The Little Ice Age: How Climate Made History 1300-1850 by Brian Fagan Page B

spectacular eruption
of the past 15,000 years. Over the next few years, Tambora's ash drifted
through the atmosphere and dimmed the sun. The year 1816 appears as a
sharp cold spike in the climatologists' temperature diagrams, the "year
without a summer" when snow fell in New England in June and Europe
shivered through a frigid September. Major volcanic eruptions almost invariably brought colder summers and bad harvests, natural phenomena
unconnected with the endless perturbations of the Little Ice Age. During
the seventeenth century, an unusual frequency of volcanic events contributed to the volatility of climate change.

    What caused the Little Ice Age? Did small changes in the earth's axis affect
global temperatures for five centuries? Or did cyclical fluctuations in solar
radiation lead to greater cooling? The answer still eludes us, largely because
we have barely begun to understand the global climatic system and the interactions between atmosphere and ocean that drive it. There are few certainties. One is that we still live in the Great Ice Age, somewhere near the
midpoint of an interglacial, one of the many that have developed over the
past three-quarters of a million years. In the fullness of time-according to
some estimates, in the next 23,000 years-the world will most likely return to another glacial cycle, with temperatures as extreme as those of
18,000 years ago, when much of Europe really was in a deep freeze.
    Slow, cyclical changes in the eccentricity of the earth's orbit and in the
tilt and orientation of its spin axis have constantly changed patterns of
evaporation and rainfall and the intensity of the passing seasons over the
past 730,000 years. As a result, the world has shifted constantly between
extreme cold and short warmer periods. The geochemist Wallace
Broecker believes these changes caused the entire ocean-atmosphere system to flip suddenly from one mode during glacial episodes to an entirely
different one during warmer periods. He argues that each flip of the
"switch" changed ocean circulation profoundly, so that heat was carried
around the world differently. In other words, Ice Age climatic patterns
were very different from those of the past 10,000 years.

    The Great Ocean Conveyor Belt, which circulates saltwater deep below the
surface of the world's oceans. Salt downwelling in the North Atlantic Ocean
plays a vital part in this circulation.
    If Broecker is correct, then today's climatic mode results from what he
calls the "Great Ocean Conveyor Belt." % Giant, conveyor-like cells circulate
water through the world's oceans. In the Atlantic, warm, upper-level water
flows northward until it reaches the vicinity of Greenland. Cooled by Arctic
air, the surface waters sink and form a current that covers enormous distances at great depths, to the South Atlantic and Antarctica, and from there
into the Pacific and Indian Oceans. A southward movement of surface waters in these oceans counters the northward flow of cold bottom water. In
the Atlantic the northward counterflow is sucked along by the faster southward conveyor belt, which is fed by salt-dense water downwelling from the
surface in northern seas. The Atlantic conveyor circulation has power
equivalent to one hundred Amazon Rivers. Vast amounts of heat flow
northward and rise into the Arctic air masses over the North Atlantic. This
heat transfer accounts for Europe's relatively warm oceanic climate, which
has persisted, with vicissitudes, through ten millennia of the Holocene.
    We understand the Great Ocean Conveyor Belt in only the most general terms, but enough to know that circulation changes in the upper ocean have a profound effect on global climatic events like El Niflos. We
know, also, that the chaotic equations of the atmosphere and ocean exercise a powerful influence on the swirling atmospheric streams, surface
downwelling, and shifting currents of the North Atlantic. Broecker and
others have recently