BLUE DEATH IN BENGAL - Tambora: The Eruption that Changed the World - page 42

upon an identical course, breasting the trade winds from the resource-rich East Indies to com-
mercial ports in the Indian Ocean. The vanguard of Tambora's stratospheric plume arrived
over the Bay of Bengal within days. In late April in Madras, on the southeast coast of India,
the morning temperatures plummeted over the course of a single week from 50° to 26°F, a
harbinger of Tambora's profoundly disruptive effects on the weather systems of India. 9
A 1999 study of ash deposits in the Arabian Sea of the coast of modern-day Pakistan
indicates a heavy aerosol load in the subcontinental atmosphere in the aftermath of Tam-
bora's eruption. 10 By the time of its arrival in the Indian subcontinent, however, the Tambora
cloud had already lost the greater part of its initial volume. Of the vast mass of rock and ash
blasted into the upper atmosphere on April 10 and 11, most particles were too large to re-
main airborne, cascading gently into the tropospheric realm of clouds and weather—there to
be washed out of the sky by rain. This great husk of matter shed, Tambora's sleek, residual
cloud—made up of pulverized mineral matter, gases, and sulfate aerosol particles less than a
micron in thickness—would hang at altitude for more than two years. 11
Tambora's impacts on the Indian monsoon were not immediate. The sheer presence of a
large volcanic cloud in the stratosphere is less important than the sequence of chemical re-
actions it sets in train. Released from their eon's residence in the earth, Tambora's sulfate
gases embraced the freedom of the oxygen-rich sky by forming new molecular combinations:
first sulfur dioxide then, with increasing oxidization and interaction with water vapor, tiny
droplets of sulfuric acid. As the months passed, the volcanic aerosols, further plumped by wa-
ter vapor drawn to their acidic content, reached a crucial tipping point: around the end of
1815, Tambora's aerosols attained a density sufficient to interact with both the sun's rays and
the radiative heat from the earth, reflecting incoming solar energy back to space (the albedo
effect), while at the same time intercepting longwave radiation from the surface.
The sum of these effects was a hotter stratosphere but a net cooling of surface temperat-
ures, initiating a three-year depression of the thermal cycle of the South Asian continent, and
ultimately the globe. This depression of summer minima and maxima—at the height of the
growing season—proved devastating to farmers in the temperate zones of the North Atlantic
in 1816 and 1817. But in the tropical latitudes of South and East Asia, raw surface temper-
ature decline was less important than the impact of a disrupted thermal synchrony between
land, sea, and sky on the life-giving monsoon.
The ecology of the Bengali river delta is inseparable from its monsoonal climate. From
their first encounter with the South Asian continent, European travelers identified the mon-
soon as its defining cultural and economic driver. Dry through much of the year, the land
would be uninhabitable but for the awesome three-month deluge that replenished aquifers
and generated crops. In addition, monsoonal winds brought trade. For centuries before the
British arrival in the 1700s, Arab merchants—and later the Portuguese and Dutch—had sailed
the monsoonal courses to India and Africa, where they bought spices and slaves to sell on the
Mediterranean market.
In the dry, cooler months from November to March, the prevailing winds in India come
from the north. Then in May, the geophysical mechanisms of the Indian wet monsoon begin
to stir. 12 As the Earth's summer tilt draws the sun's maximum heat to the latitudes just north
of the equator, the land responds more quickly to the increased solar input than does the vast
thermal sink of the Indian Ocean. The temperature gradient between land and ocean steep-
ens, awakening the cloud-bearing monsoonal winds from the south, which rush en masse into