Greenhouse effect: In the laboratory
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As well as taking measurements in the Earth’s atmosphere, scientists have also investigated some components of the greenhouse effect in the laboratory. Scientific instruments can measure how much energy is absorbed at certain frequencies by different gases. Professor Frank James explains John Tyndall’s early climate experimentsin the 1850s. Tyndall declared that without greenhouse gases (such as water vapour and carbon dioxide) the Earth would be ‘held fast in the iron grip of frost’.
Just as visible light comes in all the colours of the rainbow, the infrared heat energy given off by the Earth comes in different shades, or frequencies. Different gases absorb different frequencies. Over a hundred years ago, scientists devised instruments called spectrometers to measure the absorption properties of atmospheric gases. Spectrometers used today have much greater precision and can measure a broader range of frequencies. By examining these measurements, scientists discovered that oxygen and nitrogen – the most abundant atmospheric gases – don’t absorb infrared energy. But gases such as water vapour, carbon dioxide and methane each absorb a distinctive set of infrared frequencies, making them efficient greenhouse gases.
Scientists carry out laboratory experiments in which they focus an infrared heat source on one end of a tube filled with the gas they wish to study. A spectrometer at the other end of the tube measures in detail the amount of infrared energy passing through. The amount of energy not escaping from the tube will have been absorbed by the gas under examination. This enables scientists to determine not only which gases are capable of absorbing infrared heat energy, but also how much of the energy each gas absorbs. Measurements show that of the three main greenhouse gases, molecule for molecule, methane is the strongest, followed by carbon dioxide and then water vapour.
Joseph Fourier was a mathematician fascinated by heat. At the beginning of the 19th century he developed mathematical techniques to investigate heat transfer. As part of his research he mapped out all the factors that affect the Earth’s heat energy balance. He suggested that the atmosphere could regulate our planet’s surface temperature by preventing heat energy, also known as infrared energy, from escaping to space. He anticipated the role of what we now call the greenhouse effect in controlling the climate, but did not fully explain its mechanisms. His ideas were developed further by John Tyndall and Svante Arrhenius.
In 1859, John Tyndall was the first to find evidence at a molecular level of the greenhouse effect. He was interested in explaining ice ages and believed the answer might be found in the atmosphere’s composition. He designed an apparatus to show that small amounts of ‘perfectly colourless and invisible gases and vapour’ found in the atmosphere – such as water vapour, carbon dioxide and methane – can absorb infrared heat energy. He suggested that these gases, now called greenhouse gases, play a significant role in controlling the planet’s climate.
Like John Tyndall, Svante Arrhenius, a chemist, was interested in finding out the cause of ice ages. In 1896 he published a paper that examined the effect of carbon dioxide concentration in the atmosphere on the surface temperature of the Earth. He predicted that any doubling of the percentage of carbon dioxide in the air would raise the temperature of the Earth’s surface by 5 to 6 °C. In 1908 he adjusted his model and calculated that average surface temperatures would increase by 4 °C for a doubling of carbon dioxide. These results are close to current estimates, despite the relatively rudimentary information Arrhenius’ model was based upon.
During the first half of the 20th century, Guy Callendar, a British steam engineer, set out to investigate, at home during his spare time, the influence of human activity on the greenhouse effect using his own weather station, world weather records, measurements of carbon dioxide and calculations. In 1938 he suggested that rising global temperatures and rising atmospheric carbon dioxide levels owing to increased coal burning and human activity are closely linked. His work is seen by many as the starting point of modern climate science and it had a significant influence on scientists who investigated the greenhouse effect further.