Solar radiation is relatively unaffected in its journey from the Sun to the top of Earth’s atmosphere. As it travels through the atmosphere, some wavelengths, like ultraviolet light, are selectively absorbed, so what reaches the surface is mostly visible light with wavelengths between ~380 – 700 nanometers (10-9 meter), often referred to as shortwave energy. The Earth’s surface absorbs this incoming solar radiation, is warmed, and then re-emits energy at longer wavelengths within the infrared portion of the electromagnetic spectrum. This infrared radiation, which has wavelengths between ~900 – 5000 nanometers often is called outgoing longwave radiation. As the longwave energy emitted by the Earth’s surface travels upward through the atmosphere, various gases absorb much of it, with water vapor and carbon dioxide being the two most prominent examples. These gases are heated by the longwave energy and emit longwave radiation themselves. Much of this energy returns to the surface and keeps the lower atmosphere warmer than it would be in the absence of the greenhouse gases.
A greenhouse is a building with a glass roof, used to grow plants. The glass lets sunlight enter and heat the interior. Plants and other objects inside radiate longwave energy, but the glass absorbs much of this energy and radiates longwave energy back into the building. The greenhouse is, therefore, warmer than it would be without the glass roof and many plants grow more effectively in a warmer environment. The “Greenhouse Effect” is the term used for the analogous situation between the glass on a greenhouse and Earth’s atmosphere, with both letting shortwave energy pass but not longwave. (Actually, much of the additional heating within greenhouses is due to the lack of wind mixing the air, but that part of the story is not analogous to the atmosphere.)
Much more information on the greenhouse effect can be found elsewhere in Encyclopedia of Earth. The purpose of this article is to briefly describe the progression in our understanding over time of the phenomena involved, as applied to the atmosphere, and how humans have affected the global temperature. There is no one person responsible for the concept of the greenhouse effect and how it relates to anthropogenic warming; it developed over the course of several centuries. (More in-depth histories of climate change research and related topics can be found in the Further Reading section at the end of this article, especially Fleming (1998) and Le Treut et al. (2007).)
In 1681, French physicist Edme Mariotte (c. 1620-1684) suggested that sunlight easily passes through glass while heat from other sources does not. Horace Bénédict de Saussure (1740-1799), a Swiss natural scientist, used Mariotte’s insight to design and build a “heliothermometer” to measure the relative intensity of sunlight. It was an insulated box with one side consisting of three layers of glass separated by air layers. Sunlight penetrated the glass relatively easily, but the heat generated inside was trapped and measured by a thermometer. Measurements by de Saussere in the 1770s using heliothermometers at different elevations in the Alps showed that solar radiation intensifies with altitude.
In the early 1800s, French mathematician and physicist Joseph Fourier (1768 – 1830) developed a simple mathematical model of the temperature of the atmosphere. He found the atmosphere to be analogous to de Saussure’s heliothermometer in that sunlight penetrates it easily but heat from the ground has a harder time getting through the air. He commented that the same principles could be applied to the heating of greenhouses and other closed spaces.
Irish and British physicist and chemist John Tyndall (1820–1893) added details to Fourier’s findings. Others had studied the radiative properties of solids and liquids and Tyndall sought to extend this knowledge to how heat and radiation affect gases. He designed and built a “ratio spectrophotometer” in which he put various gases and determined their ability to absorb heat. He found that radiant heat (i.e., longwave energy) was transmitted readily through nitrogen and oxygen, the main constituents of air. More complex molecules, however, absorbed heat readily. These included water vapor and carbon dioxide, though most of his attention was directed at water vapor. Tyndall showed that water vapor is relatively transparent to sunlight but effectively traps radiation from Earth’s surface. He compared water vapor in the air to a dam, storing heat in the atmosphere. The idea of ice ages was relatively new at the time and Tyndall felt that changes in the composition of the atmosphere might account for climatic changes over geological time.
The next major figure in the story of the greenhouse effect is Svante Arrhenius (1859–1927). He was a Swedish chemist and physicist who won the Nobel Prize in Chemistry in 1903 for his contributions to “the electrolytic theory of dissociation” (not for his climate studies). In the 1890s, Arrhenius presented a mathematical model of the atmosphere that incorporated the radiative effects of water vapor and carbon dioxide. He used the model to suggest that changes in CO2 concentrations could account for the temperature variations needed to bring about ice ages. Arrhenius explicitly compared Earth to a greenhouse, what he called the “hot-house theory,” where shorter visible wavelengths from the Sun easily pass through, while the “dark rays” from the surface are trapped by the air. He also predicted that a doubling of CO2 in the atmosphere would raise the average temperature by about 4°C. (While this prediction is reasonably close to present estimates, some of his reasoning is considered too simplistic by today’s standards, so the match is due more to luck than accuracy.) Arrhenius did become interested in human contributions of carbon dioxide in the atmosphere and concluded that it might slow the onset of the next ice age. He also argued that more CO2 will lead to more plant growth, which he saw as a benefit to humans.
In the late Nineteenth and early Twentieth Centuries, two geologists contributed significantly to our understanding of the carbon cycle as it applies to the atmosphere. Arrhenius incorporated the ideas of his friend and colleague Arvid Högbom (1857-1940). Högbom discussed the variations of CO2 over geological time. At such long time scales CO2 in the atmosphere was, he felt, largely controlled by volcanic eruptions and the formation and destruction of rocks such as limestone. Carbon dioxide in plants and animals and human contributions from coal burning were considered relatively inconsequential. American geologist T. C. Chamberlain (1843-1928) developed somewhat different ideas on carbon moving through the environment on geological time scales. He incorporated mountain building and denudation, weathering of rocks, and ocean circulation, among other processes. Chamberlain also looked at the relationships between carbon dioxide and water vapor. Increasing CO2 warms the surface, which increases evaporation and increases water vapor in the air. More water vapor, of course, leads to still higher temperatures.
The person most responsible for our understanding of human impacts on the greenhouse effect is Guy Stewart Callendar (1897-1964), a British engineer with a side interest in meteorology. Starting in the late 1930s, Callendar began publishing papers on anthropogenic carbon dioxide levels and their impact on temperature, incorporating information on CO2 production from coal burning, rising levels of carbon dioxide over time, and the uptake of CO2 by oceans and the biosphere. He documented that temperature had risen slightly over the preceding decades and concluded that the warming was due to higher levels of CO2 due to human activity. Like Arrhenius, Callendar felt that a warmer world would benefit humans by improving agriculture and, in the long run, delay the onset of the next ice age.
Starting in the 1950s, Canadian and American physicist Gilbert Plass (1920-2004) echoed Callendar’s conclusions that human impacts on the atmosphere have begun a period of global warming. Plass used a more sophisticated theoretical understanding of infrared absorption, and was an early user of computers to model the atmosphere.
Also in the 1950s, the popular press reported on global warming, with articles on higher temperatures, shrinking glaciers and warmer winters. A series of relatively cool years in the 1960s started speculation that “global cooling” had begun and this continued into the 1970s. But, by the 1980s the warming trend returned and the consensus of expert opinion was that Earth is warming.
American oceanographer Roger Revelle (1909-1991) was instrumental in promoting the idea of global warming to the scientific community and to policy makers. In 1958, his associate Charles Keeling (1928-2005) set up the first continuous monitoring station for atmospheric carbon dioxide, at the Mauna Loa Observatory, in Hawaii. Callendar had assembled various measurements of CO2 going back to the 1870s, but Keeling’s data dramatically show the steady increase in CO2 levels.
By the 1980s, research on the role of humans in global warming became part of mainstream science. That is where this brief historical overview stops.
- Fleming, James Rodger, 1998, Historical Perspectives on Climate Change, New York: Oxford University Press.
- Fleming, James Rodger, 2007. The Callendar Effect: The Life and Work of Guy Stewart Callendar (1898-1964), the Scientist Who Established the Carbon Dioxide Theory of Climate Change. American Meteorological Society, 155 p.
- Fleming, James Rodger, Climate Change and Anthropogenic Greenhouse Warming: A Selection of Key Articles, 1824-1995, with Interpretive Essays http://wiki.nsdl.org/index.php/PALE:ClassicArticles/GlobalWarming
- Eve, A. S. and Creasy, C. H., 1945, Life and Work of John Tyndall, London: Macmillan & Co., 404 p.
- Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather, 2007: Historical Overview of Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Weart, Spencer R., 2003, The Discovery of Global Warming, Cambridge: Harvard University Press, 228 p.