ice packs to the north. Poor harvests and harsh weather, together with the promise of better life elsewhere, initiated emigration from Scotland that would continue for centuries. Perhaps one of the greatest difficulties faced by people of the time was unpredictability. While it might be possible to adapt agriculture and housing to consistently lower temperatures, it was very difficult to cope with wild swings between very cold and very warm. And weather records, which are quite complete for much of Europe and parts of North America from the early 1600s on, show that climate extremes were often juxtaposed. The coldest winter in central England between 1659 and 1979 was the winter of 1683–84; one of the warmest was just two years later in 1685–86.
Not all of the news from the Little Ice Age was bad. Countries such as Holland benefited from the shift of fish stocks southward into the North Sea. In response to coastal flooding, exacerbated by the violent storms of the sixteenth and seventeenth centuries, the Dutch also became experts in the technology of reclaiming low-lying land from the sea. Subsistence farming, always risky at the best of times, declined during the Little Ice Age, and more specialized agriculture sprang up, especially in Britain. Growing cash crops for the city was usually a better way to survive than trying to be self-sufficient. Even glass windows are believed by some to be a response to the cold temperatures of the Little Ice Age—they helped keep the cold out but still provided a view of the outside world. And while no one has yet suggested any connection between climate and the American Revolution, it is quite likely that the chronic shortages of bread and grain in France in the late 1700s, which were due at least partly to the bitter winters, unpredictable climate shifts, and generally bad weather of the time, fed the underlying discontent that led to the French Revolution.
The Little Ice Age even influenced art. Snowy winter scenes suddenly appear abundantly in sixteenth- and seventeenth-century European paintings. Skaters glide on canals and lakes that have not been frozen in living memory (figure 26). In 1970, Hans Neuberger, a meteorologist at Pennsylvania State University, analyzed more than twelve thousand paintings from American and European museums, all dating between 1400 and 1967. He classified them by region and date, and looked at them with a meteorologist’s eye. Neuberger was particularly interested in depictions of the sky, and while not every painting was amenable to this kind of examination, many were—even if they only showed a glimpse of the sky from a window. While artists surely take some license with their subjects, Neuberger’s analysis uncovered some clearly defined trends—for example, none of the British paintings he studied showed a completely clear sky, while some 12 percent of those from Mediterranean countries did. Half of the British paintings showed the sky completely overcast, a higher fraction than any of the other regions he studied. His data show an increase in cloudiness between 1400 and 1550, and then an abrupt further increase—more than 50 percent—especially in the abundance of low, scudding clouds. Cloudiness peaks during the seventeenth century but remains high throughout the Little Ice Age. Neuberger also pointed out that most paintings from the last few hundred years of the Little Ice Age—on average its coldest period—are very dark compared to both earlier and later art. This could just be a popular stylistic device, but Neuberger suggests that it might also be a reflection of the generally cloudy, low-illumination conditions that prevailed.
Figure 26.Painting by Sir Henry Raeburn of the Reverend Robert Walker skating on Duddingston Loch, near Edinburgh, Scotland, toward the end of the Little Ice Age. Duddingston Loch rarely freezes in winter now. Reproduced with permission of the National Gallery of Scotland.
And in 2003, Henri Grissino-Mayer, a tree-ring expert at the University of Tennessee, and Lloyd Burckle, at Columbia University, suggested yet another possible connection between the arts and the Little Ice Age: they proposed that climate may be partly responsible for the exquisite sound of Stradivarius violins. By examining tree rings, Grissino-Mayer found that growth in European high-altitude forests slowed because of the cold of the Little Ice Age. He also discovered that between 1625 and 1720, the trees showed exceptionally narrow growth rings, producing dense and strong wood—properties that may have enhanced the quality of the instruments made by the renowned Italian craftsman. Stradivarius produced his most famous violins between 1700 and 1720.
A hallmark of ice age climate change, at least when viewed from the perspective of its impact on human societies, is abruptness. With little or no warning, there have been drastic shifts in temperature, storminess, and precipitation, both regionally and globally. Frequently, these shifts, although very rapid, leave the cli
mate system in a new mode that persists for a relatively long time. The Mediaeval Warm Period and the Little Ice Age, as well as the occasional abrupt fluctuations that occurred within them, are good examples from the past millennium. As already discussed, there are even more striking events, such as the Younger Dryas interval, on a longer timescale. There is little doubt that similar changes will occur in the future, and understanding the underlying causes of such events is important if there is to be any hope of predicting them or mitigating their impact on society. Abrupt climate change is currently a hot topic in the environmental sciences, and a large cadre of scientists from diverse disciplines are working on the problem. In a relatively short time, much has already been learned, and although definitive answers—always elusive in science in any case—are not available, some general conclusions are.
Most of the attempts to understand why rapid climate shifts happen involve several concepts that are quite familiar. The first is the idea of a threshold, a state that, if crossed, more or less automatically shifts the climate into a different mode. To cross a threshold, however, requires something else—usually an external “forcing” or trigger, some process or phenomenon that will change the system, either slowly or rapidly, until it reaches and crosses the threshold and flips into the new mode. Such changes also usually require some type of positive feedback, a multiplying effect that ensures that the change will be global or universal. None of these ideas is particularly new—recall James Croll’s belief that variations in the Earth’s orbit would act as an external forcing, cooling the Earth until snow persisted on the ground throughout the year. Year-round snow was the threshold, and it would also provide positive feedback: it would amplify the cooling by reflecting more solar energy back into space, initiating rapid expansion of ice sheets and a new glacial interval. By building such ideas into complex computer simulations of the global climate—an enormous task that requires great computing power—and by using accurately determined ocean and atmospheric conditions, it has been possible to examine the effects of different types of external forcing on the entire system—a kind of “what if ” approach.
One conclusion of the simulation studies, already known in a general way from earlier work, is that the ocean current system is very important for distributing heat. In particular, changes in the way ocean circulation occurs in the North Atlantic Ocean have been implicated in some of the large and abrupt temperature changes observed in the Greenland ice-core data over the past few tens of thousands of years. As discussed briefly in chapter 6, the warm surface water of the Gulf Stream moves northward in the Atlantic, evaporating and cooling as it goes. Both evaporation, which increases the salt content of the water left behind, and cooling, which contracts it, cause the density of the water to increase. By the time it reaches high latitudes, it has become so dense that it sinks, displacing the underlying lighter water. There are a few other places, such as the Antarctic, where very cold surface water also becomes dense enough to sink, but the process is most important in the North Atlantic—so important that it is a driving force in the circulation of the entire ocean. The cold sinking water spreads southward across the equator in a deep layer, south to the Antarctic and around into the Indian and Pacific Oceans. In places, it upwells again to the surface, and surface currents make up the return flow, eventually again joining the Gulf Stream to complete the circuit. If something were to shut down the sinking of North Atlantic seawater, the whole ocean circulation system would slow down and either stop completely or reorganize. The Gulf Stream would no longer carry warm tropical water into the North Atlantic. Greenland and Europe would lose the warming benefit of this current, and their climates would abruptly become much colder.
That sounds very nice and simple in theory. Could it actually happen? Many researchers now believe that just such a scenario was responsible for the Younger Dryas cold period discussed in the previous chapter, and probably also for many other cold snaps that can be identified in the Greenland ice cores. Hints can be found in sediment cores from the North Atlantic that during these intervals, the flow of Gulf Stream water slowed, and the amount of new dense bottom water being produced declined. In addition, ice cores from the Antarctic show a slight warming at high southern latitudes, an effect that has been linked with a weak or nonexistent Gulf Stream. Under conditions similar to those at present, the Gulf Stream cools the Antarctic slightly by drawing warm water out of the Southern Hemisphere and transporting it northward; if the Gulf Stream slowed or stopped, a small amount of warming would be expected.
That still leaves the question of cause. The search for reasons for the on-again off-again nature of the Gulf Stream has focused on processes that could change seawater density, because, as explained above, density plays an important role in ocean circulation. For a given batch of ocean water, density depends on temperature and dissolved salt content (and for this reason the circulation is referred to as thermohaline circulation). If some process were to decrease the density of North Atlantic surface water, it would eventually cross a threshold value and float rather than sink, shutting down the thermohaline circulation. This could happen by addition of low-density fresh water from rapidly melting glaciers, as was discussed in chapter 6. Just such a scenario has been proposed for abrupt temperature decreases recorded in Greenland ice cores near 12,800 (beginning of the Younger Dryas) and 8,200 years ago, both of which correspond to sudden changes in glacial Lake Agassiz’s drainage that added large volumes of fresh water to the North Atlantic. It is also possible—paradoxically—that present-day global warming will lead to cooler temperatures in northern Europe through a similar effect. Both accelerated addition of fresh water due to melting of the Greenland ice sheet and the general warming of seawater because of globally higher temperatures will decrease the density of surface water in the North Atlantic. There is some evidence that the amount of sinking cold water in the North Atlantic has decreased slightly in recent years, but the measurements have not been carried out over a long enough period to determine whether this is a long-term trend or just a minor deviation from the average.
Although a strong case can be made that large-scale changes in North Atlantic ocean circulation were responsible for at least some of the rapid temperature changes recorded in Greenland ice cores, there is no evidence that the less severe climate variations of the past millennium, disruptive as they were for European civilization, had a similar origin. As mentioned earlier in this chapter, one idea is that the sun’s activity may have been the important forcing factor. Possibly that could have tipped the North Atlantic Oscillation into a mode that dominantly brought cold weather to the region. Even volcanic activity has been implicated, not as a cause of Little Ice Age cold, but as a process that occasionally and temporarily exacerbated the already-cool climate. That volcanic activity can have a measurable effect on temperatures worldwide is no longer in dispute—the volcanic dust and sulfurous gases blasted into the stratosphere during the 1991 eruption of Mt. Pinatubo in the Philippines so reduced solar energy reaching the Earth’s surface that global temperatures were lowered by about half a degree Celsius for over a year. That doesn’t sound like much, but it is a sizeable fraction of the average temperature reduction during the Little Ice Age. The seventeenth century saw at least five large, explosive eruptions, beginning with the most massive, in the Peruvian Andes, early in 1600. Ash from this eruption is easily identifiable in both Greenland and Antarctic ice cores. Records from Europe and North America count the following summer, in 1601, as the coldest for hundreds of years. In 1815, as the Little Ice Age was drawing to a close, there was an even larger eruption on the island of Sumbawa in Indonesia. Again, the following summer was frigid. The year 1816 became known as the “year without a summer.” Snow fell in New England in June, and crops failed in Europe.
If there is a lesson to be learned from our knowledge of the past millennium’s climate history, it is that surprises abound even for this very short snippet of geological time, which, when viewed from the long-t
erm perspective of the entire Pleistocene Ice Age, enjoyed a relatively warm and stable interglacial climate. Modern societies for the most part are better equipped to deal with such surprises than were those of even a hundred years ago, but are not entirely immune. Just-in-time logistics systems and highly concentrated and specialized agriculture are as likely to be disrupted by abrupt climate change as some earlier technologies. Energy grids even now have difficulty coping with high demand during heat waves, when millions of air conditioners are operating at full capacity. Just as troubling is our inability to predict, even in a general way, what may happen to the climate system as a result of human influences. A great, unintended experiment in “climate forcing” is under way as we add more and more greenhouse gases to the atmosphere. Whether or not we shall reach one of those thresholds that seem to separate different climate modes, and what will happen if we do, is still unknown.
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