The Little Ice Age: How Climate Made History 1300-1850 (23 page)

The Franz Josef glacier in New Zealand's Southern Alps thrusts down a
deep valley backed by mountains that rise to the precipitous heights of
Mount Tasman, 3,494 meters above sea level.3 The path to the ice face
winds through a barren valley floor, across fast-running streams fed by
the melting glacier. As you walk up to the ice, you wend your way
through massive blocks of hard rock polished into humps by the abrasive debris collected by the ice as it advanced and retreated along the
valley. When you reach the face, you gaze up at a pale green river of ice
glistening in the sun, a microcosm of the glacial fluctuations of the Little
Ice Age.

Franz Josef is a glacier on the move. Nine centuries ago, it was a mere
pocket of ice on a frozen snowfield. Then Little Ice Age cooling began
and the glacier thrust downslope into the valley below, smashing into the
great rain forests that flourished there. The ice crushed everything in its
path, felling giant trees like matchsticks. By the early eighteenth century,
Franz Josef's face was within three kilometers of the Pacific Ocean, a river
of aggressive ice pointing like an arrow toward the coast.

Today, Franz Josef, like other New Zealand glaciers, is in retreat. Thousands of tourists walk up to the face every year, lingering at sunset to enjoy the sight of the mountain peaks bathed in rosy hues while the lower
slopes lie in deep, purple shadow. Their pilgrimage takes them through
rocky terrain that was completely covered with ice during the eighteenth
and early nineteenth centuries. They can see how the glacier is a barometer of the greater cold of two centuries ago, followed by modern-day
warming. Signs along the access path mark the spots where the end
moraines halted at their maximum extent, then document the spectacular
retreats and glacial fluctuations since 1850. The glacier retreated steadily
until about 1893, when a sudden forward thrust destroyed the tourist
trail to the face. In 1909, advances of up to fifty meters a month were reported. Franz Josef then retreated again before recovering about half the
ground lost earlier in the 1920s. By 1946, the glacier was at least a kilometer shorter than it had been three-quarters of a century before. The
pattern of advance and retreat continues to this day, with the retreats
more prolonged than the advances.

The New Zealand Alps are one of the few places in the world where
glaciers thrust into rain forest. It was here, at the foot of Franz Josef, that
I realized the Little Ice Age at its apogee was a truly global phenomenon,
not just something of concern to Alpine villagers on the other side of the
world.

The high tide of glacial advance at Franz Josef came between the late
seventeenth and early nineteenth centuries, just as it did in the European
Alps. Glaciers in the Alps advanced significantly around 1600 to 1610,
again from 1690 to 1700, in the 1770s, and around 1820 and 1850. Ice
sheets in Alaska, the Canadian Rockies and Mount Rainier in the northwestern United States moved forward simultaneously. Glaciers expanded
at the same times during the nineteenth century in the Caucasus, the Hi malayas, and China. The Qualccaya ice core in Peru's southern Andes
provides evidence of frequent intense cold from A.D. 1500 to 1720, with
prolonged droughts and cold cycles from 1720 to 1860.

As Franz Josef shows, the cycles of glacial advance and retreat were never
clear-cut, often rapid, and always irregular in duration. Nor did the maximum advances coincide from one region to the next. The northern glaciers
in both Europe and North America advanced late in the Little Ice Age and
retreated early. (Iceland is an exception: its glaciers reached their greatest extent in the late nineteenth century.) In contrast, the more southerly glaciers,
like those in the New Zealand Alps, advanced early, retreated and advanced
again and again to the same extended positions before shrinking decisively
in the late nineteenth and early twentieth centuries.

The increasing cold affected not only glaciers but mountain snow levels, which extended lower than today. Snow cover lasted longer into the
spring. High mountains in the Andes of Ecuador were perennially snowcapped until at least the late nineteenth century. Travelers in Scotland reported permanent snow cover on the Cairngorm Hills at about 1,200 to
1,500 meters, which would require temperatures 2 to 2.5°C cooler than
those of the mid-twentieth century. Wrote traveler John Taylor of the
Deeside area in about 1610: "the oldest men alive never saw but snow on
the tops of divers of these hills, both in summer as well as in winter."4 A
temperature drop of only 1.YC (about that recorded in central England
at the time) would have been sufficient to bring the snowline down to
about 1,200 meters in the Scottish mountains and would have allowed
glacier ice to form in some shaded gullies.5

The colder conditions had striking biological consequences that we
can only extrapolate from modern plant, tree, and animal movements.
Trees like birch and pine extended into new territory when conditions became warmer near the northern forest line, then retreated as conditions
grew colder, a process that was not necessarily instantaneous. Between
1890 and the 1940s, the North Atlantic Oscillation was in a high mode,
bringing milder weather and a constant flow of depressions across northern Europe. During these warmer years, many European birds extended
their ranges northward, for they are highly sensitive to the depth and duration of snow cover, the length and warmth of summers, and the harshness of winters. Animal distributions can also reflect the availability of their favorite foods. For example, puffins declined sharply around Britain
between 1920 and 1950 because the fish species they ate preferred colder
water. When sea temperatures dropped after the 1950s, the sand eels returned, and northern puffin colonies increased again.

Iceland welcomed all manner of European bird species during the first
half of the twentieth century, among them breeding pairs of blackheaded
gulls, swallows, starlings, and fieldfare. Even more striking is the distribution of the serin, a bird that flourished among the sunny borders of
woodlands throughout the western Mediterranean during the eighteenth
century. By 1876, serins had colonized much of central Europe; they now
breed as far north as the Low Countries, northern France, and Scandinavia. When northern latitudes cooled slightly in the 1960s, species like
the snowy owl moved southward to nest in Shetland, and great northern
divers returned to Scotland from the north.

All these changes, like those of moths and butterflies, are minor compared with the shifts in animal distributions that must have taken place
during the coldest episodes of the Little Ice Age. Were these shifts noticed
at the time? Except for economically important species like the herring
and cod, any such observations were not written down-at least, they are
unknown to modern scholars. But if people noticed a slow change in
their local animals and plants, surely few of them knew of the remarkable
lack of sunspots between 1645 and 1715.

Sunspots are familiar phenomena. Today, the regular cycle of solar activity waxes and wanes about every eleven years. No one has yet fully explained the intricate processes that fashion sunspot cycles, nor their maxima and minima. A typical minimum in the eleven-year cycle is about six
sunspots, with some days, even weeks, passing without sunspot activity.
Monthly readings of zero are very rare. Over the past two centuries, only
the year 1810 has passed without any sunspot activity whatsoever. By any
measure, the lack of sunspot activity during the height of the Little Ice
Age was remarkable.

The seventeenth and early eighteenth centuries were times of great scientific advances and intense astronomical activity. The same astronomers
who observed the sun discovered the first division in Saturn's ring and five
of the planet's satellites. They observed transits of Venus and Mercury,
recorded eclipses of the sun, and determined the velocity of light by observing the precise orbits of Jupiter's satellites. Seventeenth-century scholars published the first detailed studies of the sun and sunspots. In 1711,
English astronomer William Derham commented on "great intervals"
when no sunspots were observed between 1660 and 1684. He remarked
rather charmingly: "Spots could hardly escape the sight of so many Observers of the Sun, as were then perpetually peeping upon him with their
Telescopes ... all the world over."6 Unfortunately for modern scientists,
sunspots were considered clouds on the sun until 1774 and deemed of little importance, so we have no means of knowing how continuously they
were observed.

The period between 1645 and 1715 was remarkable for the rarity of
aurora borealis and aurora australis, which were reported far less frequently than either before or afterward. Between 1645 and 1708, not a
single aurora was observed in London's skies. When one appeared on
March 15, 1716, none other than Astronomer Royal Edmund Halley
wrote a paper about it, for he had never seen one in all his years as a scientist-and he was sixty years old at the time. On the other side of the
world, naked eye sightings of sunspots from China, Korea, and Japan between 28 B.C. and A.D. 1743 provide an average of six sightings per century, presumably coinciding with solar maxima. There are no observations whatsoever between 1639 and 1700, nor were any aurora reported.

In the 1890s, astronomers F. W. G. Sporer and E. W. Maunder drew
attention to this long sunspot-free period in the late seventeenth and
early eighteenth centuries. If seventeenth-century observers were to be
believed, almost all sunspot activity ceased for seventy years, a dramatic
departure from the modern sun's behavior. This lacuna in sunspot activity
has since been known as the "Maunder Minimum."

In later papers, Maunder made some striking assertions. First, very few
sunspots were seen over the seventy years between 1645 and 1715. Second,
for nearly half this time (1672-1704) no sunspots were observed on the northern hemisphere of the sun whatsoever. Only one sunspot group at a
time was seen on the sun between 1645 and 1705. Last, the total number
of sunspots throughout the seventy years was less than the number that occur in a single active year today. Maunder quoted extensively from contemporary observations, among them that of the French astronomer Picard in
1671, who "was pleased at the discovery of a sunspot since it was ten whole
years since he had seen one, no matter how great the care which he had
taken from time to time to watch for them." 7 Maunder himself pointed out
that this apparent anomaly in the sun's history might have had important
consequences for terrestrial weather, perhaps far more important than the
regular eleven-year cycles of solar activity in normal times.

Better catalogs of historical solar aurorae, hitherto unknown sunspot
observations by early Asian scholars and new tree-ring data have all upheld the validity of the Maunder Minimum. Radiocarbon-dated tree
rings are a valuable source of information on fluctuations in solar radiation. Carbon 14 is formed in the atmosphere through the action of cosmic rays, which are in turn affected by solar activity. When the sun is active and sunspot cycles are at their maximum, some of the incoming
galactic rays are prevented from reaching the earth, resulting in less 14C
in the tree rings of the day. When the cycle is low, terrestrial bombardment by cosmic rays increases and 14C levels rise. The dated tree-ring sequences document a well-defined fall in 14C levels and a peak in solar activity between about A.D. 1100 and 1250, the height of Europe's
Medieval Warm Period. Carbon 14 levels rose significantly as solar activity slowed between 1460 and 1550 (the Sporer Minimum), then fell for a
short time before rising again sharply between A.D. 1645 and 1710,
peaking in about 1690, an anomaly so marked that it is named the De
Vries Fluctuation, after the Dutch scientist who first identified it. This
anomaly coincides almost exactly with the Maunder Minimum.

Was there a relationship between solar activity and the Little Ice Age?
There is certainly a nearly perfect coincidence between major fluctuations
in global temperature over the past 1,000 years and the changes in 14C
levels identified in tree rings. This implies that long-term changes in solar
radiation may have had a profound effect on terrestrial climate over
decades, even centuries. Certainly the sun has never been constant, and
over the past 1,000 years it has shone with periods of greater and lesser
activity far more extreme than the levels of today.8 We may never be able to decipher the direct linkages between sun and short-term climatic
change, but there are compelling connections between the prolonged periods of low solar activity and the maxima of the Little Ice Age.