“THIS END OF THE WORLD WEATHER” - Tambora: The Eruption that Changed the World - page 23

Figure 3.3. This diagram shows the penetration of loftier volcanic matter into the stratosphere. There,
as volcanic sulfur dioxide is chemically transformed into sulfuric acid, an aerosol layer forms, reducing
incoming radiation from the sun and cooling the surface, even as the stratosphere itself is warmed. (Ad-
apted from M. Patrick McCormick et al., “Atmospheric Effects of the Mt. Pinatubo Eruption,” Nature 373
[February 2, 1995]: 400; © Macmillan Publishers Ltd.)
The relation between volcanism and climate depends on eruptive scale. Volcanic ejecta
and gases must penetrate skyward high enough to reach the stratosphere where, in its
cold lower reaches, sulfate aerosols form. These then enter the meridional currents of the
global climate system, disrupting normal patterns of temperature and precipitation across
the hemispheres. Tambora's April 1815 eruption launched enormous volumes of long-sup-
pressed volcanic rock and gases more than 40 kilometers into the stratosphere. This volcan-
ic plume—consisting of as much as 50 cubic kilometers of total matter—eventually spread
across one million square kilometers of the Earth's atmosphere, an aerosol umbrella six times
the size of Mount Pinatubo's 1991 cloud.
In the first weeks after Tambora's eruption, a vast volume of coarser ash
particles—volcanic “dust”—cascaded back to Earth mixed with rain. But ejecta of smaller
size—water vapor, molecules of sulfur and fluorine gases, and fine ash particles—remained
suspended in the stratosphere, where a sequence of chemical reactions resulted in the form-
ation of a 100-megaton sulfate aerosol layer. Over the following months, this dynamic,
streamer-like cloud of aerosols—much smaller in size than the original volcanic mat-
ter—expanded by degrees to form a molecular screen of planetary scale, spread aloft the
winds and meridional currents of the world. In the course of an eighteen-month-long journey,
it passed across both south and north poles, leaving a telltale sulfate imprint on the ice for
paleoclimatologists to discover more than a century and a half later.
Once settled in the dry firmament of the stratosphere, Tambora's global veil circulated
above the weather dynamics of the atmosphere, comfortably distanced from the rain clouds
that might have dispersed it. From there, its planet-girdling aerosol film continued to scatter
shortwave solar radiation back into space until early 1818, while allowing much of the long-
wave radiant heat from the earth to escape. The resultant three-year cooling regime, unevenly
distributed by the currents of the world's major weather systems, barely affected some places
on the globe (Russia, for instance, and the trans-Appalachian United States) but precipitated
a truly drastic 5-6°F seasonal decline in other regions, including Europe.
The first extreme impact of a major tropical eruption is felt in raw temperature. But in
western Europe, biblical-style inundation during the 1816 summer growing season wrought