THE POLAR GARDEN - Tambora: The Eruption that Changed the World - page 77

of 600 fathoms. After some hours' experimentation, Scoresby observed that the maximum
temperature of the water through which the marine diver passed lay well beneath the surface.
Also remarkable was the fact that this maximum temperature, 37°F, was the “greatest heat of
the water, which [I] have observed in these regions.” The experiment, concluded Scoresby,
“proves the existence of a current from the southward running beneath, at the same time the
current from the NE to the SW runs upon and near the surface, whereby the whole body of
the polar ice is carried.” 22 What Scoresby had unwittingly identified was, in fact, the main
engine of northern hemispheric climate.
Figure 6.3. Scoresby's “Marine Diver.” (William Scoresby, Account of the Arctic Regions [Edinburgh, 1820];
Courtesy of the Rare Book & Manuscript Library, University of Illinois at Urbana-Champaign.)
Figure 6.4. Thisschematized diagram oftheglobal thermohaline circulation includes adramatic U-turn in
the North Atlantic in the vicinity of the undersea Greenland-Scotland Ridge. In the Greenland Sea west of
Spitsbergen, where William Scoresby sailed in 1816, millions of gallons of warm, salty water flow north-
ward, while the overturning southward flow is less salty and more than 10°F cooler. (Jack Cook; ©Woods
Hole Oceanographic Institution.)
The so-called Atlantic Meridional Overturning Circulation (AMOC) is a submarine current
system that transports tropical warmth to the North Pole via the gulf stream and, in the course
of its many thousand miles' journey, moderates extremes of air temperature at all latitudes.
The AMOC, in turn, belongs to the conveyor belt of thermal deep sea currents that girdle
the globe from pole to pole. As warm waters flow into the North Atlantic, they grow colder
and saltier, and hence more dense. The heavy water sinks, drawing lighter, deep water to
the surface. With his diving machine, Scoresby was able to capture a snapshot image of this
dynamic process of liquid thermal exchange, and a vivid impression of opposing southward