According to one contemporary account, no sunspots of this magnitude had been witnessed in the United States since 1779. Moreover, observers could stare at the spot without the usual protection of shaded glasses, because the atmosphere lately had been filled with a curious thick haze—“a fine dust,” reported a Virginia newspaper, “very injurious to respiration.” “It had nothing of the nature of a humid fog,” noted an American physician. “It was like that smoking vapour which overspread Europe about thirty years ago.” And while sunspots typically are visible to the naked eye only when the sun is barely above the horizon, when the atmosphere has scattered much of the sunlight, this spot could be seen throughout much of the day. In fact, the aerosol haze from Tambora may have lengthened by as much as five times the usual window for viewing sunspots after sunrise and before sunset.
Since most Americans had never witnessed the sunspots that routinely move across the face of the sun, this highly visible spot—much larger than usual—generated more apprehension than the haze. Some feared it was an omen of impending apocalypse, a “calamitous sign in heaven,” or a warning that “the sun may, in time … become wholly incrusted” with spots, “so as to plunge us at once into the unutterable darkness that characterized the primitive chaos.” Others predicted the huge spot would weaken the sun’s rays and permanently cool Earth’s atmosphere. While the editors of the North American Review dismissed such speculation, they did admit that “the observation … that the light of the sun is less brilliant and dazzling than usual, is unquestionably well founded. We have remarked at different times during the present season, on days when the sky was perfectly clear, that there was a degree of feebleness and dimness in the Sun’s rays, not unlike the effect produced by a partial eclipse.”
Yet the first four months of 1816 were not noticeably colder than normal in the Eastern United States. In New England, the winter had been one of the mildest in a decade, with significantly less snow than usual. “The winter was open,” noted Noah Webster in his diary at Amherst, Massachusetts. “A snow in January, which was sufficient for sledding, was swept away in a few days. The ground was uncovered most of the winter.” Judging by the measurements of several amateur meteorologists at Northeastern colleges, January’s temperatures appeared to have been slightly above normal, with a warming trend at the end of the month. In Maine, the days were so pleasant “that most persons allowed their fires to go out and did not burn wood except for cooking.” Similarly, the Connecticut Courant reported that “January was mild—so much so as to render fires almost needless in sitting rooms.” (Thomas Jefferson, on the other hand, wrote to a friend from his retreat at Monticello, just west of Charlottesville, Virginia, shortly after New Year’s Day that he was “shivering and shrinking in body from the cold we now experience.”)
February brought generally mild temperatures with only a few snowstorms. “The first of March was very warm,” noted Adino Brackett, a farmer and schoolteacher in Lancaster, New Hampshire, “and almost all the snow went off.” The weather then turned clear and cold for several weeks, but the month ended with another warm spell and a rare appearance of early spring thunderstorms in the Northeast. There had been sharp cold snaps along the East Coast in mid-March, however, including a bout of sleet in Richmond, Virginia, that left fruit trees covered in icicles. As winter departed, the first week of April was slightly warmer than usual in New England, with very little precipitation.
Although it appears counterintuitive, the stratospheric aerosol cloud from Tambora was partly responsible for both the mild winter of 1815–16 in North America and the stormy conditions across central Europe. The aerosol cloud not only scattered sunlight, preventing it from reaching Earth’s surface, it also absorbed some of the incoming energy, reradiating it as heat. This warmed the stratosphere immediately above the cloud. If the aerosol cloud had warmed the stratosphere evenly around the globe, its effect would have been minimal. In the depths of winter, however, the high northern latitudes are plunged into continual darkness for several months. Without sunlight to absorb, the aerosol cloud could not heat the Arctic stratosphere; yet it continued to heat the stratosphere in the sunlit middle and lower latitudes.
A strong, cyclonic vortex forms near the North and South poles each winter. Strong west-to-east winds surround the vortex and expand to cover much of the high latitudes. These winds are created by the difference in winter temperatures between the sunlit middle and perpetually dark high latitudes: Air always flows from warmer temperatures toward colder ones, but Earth’s rotation turns the air off its path, towards the right in the Northern Hemisphere and the left in the Southern, to produce westerly winds. These westerly winds prevent cold, polar air from moving into the middle latitudes. When the vortex is particularly strong, lower atmospheric pressures exist near the pole; higher pressures are found in the middle latitudes; and the westerly winds provide an effective barrier. Should the vortex weaken, the pressure rises near the poles and falls in the middle latitudes, leading to frequent outbreaks of polar air. In the Northern Hemisphere, scientists have defined the North Atlantic Oscillation index to describe this seesaw of pressures between the poles and the middle latitudes, with a high index associated with a strong vortex.
Because the aerosol cloud from Tambora heated the stratosphere in the middle latitudes, but not in the Arctic, it enhanced the stratospheric westerly winds around the polar vortex. This effect soon made its way from the stratosphere to the troposphere, strengthening the barrier to Arctic air and leading to a stronger than normal high-pressure system in the Atlantic Ocean near the Azores Islands. The unusually warm winter throughout New England likely resulted from fewer incursions of polar air into the region. Data from tree rings and other proxies for temperature indicate that the average winter temperature in 1815–16 was as much as three degrees Fahrenheit warmer than normal in a band extending southwest from Alaska through central and southern Canada, across the Great Lakes, and into New England.
By strengthening the polar low and the Atlantic high-pressure system, the aerosol cloud also accelerated the trans-Atlantic westerly jet stream that steers weather systems from North America towards Europe. The jet stream also shifted north, bringing more systems to central and northern Europe and fewer to the Mediterranean Sea and North Africa. The westerly inflow of air from the Atlantic provided a steady source of moisture for these systems, which released that moisture over Europe in a series of snow- and rainstorms. The aerosol cloud effectively increased the North Atlantic Oscillation index; as weather forecasters are well aware, high values of this index are often associated with stormy winters across northern and central Europe. Using climate models to simulate the effects of past volcanic eruptions, scientists have found a consistent link between large eruptions and increases in the index the following winter, with the models producing a nearly constant stream of storms across the Atlantic as a result. The unsettled conditions across Europe in the winter of 1815–16 were likely the result of the aerosol cloud’s effect on the North Atlantic Oscillation.
Although the primary effect of the aerosol cloud was to cool global temperatures, its strengthening of the wintertime Arctic vortex delayed the appearance of severely cold temperatures in the United States. Once the long, polar winter night ended, however, the vortex weakened. Sunlight returned to the Arctic, and the aerosol cloud began to heat the stratosphere there as well as at lower latitudes. The westerly wind barrier around the vortex largely vanished, and cold air became free to move away from the pole—south, towards the United States and Europe. The cooling effects of the aerosol veil again became dominant, setting the stage for a chilling spring and a disastrous summer.
Nevertheless, the short-term effect of the mild winter of 1815–16 in the United States was to fuel the ongoing debate over whether American winters were growing warmer. Renowned Puritan cleric and naturalist Cotton Mather had first advanced this hypothesis in the late seventeenth century, less than a hundred years after the first English settlers arrived in Ma
ssachusetts Bay. “Our own Winters are, observably as Comfortably Moderated since the Land has been Peopled, and Opened, of Late Years,” wrote Mather. “Our Snows are not so Deep, and Long … and our Winds blow not such Rasours, as in the Days of our Fathers when the Hands of the Good Men would Freeze unto the Bread upon their Tables.” (Occasionally Mather veered into flights of hyperbolic excess in describing the rigors of winters past; he once claimed that when his grandfathers tossed water into the air, it “would be Turned into Ice e’re it came unto the Ground.”) Mather ascribed the changing climate to the settlers’ destruction of forests and their cultivation of ever-greater tracts of land, which presumably allowed the sun’s rays to better penetrate and warm the earth.
Nearly a century later, Thomas Jefferson seconded Mather’s deforestation theory, although the two men would have agreed on little else. An obsessive record-keeper who spent a lifetime searching for meaning in America’s physical environment, Jefferson faithfully recorded the temperature nearly every day—and often twice a day—for fifty years. (He even noted the weather in Philadelphia on July 4, 1776, when members of the Continental Congress signed the Declaration of Independence: 76 degrees at one o’clock in the afternoon.) Based upon his personal observations and anecdotal evidence, Jefferson suggested in 1781 that Virginia’s climate was indeed changing. Not only were winters less severe than they had been several decades earlier, but summers were cooler than before. “Both heats and colds are become much more moderate within the memory even of the middle-aged. Snows are less frequent and less deep.… The elderly inform me the earth used to be covered with snow about three months in every year. The rivers, which then seldom failed to freeze over in the course of the winter, scarcely ever do so now.” Twenty-five years later, this notion apparently had become so widespread that Jefferson could write that “it is a common opinion that the climates of the several States, of our Union, have undergone a sensible change since the dates of their first settlements; that the degrees both of cold and heat are moderated.”
Among those who concurred were French historian and philosopher Constantin-François de Chasseboeuf (who renamed himself the Comte de Volney). After traveling through the United States in 1795–98, Volney attributed the perceived climate change in North America to deforestation. To support his theory, he quoted an early history of Vermont, which claimed that conditions “in the cultivated part of the country” had changed dramatically since English settlers first arrived in New England: “The seasons are different, the weather more variable, the winter become shorter, and interrupted by great and sudden thaws. Spring is a scene of continual vicissitude … Summer is not so hot, but it lasts longer. Autumn is most tardy in beginning and ending … nor does winter become settled and severe before the end of December.”
“It is a popular opinion that the temperature of the winter season, in northern latitudes, has suffered a material change, and become warmer in modern, than it was in ancient times,” concluded Noah Webster in a speech to the Connecticut Academy of Sciences in 1799. “This opinion has been adopted and maintained by many writers of reputation”—Webster cited Jefferson, Dr. Samuel Williams, a weather expert and former Harvard professor, and Massachusetts physician Edward Augustus Holyoke—“indeed, I know not whether any person, in this age, has ever questioned the fact.” Webster himself believed that “the weather, in modern winters, is more consistent, than when the earth was covered with wood, at the first settlement of Europeans in this country.” The warm weather of autumn, he argued, extended further into the winter months due to “the greater quantity of heat accumulated in the earth in summer, since the ground has been cleared of wood, and exposed to the rays of the sun.” Similarly, “the exposure of its uncovered surface to the cold atmosphere” allowed frost to penetrate the ground to a greater depth in winter, which appeared to delay the advent of summer weather.
Nonsense, countered William Dunbar, a Scottish-born scientist who had emigrated to Pennsylvania in 1771. Dunbar, who frequently exchanged meteorological observations with Jefferson, claimed that deforestation actually made summers and winters more extreme. “I would enquire,” he wrote in an article published in the Transactions of the American Philosophical Society, “whether a partial clearing extending 30 or 40 miles square, may not be expected to produce a contrary effect by admitting with full liberty, the sunbeams upon the discovered surface of the earth in summer, and promoting during winter a free circulation of cold northern air.”
Timothy Dwight, a Massachusetts cleric and educator who, like his contemporary Jefferson, loved to collect weather data, also rejected the argument that American winters were growing milder. Dwight pointed to numerous very cold and snowy winters in the thirty years since independence that rivaled any of the formidable seasons of the seventeenth or early eighteenth centuries. Besides, discussions of changing climates seemed pointless to Dwight without adequate statistical data. “Few, if any, registers were kept in former times,” Dwight noted, and fewer still had been published. “Hence the comparisons of our present climate with that of former periods must be extremely defective.”
Climate scientists now know that deforestation of large areas can cause prolonged droughts and exaggerate seasonal variations in temperature, such that summers become much warmer and winters much colder. Dunbar was partially correct in his conclusions, although he failed to understand how forest canopies maintain the climate beneath them. Forests insulate their environment not only by reflecting sunlight but also by trapping moisture; plant roots help to retain water in the ground, while the canopy prevents water vapor from escaping into the air above. Remove the forest, and the moisture in the soil quickly escapes; winds then transport the water vapor hundreds or thousands of miles away. This starts a vicious cycle: Less water in the soil leads to less evaporation into the air, which can lead—when applied to an area of hundreds of square miles or more—to less rainfall, which in turn leads to less water in the soil. What rain does fall will often be unable to penetrate into the dry, hard soil, further increasing the risk of devastating droughts.
Summers become hotter in deforested areas not only because more sunlight reaches the surface, as Dunbar argued, but also because there is less moisture in the soil to cool the ground through evaporation. Water in the soil performs the same function as sweat does in humans; with little moisture to evaporate, bare ground quickly warms in the sunlight. Without the insulating effects of the forest canopy, winter temperatures can drop rapidly as the heat stored in the soil is lost to the atmosphere. There is no evidence to support Dunbar’s link between deforestation and stronger northerly winds, although generally forests do act as a brake on the local wind speeds, regardless of the direction. The effects of deforestation on local temperatures and rainfall can be mitigated where the forests are replaced with other ground cover, such as shrubs or crops, instead of simply left as bare soil.
If deforestation had, in fact, transformed their climate, Americans were ambivalent about the desirability of that change. On the one hand, the early colonists viewed the virgin North American forests as dangerous and evil places, the preserve of the devil (and, not coincidentally, Native Americans). They and their descendants believed they had a duty to level what Nathaniel Hawthorne termed the “heathen wilderness.” Turning a dense and dark forest filled with “stagnant air” and “rank vegetation” into productive farmland to support a Christian community seemed to fulfill God’s plan for the New World. Yet by the early nineteenth century, Americans in the Eastern states increasingly viewed the landscape less as a threat than a source of beauty and natural wonder. Alarmed at the ravages wrought by the “savage hand of cultivation,” they worried that their slashing and burning of the wilderness despoiled God’s handiwork and disrupted the natural harmony between heaven and earth, and that violent and erratic weather patterns comprised their punishment.
Certainly, chauvinistic New Englanders who prided themselves on their hardiness had no desire to escape the bracing rigor of their wint
ers. Months of subfreezing temperatures accompanied by occasional blizzards built the rugged New England character, they believed, inculcating the virtues of prudence, foresight, diligence, and cooperation in farmers from Connecticut to Maine. “Of all the scenes which this climate offers,” wrote St. John de Crevecoeur in an essay on the American farmer, “none has struck me with a greater degree of admiration than the ushering in of our winters … a rigour which, when once descended, becomes one of the principal favors and blessings this climate has to boast of.” Without such a challenge, New Englanders feared losing their unique identity and growing as weak and soft as they perceived the European character.
Popular anxiety about a general warming trend faded, however, as the nation entered the second decade of the nineteenth century, the coldest ten-year period on record in the history of North America. Even before the eruption of Mount Tambora, aerosol veils from a series of volcanic eruptions were cooling temperatures around the world. In 1809, a very powerful volcano erupted at an unidentified location—probably somewhere in the tropics, based on the recent discovery of large amounts of volcanic sulfuric acid in ice cores in the Arctic. Three years later, Soufrière (“Sulfur Mine”) on Saint Vincent erupted over a six-week period, followed by Awu on Sangihe Island, slightly northeast of Tambora. In February 1814, the eruption of Mount Mayon in the Philippines killed over 2,000 people on the island of Luzon. Some of each of these aerosol clouds, particularly the latter two, would have lingered in the stratosphere in 1815. (The lifetimes of stratospheric clouds vary from eruption to eruption, but three- to five-year spans are common, with a decreasing fraction of the original cloud remaining each year.) The devastating global cooling from Tambora, an eruption more powerful than the three earlier ones put together, was likely amplified by the existing cooling trend from these previous eruptions.
The Year Without Summer: 1816 and the Volcano That Darkened the World and Changed History Page 4
