Geoscience Reference
In-Depth Information
which place cooling sulfate aerosols into the stratosphere, where they can reside for some time,
blocking some of the incoming sunlight and cooling the planet for several years. The problem in trying
to explain the observed warming with these external natural factors is that they should have led to
cooling, not warming, in recent decades: Solar output shows no increase over the latter half of the
twentieth century, and two large cooling volcanic eruptions occurred near the end of the twentieth
century: El Chichon in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991.
There is another source of natural variability, however, that could in principle have been
responsible for recent trends: internal variability—oscillatory variations of the climate system that
take place without any particular external cause such as changing solar output. These climate
behavior of the atmosphere. On longer time-scales, the atmosphere doesn't operate in isolation, but
instead interacts with other, more sluggish components of Earth's climate system, such as the oceans
and the ice sheets. These interactions lead to oscillations with a considerably longer timescale than
typical weather fluctuations. The most familiar example is what we know as the El Niño
phenomenon, an oscillation in the climate system that arises from the way atmospheric winds and
ocean temperatures in the tropical Pacific influence each other. While El Niño's origins lie in the
tropical Pacific, the phenomenon triggers, in turn, changes in wind patterns, temperature, rainfall, and
drought around the world. El Niño (and its flip side, La Niña) comes and goes every few years, but
there are other types of natural, internally generated climate oscillations that occur over time-scales
of decades and even centuries.
Work in the 1980s by climate scientists such as Klaus Hasselmann of the Max Planck Institute in
Germany and Tom Wigley of the Climatic Research Unit (CRU) of the University of East Anglia in the
United Kingdom demonstrated that the climate system was capable of generating sizable oscillations
in global temperature on timescales of a century or longer. Such long-term natural variations might
potentially have been large enough to explain the overall warming trend of the past century, which
was just under 1°F at that time.
By the early 1990s, various climate scientists were trying to determine whether there was
specific evidence for natural long-term oscillations in the climate system that could be competing
with—or even masquerading as—apparent anthropogenic (human-caused) climate change. In 1994,
for example, while I was working on my Ph.D., Michael Schlesinger of the University of Illinois and
coauthor Navin Ramankutty published findings suggesting that such oscillations do indeed exist.
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They employed a simple theoretical climate model fed with data on anthropogenic impacts (the
warming effect of greenhouse gas increases partially offset by the cooling impact of industrial sulfate
aerosols) to estimate how the globe should have warmed in the absence of natural variability. Then
they subtracted this estimate from the actual temperature record to see what residual variability,
presumably natural, was left unexplained. They found some evidence in that leftover variability for
the existence of natural temperature oscillations occurring in fifty- to eighty-year cycles. These
oscillations could explain why some regions had warmed more than other regions, and why some
areas of the North Atlantic had actually cooled in the latter half of the twentieth century. The
oscillations, however, could not explain the overall warming of the globe since the nineteenth century.
That warming was accounted for in their analysis primarily by an increase in greenhouse gas
concentrations from fossil fuel burning—anthropogenic effects. Meanwhile, in my own work I had
been employing both climate modeling and data analysis methods in an attempt to understand and
