CHAPTER 2 | THE SKY 27
years, and the results were a surprise. Th e evidence seemed to
show that Earth had begun warming up thousands of years too
early for the last ice age to have been caused by the Milankovitch
cycles.
Th ese contradictory fi ndings are irritating because we
humans naturally prefer certainty, but such circumstances are
common in science. Th e disagreement between the ocean fl oor
samples and the Devil’s Hole samples triggered a scramble to
understand the problem. Were the age determinations of one or
the other set of samples wrong? Were the calculations of ancient
temperatures wrong? Or were scientists misunderstanding the
signifi cance of the evidence?
In 1997, a new study of the ages of the samples confi rmed
that those from the ocean fl oor are correctly dated. But the same
study found that the ages of the Devil’s Hole samples are also
correct. Evidently the temperatures at Devil’s Hole record local
climate changes in the region that became the southwestern
United States. Th e ocean fl oor samples record global climate
changes, and they fi t well with the Milankovitch hypothesis. Th is
has given scientists renewed confi dence in the Milankovitch
hypothesis, and although it is widely accepted today, it is still
being tested whenever scientists can fi nd more evidence.
As you review this section, notice that it is a scientifi c argu-
ment, a careful presentation of theory and evidence in a logical
discussion. How Do We Know? 2-4 expands on the waysThe Evidence
By the middle 1970s, Earth scientists were able to collect the
data that Milankovitch had lacked. Oceanographers drilled deep
into the seafl oor to collect long cores of sediment. In the labora-
tory, geologists could take samples from diff erent depths in the
cores and determine the age of the samples and the temperature
of the oceans when they were deposited on the seafl oor. From
this, scientists constructed a history of ocean temperatures that
convincingly matched the predictions of the Milankovitch
hypothesis (Figure 2-11b).
Th e evidence seemed very strong, and by the 1980s the
Milankovitch hypothesis was widely considered the leading
hypothesis. But science follows a mostly unstated set of rules that
holds that a hypothesis must be tested over and over against all
available evidence (How Do We Know? 2-3). In 1988,
scientists discovered contradictory evidence.
For 500,000 years rainwater has collected in a deep crack in
Nevada called Devil’s Hole. Th at water has deposited the mineral
calcite in layer on layer on the walls of the crack. It isn’t easy to
get to, and scientists had to dive with scuba gear to drill out
samples of the calcite, but it was worth the eff ort. Back in the
laboratory, they could determine the age of each layer in their
core samples and the temperature of the rainwater that had
formed the calcite in each layer. Th at gave them a history of
temperatures at Devil’s Hole that spanned many thousands ofThe So-Called Scientifi c Method
Why is evidence critical in science? From
colliding galaxies to the inner workings
of atoms, scientists love to speculate and
devise theories, but all scientifi c knowledge
is ultimately based on evidence from observa-
tions and experiments. Evidence is reality, and
scientists constantly check their ideas against
reality.
When you think of evidence, you prob-
ably think of criminal investigations in which
detectives collect fi ngerprints and eyewitness
accounts. In court, that evidence is used to
try to understand the crime, but there is a
key difference in how lawyers and scientists
use evidence. A defense attorney can call a
witness and intentionally fail to ask a question
that would reveal evidence harmful to the
defendant. In contrast, the scientist must be
objective and not ignore any known evidence.The attorney is presenting only one side of
the case, but the scientist is searching for the
truth. In a sense, the scientist must deal with
the evidence as both the prosecution and the
defense.
It is a characteristic of scientifi c knowledge
that it is supported by evidence. A scientifi c
statement is more than an opinion or a specu-
lation because it has been tested objectively
against reality.
As you read about any science, look for
the evidence in the form of observations
and experiments. Every theory or conclusion
should have supporting evidence. If you can
fi nd and understand the evidence, the science
will make sense. All scientists, from astrono-
mers to zoologists, demand evidence. You
should, too.Fingerprints are evidence to past events.
(Dorling Kindersly/Getty Images)