Geoscience Reference
In-Depth Information
Chapter 3
Signals in the Noise
One man's signal is another man's noise.
—Origin unknown
When I first began to work with my Ph.D. adviser Barry Saltzman in the early 1990s, he, like many
other climate researchers at the time, remained unconvinced that there was yet a detectable human
influence on the climate. You might say that Barry was skeptical. Scientists should in fact strive to be
skeptics—in the truest sense of the word. That is to say, they should always apply healthy scrutiny to
any new claim or finding. True skepticism, however, demands that one subject all sides of a scientific
contention or dispute to equal scrutiny and weigh the totality of evidence without prejudice. That
should not be conflated with contrarianism or denialism, which is a kind of one-sided skepticism that
entails simply rejecting evidence that challenges one's preconceptions. Unfortunately, the term skeptic
has at times been co-opted by those who are not skeptics at all, but are instead contrarians or deniers,
predisposed to the indiscriminate rejection of evidence supporting a human influence on the climate.
In the early 1990s, after carefully weighing all the evidence, scientists could honestly disagree
with each other over whether there was a detectable human influence on the climate. They could
legitimately be skeptical about whether the human climate change signal had yet emerged. The
evidence was not as extensive as it would soon become, and the theoretical models that scientists
then employed to study Earth's climate system were still rather primitive. For these reasons, scientists
like my adviser were holding out for more evidence, while other scientists, such as NASA's James
Hansen and Stanford University's Stephen Schneider, were convinced by the evidence already in
hand that human-caused climate change was indeed now upon us. I myself was closer to Barry's
position than to Hansen's or Schneider's. In particular, I felt that natural climate variability might be
more important than some scientists thought. Indeed, it was that very assumption that motivated my
Ph.D. research topic.
Emerging from the Noise?
Natural climate variability, in the view of many climate scientists at the time, was still a plausible
competing mechanism for explaining observed climate trends. There were, in fact, two fundamentally
different types of natural climate variability that could potentially explain observed trends. One is
external to the climate system, relating to changes in the factors that govern Earth's climate. On
timescales relevant to modern global warming, the key natural external factors are the small but
measurable changes over time in the output of the Sun (a fraction of a percent, but large enough to
have a detectable effect on surface temperatures) and the effect of explosive volcanic eruptions,
 
 
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