The Year Without Summer (5 page)

While individual sunspots occur almost randomly, the total number of spots follows
a fairly predictable eleven-year cycle (a cycle that was discovered in 1844). But
sunspot activity also varies over much longer periods of time which are less predictable
and less regular than the short-term cycle. The eruption of Tambora coincided with
another minimum in sunspot activity—the Dalton Minimum of 1790–1830. The Dalton Minimum
was shorter and less intense than the Maunder Minimum, but it still resulted in a
notable decrease in sunspot activity; hence the surprise exhibited by the appearance
of a large sunspot in April 1816.

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 Massachusetts 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.”