Tyndall's climate message, 150 years on
- 28 September 2011
- From the section Science & Environment
Much less trumpeted, but in its own way more significant, is one that dates back to the middle of the 19th Century, which is being marked this week by a special conference in Dublin.
It was 150 years ago that John Tyndall, one of history's truly great physicists, published a scientific paper with the far-from-snappy title On the Absorption and Radiation of Heat by Gases and Vapours, and on the Physical Connexion of Radiation, Absorption, and Conduction.
Not a title to excite the senses at first sight, perhaps; but nowadays, the basis for a vitally important branch of science and a particularly noxious brand of political discourse.
What Tyndall had demonstrated for the first time was that gases in the atmosphere absorb heat to very different degrees; he had discovered the molecular basis of the greenhouse effect.
Its existence had been surmised by earlier generations of scientists, notably Joseph Fourier, who wrote in 1824: "The temperature [of the Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in re-passing into the air when converted into non-luminous heat."
What Fourier could not do, but Tyndall could, was design and construct apparatus capable of demonstrating and measuring the effect.
Born south of Dublin in County Carlow in 1820, Tyndall moved first to England and then to Germany, which he saw as the leader in experimental sciences at the time.
Tutors included Robert Bunsen, one of the few scientists whose invention every school kid knows by name.
In the late 1850s, Tyndall decided to investigate heat absorption by various gases.
He conceived an experimental set-up that would send infrared radiation through a tube of gas and into a detector, a thermopile, which would translate temperature differences into electrical current.
Reading the paper - posted online by organisers of this week's Tyndall Conference - is to gain an insight into the sheer cleverness and persistence required to be an experimental physicist at the time.
The first practical obstacle was that the galvanometer, the instrument that would measure currents generated in the thermopile, performed very erratically.
Of all the possible factors you might think of, Tyndall eventually traced the disruption back to the colour of the silk thread covering the wires.
Green silk perturbed the machine, presumably because it had been coloured with a copper compound; exchanged for white, the galvanometer worked fine.
The tube containing the gases had to have transparent ends.
Glass wouldn't do, because it blocks infrared. So samples of rock salt were obtained, and after many trials and many errors it was kept gas-tight with washers made of "vulcanized india-rubber very lightly smeared with a mixture of bees-wax and spermaceti".
He also needed an infrared source that would remain at a stable temperature, which was far from easy to find.
"The course of the inquiry during this whole period was an incessant struggle with experimental difficulties," he wrote.
Eventually, Tyndall had an apparatus he was happy with, and he began to investigate the heat-trapping properties of various gases.
Oxygen, nitrogen, and hydrogen were all tried and found to have no impact.
But when a trace of ozone went in with the oxygen, Tyndall saw a significant absorption.
That was dwarfed, though, by results on "olefiant gas" - or ethene, as it's rather less poetically known today.
"More than seven-ninths of the total heat was cut off by the olefiant gas, or about 81%," he recorded.
The incredulity with which Tyndall noted these readings is perfectly transmitted in his later comments.
"Those who, like myself, have been taught to regard transparent gases as almost perfectly diathermanous (transparent to heat), will probably share the astonishment with which I witnessed the foregoing effects," he wrote.
"I was indeed slow to believe it possible that a body so constituted, and so transparent to light as olefiant gas, could be so densely opake to any kind of calorific (infrared) rays; and to secure myself against error, I made several hundred experiments with this single substance."
The result held true.
In understanding the nature of today's greenhouse effect, where the natural warming (dominated by water vapour) is amplified by traces of gas small enough to be measured in parts per million or even parts per billion, this paper made two important points:
- tiny amounts of a highly absorbent gas can dominate much larger amounts of a less absorbent gas, as in the ozone example
- increasing the concentration of an absorbent gas does not always produce a proportional increase in heat uptake, because there is progressively less to be absorbed.
It's salutary to put Tyndall's paper in a historical concept.
His discovery, description and analysis of the molecular basis of the greenhouse effect came more than 30 years before the discovery of either radioactivity or the electron.
Transport was predominantly powered by human or animal muscles rather than anything mechanical.
Charles Dickens was releasing Great Expectations chapter by chapter, Livingstone was exploring the Zambezi, and women were barred from voting in just about every nation that maintained elections.
So when you take into account Tyndall's other achievements - inventing devices to purify air (which co-incidentally led to a way of preserving food), explaining why light is scattered by particles (known as the Tyndall Effect), confirming that ozone is composed of oxygen atoms, improving understanding of heat production in chemical reactions, and many other things besides - it's perhaps surprising that he's not better known.
His love of the outdoors led to a glacier and a mountain being named for him too.
But public appreciation, it seems, remains dim by comparison with his peers (though London's Science Museum has taken something of a step forward by putting some of his equipment on public display, and explaining the key experiment online).
Equally perturbing - and perhaps not unrelated - is the persistence of the view that emissions of carbon dioxide and other gases cannot influence global temperatures because they are so small.
In the light of Tyndall's findings, amply replicated and refined and explained since, maintaining such a view is virtually the definition of anti-science.
Tyndall's lab experiments do not prove that humanity's CO2 emissions are warming the planet, any more than lab experiments being conducted now at Cern can prove that changes in cosmic ray flux are warming the planet - because in the real world, other factors can influence and outweigh those lab findings.
But Tyndall did show how man-made global warming can work; and he did so 150 years ago.
Time, perhaps, to accept the fact, laud the achievement, and deal with what it means.
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