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.
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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al-Biruni, Bishop James Ussher (1581–1686) estimated the age of the Earth by meticulously studying the Bible and other historical documents.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 (17261797). In the year 1788, Hutton was accompanied on a field trip by his friend, the mathematician, John Playfair (17481819). 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.”
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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 (18251895). 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