THE POLAR GARDEN - Tambora: The Eruption that Changed the World - page 78

and northward currents. Becalmed of the Greenland coast west of Spitsbergen (Svalbard),
he was well situated to observe the AMOC in the deep polar basins of the Norwegian and
Greenland seas where its overturning motion is in fact triggered. While the sinking northward
current brought heat to the Arctic seas, melting the ice pack, the cold surface drift escorted
the broken ice south into the Atlantic Ocean, wreaking havoc on transatlantic shipping lanes
in 1816 and 1817. Because of his vast experience in northern waters, Scoresby was thus able
not only to observe the dynamics of an overturning oceanic current funneling warm water
north into the polar sea but to make note of its extreme behavior in that season.
With the redoubtable Scoresby's marine diver experiment, we inch closer to the solution
to why, in the summers of 1816 and 1817—while the inhabitants of temperate zones from
China to New England shivered and starved—the Arctic Circle basked in relative warmth and
shed its ice at amazing, unprecedented rates. Crucial to understanding the relation between
Tambora's eruption in 1815 and the reports of massive polar ice loss in the summers follow-
ing is Scoresby's observation that, in 1817, the Arctic water was at its “greatest heat” in his
experience. A survey of ships' logs in the period suggests environmental strains on the Arctic
even beyond Scoresby's reckoning. Godthaab in southern Greenland experienced temperat-
ures 5.5°C above average during the entire volcanic decade of 1810-19. 23 The record air tem-
peratures in Greenland, as well as Scoresby's observation of unusually heated water, suggest
that in the aftermath of Tambora, the AMOC was operating with increased intensity.
Answers to why this should be so may be found in studies of the 1991 eruption of Mount
Pinatubo in the Philippines. The Pinatubo eruption has served scientists well as a model from
which environmental impacts of the nearby Tambora event, unobserved by modern scientific
instruments, might be extrapolated. A notable consequence of Pinatubo's eruption, and the
global cooling it produced, was the “substantial decrease” in rainfall overland for a year fol-
lowing the eruption and a subsequent “record decrease” in runoff to the oceans. The cause
was the chilled, volcanic atmosphere, which repressed evaporation and reduced the amount
of water vapor in the air. Put in its broadest terms, reduced solar radiation in Pinatubo's after-
math altered the flow of energy through the coupled ocean-atmosphere system, with signific-
ant implications for the global hydrological cycle. Accordingly, the first post-Pinatubo year,
1992, witnessed thelargest recorded percentage oftheglobal landmass suffering drought con-
ditions. A recent computer simulation of the influence of volcanic activity on global climate
since 1600 produced the same “general precipitation decrease” in the high latitudes of the
northern hemisphere, especially pronounced over land. 24
Figure 6.5. A model incorporating historical streamflow records of the world's largest 925 rivers shows
the dramatic decrease in freshwater runoff to the oceans following Pinatubo's eruption in June 1991.
(Kevin Trenberth and Aiguo Dai, “Effects of the Mount Pinatubo Volcanic Eruption on the Hydrological
Cycle as an Analog of Geoengineering,” Geophysical Research Letters 34 [2007]: L15702; © American Geo-
physical Union.)