Climate Science Comes of Age - The Hockey Stick and the Climate Wars - page 11

among these components. Of course, no theoretical model is ever perfect; even the best model is only
an idealization of the actual world. There are always real-world processes that cannot be captured—
for example, in the case of a numerical climate model, individual clouds or small-scale air currents
like dust devils—that are simply too small for the model to resolve. The key question is, can the
model be shown to be useful? Can it make successful predictions?
Climate models had passed that test with flying colors by the mid1990s. James Hansen, in the
late 1980s, successfully predicted the continuing warming that would be observed by the mid-1990s. 1
Even something the model couldn't have predicted in advance—the 1991 eruption of Mount Pinatubo
in the Philippines—provided yet another key test. As soon as the eruption occurred, Hansen put what
was known about the reflective qualities of volcanic sulfur particulates (known as “sulfate aerosols”)
into the simulations. The aerosols cooled surface temperatures for several years in the model by
shielding the surface from a fraction of incoming sunlight, leading Hansen to make what turned out to
be a successful prediction of the temporary cooling that was seen over the ensuing few years. 2
(5) Finally, perhaps most significant of all, only when human factors were included could the
models reproduce the observed warming—both its overall magnitude and, equally important, its
geographical pattern over Earth's surface and its vertical pattern in the atmosphere. The primary such
human factor was increasing greenhouse gas concentrations due to fossil fuel burning and other human
activities. A secondary human factor, sulfate aerosols emitted from industrial smokestacks, also
played a role, however. Like volcanic sulfate aerosols, these industrial aerosols have a cooling
effect. Unlike volcanic aerosols, which reach the lower stratosphere, allowing them to spread out into
a layer covering the globe, industrial aerosols remain confined to the lower atmosphere, leading to
localized patterns of cooling that offset global warming in some regions. The pattern of warming
predicted by the models from the combination of these two human effects on the climate provide a
unique “fingerprint” of what the human influence on climate should look like if the models were
correct—and the fingerprint matched. The surface and lower atmosphere showed an irregular pattern
of warming, while the atmosphere aloft was cooling, just as the models indicated it should. 3 The
fingerprint predicted for natural factors alone—for example, from fluctuations in solar output—on the
other hand, failed to match the observations. It was the work of Ben Santer and other climate
scientists during the mid-1990s in establishing this fingerprint of human influence that provided the
“smoking gun,” the fifth and final link in the chain of reasoning that allowed the IPCC to declare in
1995 that there was indeed at least a “discernible” human influence on climate.