Global Warming and the Age of the Earth: A Lesson on the Nature of Scientific Knowledge

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The world stands
on the verge of committing itself to limits on the emission of carbon
dioxide that would drastically reduce the use of fossil fuels. If
this fateful decision is made, the economies of developed nations
will be strangled. Human prosperity will be reduced. Our ability
to solve pressing problems, both human and environmental, will be
severely limited. We have been told that these shackles must be
imposed to forestall a hypothetical global warming projected to
occur some time in the distant future. But to date the only unambiguous
evidence for planetary warming is a modest rise in temperature (less
than one degree Celsius) that falls well within the range of natural
variation.

The validity
of warming predictions depends upon the questionable reliability
of computer models of the climate system. But Earth’s climate system
is complex and poorly understood. And the integrity of the computer
models cannot be demonstrated or even tested. To anyone with an
awareness of the nature and limitations of scientific knowledge,
it must appear that the human race is repeating a foolish mistake
from the past. We have been down this road before, most notably
in the latter half of the nineteenth century when it appeared that
mathematics and physics had conclusively answered the question of
the Earth’s age. At that time, a science that had been definitely
“settled” fell apart in the space of a few years. The mathematical
models that appeared to be so certain proved to be completely, even
ridiculously wrong.

The age of
the Earth is one of the great questions that have puzzled people
for thousands of years. In Meteorologica,
Aristotle (384–322 BC) asserted that the world was eternal. But
with the advent of Christianity and Islam, scholars began to assume
that humanity was coeval with the Creation of the world. It followed
that the age of the Earth could be estimated from a careful examination
of sacred writings.

The first person
to make a quantitative estimate of the Earth’s age was the Islamic
scientist al-Biruni (c. 973–1050). al-Biruni based his chronology
on the Hindu, Jewish, and Christian religious scriptures. He divided
the history of the world into eras, and concluded that it had been
less than ten thousand years since the Creation.

Working in
the tradition begun by al-Biruni, Bishop James Ussher (1581–1686)
estimated the age of the Earth by meticulously studying the Bible
and other historical documents. In The Annals of the World Deduced
from the Origin of Time, Ussher pinpointed the date of Creation
as the “night preceding the 23rd of October, 4004 BC.” Ussher’s
scholarship was impressive, and his dates were accepted as the standard
chronology. Bible editors began to place Ussher’s dates in the margins
of their texts.

Isaac Newton
(1642–1727), the greatest scientist of the age, was also a Biblical
fundamentalist who believed in a young Earth. Newton explained to
his nephew, John Conduitt, that the Earth could not be old because
all human technology was of recent invention. Like Ussher, Newton
wrote his own universal history, Chronology
of Ancient Kingdoms Amended
, that was published posthumously
in 1728.

The procedures
for establishing a scientific estimate of the age of the Earth were
laid out in the seventeenth century by the Danish anatomist, Nicolaus
Steno (1638–1686). Steno was the first person to state unequivocally
that the history of the Earth was not to be found in human chronicles,
but in the Earth itself. Steno’s principles of geologic investigation
became the basis for establishing the relative age of rock sequences
and the foundation of historical geology.

Armed with
Steno’s principles, eighteenth century naturalists began to seriously
consider the implications of the rock record. It became apparent
to them that an immense amount of time was required to deposit the
rock layers that covered the Earth’s surface.

One of the
first to recognize the scope of geologic time was the Scottish philosopher
James Hutton (1726–1797). In the year 1788, Hutton was accompanied
on a field trip by his friend, the mathematician, John Playfair
(1748–1819). They traveled up the coastline of Scotland to
Siccar Point, and Hutton described the history implied by the sequence
of rocks exposed there. After listening to Hutton’s exposition,
Playfair later wrote “the mind seemed to grow giddy by looking so
far into the abyss of time.”

By the time
Charles Darwin (1809–1882) published Origin
of Species

in 1859, geologists were of the opinion that the Earth was practically,
although not literally, of infinite age. With infinite time at this
disposal, Darwin was able to invoke the slow mechanism of natural
selection as an explanation for the organic evolution evidenced
in the fossil record.

To demonstrate
the vast extent of geologic time, Darwin offered the erosion of
the Weald, a seaside cliff in England, as an offhand example. Darwin
assumed an erosion rate of an inch a century, and then extrapolated
that some 300 million years were apparently necessary to explain
the total amount of erosion that had occurred.

But Darwin’s
estimated erosion rate of one inch per century was little more than
speculation. The number was unconstrained by any measurement or
scientific observation. Nineteenth-century geologists lacked any
quantitative method for establishing dates. The rocks of the Earth’s
crust might represent the passage of ten million years. But just
as easily, the amount of time could have been a hundred, a thousand,
or ten thousand million years.

Darwin and
his geological colleagues were soon taken to the woodshed by the
greatest physicist of the nineteenth century, William Thomson (1824–1907).
Better known as Lord Kelvin, Thomson was a man of prodigious gifts
who possessed enormous intellectual stature. He published his first
scientific paper at age sixteen, and had been appointed a chaired
professor at the University of Glasgow at the precocious age of
twenty-two.

In 1861, Lord
Kelvin began to seriously address the question of dating the Earth.
He was aware that the Earth radiated internal heat. This process
could not have been going on forever. By maintaining that the Earth
was infinitely old, the geologists in effect were postulating that
energy was not conserved. This violated the First Law of Thermodynamics,
and Kelvin was aroused to do battle.

In the nineteenth
century, the only known source for the internal heat of the Earth
was the original mechanical heat of accretion. Reasoning that the
Earth had been molten at the time of its formation, but cooling
ever since, Kelvin was able to construct an elegant mathematical
model that constrained the age of the Earth on the basis of its
measured geothermal gradient. Much the same method is used today
by coroners who estimate the time of death by taking the temperature
of a cadaver.

In 1862, Kelvin
published his analysis in a paper titled On
the Secular Cooling of the Earth
. He arrived at a best estimate
for the age of the Earth of 100 million years. Kelvin’s estimate
was no idle speculation. It was based on a precise mathematical
model constrained by laboratory measurements and the laws of thermodynamics.

Kelvin attacked
Darwin directly. He raised the question: were the laboratory measurements
and mathematical calculations in error, or was it more likely “that
a stormy sea, with possibly channel tides of extreme violence, should
encroach on a chalk cliff 1,000 times more rapidly than Mr. Darwin’s
estimate of one inch per century?”

Darwin was
devastated. He wrote to his mentor, Charles Lyell, “for heaven’s
sake take care of your fingers; to burn them severely, as I have
done, is very unpleasant.” Geologists were left sputtering. They
had no effective rebuttal to Kelvin’s calculations. Within a few
years, the geological establishment began to line up with Lord Kelvin.
Among the influential converts was Archibald Geikie, President of
both the British Association for the Advancement of Science and
the Geological Society of London.

Researchers
began to look for evidence that would confirm Kelvin’s calculations.
In 1865, Geologist Samuel Haughton had estimated the age of the
Earth as 2300 million years, a number reasonably close to the modern
value of 4500 million years. But under the influence of Kelvin’s
authority, in 1878 Haughton drastically shortened his earlier calculation
to 153 million years.

A lone voice
of dissent was raised by the biologist, Thomas Huxley (1825–1895).
Huxley pointed out that there was a fundamental weakness in Kelvin’s
mathematical model. “Mathematics may be compared to a mill of exquisite
workmanship, which grinds you stuff of any degree of fineness; but,
nevertheless, what you get out depends on what you put in.” Put
in more modern terms, Huxley’s observation amounted to “garbage
in, garbage out.”

But as the
end of the nineteenth century approached, the scientific community
was beginning to regard Kelvin’s estimate of 100 million years as
a near certainty. Writing in the American Journal of Science
in 1893, geologist Warren Upham characterized Kelvin’s estimate
of the age of the Earth as the most “important conclusion in the
natural sciences…[that] has been reached during this century.”

The science
was definitely settled in 1899 by the Irish physicist, John Joly
(1857–1933). Joly hit upon a robust method for calculating the age
of the Earth that was entirely different from Kelvin’s. Joly’s calculation
was childishly simple, yet apparently foolproof. He estimated the
age of the Earth by dividing the total salt content of the oceans
by the rate at which salt was being carried to the sea by the rivers.
He found that it would take 80 to 90 million years for the ocean’s
salt to accumulate.

In consideration
of the uncertainties involved, Joly’s age estimate was essentially
identical to Thomson’s. With different methods yielding the same
result, it seemed evident that the result was conclusive: the Earth
was 100 million years old. It seemed that to deny this reality,
was to deny not only the authority of the scientific establishment
but the very laws of nature themselves.

The ingenious
calculations of Kelvin and Joly were soon to be overturned by an
improbable empiricism. In the thirteenth century, modern science
began when philosophers came to the realization that logic alone
could never uncover the secrets of the cosmos, no matter how seductive
its appeal. Contemplation of the mysterious properties of the magnet
convinced Roger Bacon and his contemporaries that nature contained
occult or hidden forces that could never be discerned or anticipated
rationally, only discovered experimentally.

In 1896, Henri
Becquerel accidentally discovered radioactivity when he found that
photographic plates were exposed when placed next to certain minerals.
By 1904, it became apparent that there were radioactive minerals
inside the Earth releasing heat. Lord Kelvin’s assumption of no
internal heat sources was wrong. At the beginning of the twentieth
century, it was not even clear if the Earth was cooling or heating.
Thomson’s calculations were precise, but he had no way of knowing
about radioactivity.

Radioactivity
also provided a rigorous way to calculate the age of the Earth.
The accepted modern estimate for the age of the Earth is 4500 million
years. The nineteenth-century estimate of 100 million years that
seemed so certain was wrong, not just by 20 or 30 percent, but by
a factor of 45. In retrospect, the reason that Thomson’s estimates
had been independently confirmed is that geologists looked for data
that would support Thomson’s physics. The consensus that had emerged
was the product of a human psychological process, not objective
science. The nature of science is such that people who look for
confirming evidence will always find it.

Compared to
modern climate models, William Thomson’s models were simple, and
contained only a few assumptions. In contrast, global warming models
are hideously complex, and contain numerous hidden assumptions,
many of which are highly uncertain. The most significant of these
is whether water vapor will exert a negative or positive feedback
on the warming induced by carbon dioxide. All the major climate
models assume the feedback will be positive, exaggerating any possible
warming. But recent research indicates the feedback may be negative.
We don’t know.

There is also
much we do not understand about why Earth’s climate changes. It
is possible that cosmic rays, modulated by the Sun’s magnetic field,
cool Earth by inducing the formation of clouds. We don’t know why
Ice Ages end so spectacularly and suddenly. Once they begin, Ice
Ages should continue indefinitely, as cooling is reinforced by a
number of positive feedbacks.

We ought to
be intelligent enough to acknowledge that we don’t know what we
don’t know. Science is never settled. We should keep in mind
Seneca’s admonition. “Nature does not reveal all her secrets at
once. We imagine we are initiated in her mysteries: we are, as yet,
but hanging around her outer courts.”

There has never
been a time when the need for understanding the limits and nature
of scientific knowledge is so compelling, or the ramifications of
ignorance so consequential. Those who ignore history are apt to
repeat its mistakes.

December
3, 2009

David
Deming [send him mail] is a
geologist and associate professor of arts and sciences at the University
of Oklahoma. His books on the history of science, Origin of Science
I and II, are scheduled to be published by McFarland in 2010.

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