Geoscience Reference
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at the Geophysical Fluid Dynamics Laboratory at Princeton, had used
numerical models to confirm the long-standing theory that increases in
atmospheric CO
2
would lead to increases the global mean temperature.
4
By
the late 1960s, Charles David Keeling had collected a decade of annually
oscillating CO
2
data that showed— as he, Roger Revelle, and Hans Suess
had predicted— that CO
2
had risen significantly since 1958. According to
the Manabe-Weatherald model, if the increase continued to the point of
doubling, global temperature could rise approximately 2.3ºC by the turn
of the century, all other things being equal.
But all other things were not equal. Meteorologists' day-to-day tem-
perature measurements revealed that although the early twentieth century
had experienced a modest overall warming, in the years between 1940 and
1970 the earth had actually cooled. The twentieth century, climatologists
recognized, had seen remarkably stable climatic conditions that starkly
contrasted with the variability of the geological past. Some feared that the
earth's string of interglacial good luck was finally coming to a close, possi-
bly with help from its human inhabitants. At a conference of the American
Association for the Advancement of Science in 1968, Reid Bryson of the
University of Wisconsin explained that this seemingly anomalous cooling
could be attributed to particles of smoke and dust in the atmosphere, com-
ing not just from volcanoes— which many climatologists agreed could be
agents of climatic change— but also from human activities, most notably
the slash-and-burn agricultural practices of less developed countries in
the tropics. Cyclical variances in solar radiation and increases in volcanic
activity seemed to many to be the prime movers of this overall cooling,
but Bryson and others warned that deforestation and other changes in
ground cover were modifying the amount of sunlight that reached the
earth's surface, increasing the albedo (reflectivity) of large portions of the
earth's surface and thus inadvertently lending nature an insidious hand
in the cooling process.
5
Other studies supported this view. In 1971, Ishtiaque Rasool and Ste-
phen Schneider of the National Center for Atmospheric Research put spe-
cific forms of pollution into the equation, with surprising results. Rasool
and Schneider (the same Schneider associated with the
Study of Man's
Impact on Climate
) incorporated aerosols from industry— sulfur dioxide
(SO
2
) the most important among them— into a simple atmospheric model,
and they, like Bryson, found that the cooling effect of the aerosols more
