Geosciences Memory Online: Solar Variability and Climate Change A Historical Overview

A novel experiment in currently underway in the geophysical sciences [Cliver and Bierly, 1977]. Thanks to funding from the Sloan Foundation, sites on the World Wide Web have been established by the AGU, the American Meteorological Society, and the American Institute of Physics to document recent path-breaking developments. This article advertises the site for Solar Variability and Climate Change. As the title "Geosciences Memory Online" suggests, the site will serve as an online repository for the preservation and documentation of recent (c. 1960-1990) progress in the field. We invite scientists who have contributed to the emergence of the field of Solar Variability and Climate Change to join (or begin!) discussions at the site and to submit recollections as well as unpublished documentation such as grant proposals, photographs, correspondence, and early drafts of papers. In this way you will enjoy a unique opportunity to write your own history as a continuing, interactive conversation.

The debate over a solar contribution to climatic change is by no means of recent vintage, and a sketch of the longer-term history of the subject will provide context for more recent developments. As early as the late eighteenth century, widespread concern for the deterioration of the earth's climate led to speculation on the sun's role in climate change. [Feldman, 1993; Fleming, 1990.] Drawing analogies with variations in the brightness of stars, the astronomer William Herschel suggested that greater sunspot activity would result in warmer earth climates. Herschel supported his hypothesis by reference to price series for wheat published in Adam Smith's Wealth of Nations. [Hufbauer, 1991.]

Later, the eminent American physicist Joseph Henry demonstrated by thermopile measurements that, contrary to Herschel's assumption, sunspots were cooler than the unblemished portions of the solar disk. Already this early, certain characteristics of the debate over solar variability and climate change appear, that will persist into the present: a reliance on terrestrial proxies of solar activity (here the grain price series), the significance of what later would be known as the Maunder Minimum (noticed by Herschel), attempts to correlate the sunspot cycle (discovered in 1844) with weather patterns, the use of evidence drawn from the behavior of other stars, and the application of new instruments to the problem (the thermopile).

By the end of the 19th century a small community of scientists was pursuing questions surrounding the possibility of solar variability and climate change. Their attempts to correlate weather patterns with the sunspot cycle suffered from inaccurate and unstandardized weather data, the difficulty of choosing among multiple aspects of the weather for correlation with the solar cycle, and lack of suitable statistical techniques for data analysis. These weaknesses did not prevent scientists from seeing in their data the desired correlations -- as is illustrated by the late 19th-century British school of "cosmical meteorology," whose Balfour Stewart grandly exclaimed of sun and planets: "They feel, they throb together."[Gooday, 1994.]

These difficulties persisted into the twentieth century as researchers continued to gather evidence for solar variation and its influence on climate. A.E. Douglass, founder of the science of dendrochronology, discovered remarkable correlations of tree-ring breadth with both the sunspot cycle and the Maunder Minimum.[Webb, 1986.] Samuel Langley and Charles Greeley Abbot of the Smithsonian recorded direct measurements of the solar constant (the level of the sun's radiation) over several decades. They concluded that this "constant" varies by about 0.3% on the short-term scale of several days and that on the longer term, the more active sun is brighter by about 1%. [Hufbauer, 1991] More recent analysis has shown that Abbott's measurements suffered from atmospheric transmission and from weaknesses in instrumentation [White, 1977]. (Nevertheless, modern observations have confirmed that the sun is brighter during active periods, because the solar faculae -- bright regions -- more than compensate for the greater numbers of sunspots during those parts of the solar cycle.)

This long search reached a turning point in the 1960s, when convincing evidence of solar variability and of correlation between solar variation and climate change first became available. In 1961 Minze Stuiver found evidence of solar variability in ¹⁴C variations in the tree-ring record over the past millenium [Stuiver, 1961]: at times of greater solar activity the solar wind and its magnetic field shield the earth from ¹⁴C-generating cosmic rays, resulting in lower ¹⁴C absorption by living organisms. Later studies of the preceding 7.5 millennia revealed fluctuations in ¹⁴C concentration implying that variations in solar activity comparable to Maunder Minimum are commonplace over scales of 100 to 1000 years. Studies of ¹⁰Be have established proxy records of longer-term solar variability. [Hoyt and Schatten, 1997.]

Secondly, in the late '60s the first measurements of solar irradiation were made from above the atmosphere. These and subsequent space-based measurements, far more sensitive than had been possible from ground-based platforms and relying on the most advanced instrumentation, showed that the solar "constant" did indeed vary. However, the time scale involved was too short to support conclusions about long-term climatic change. [Hufbauer, 1991]

John Eddy tied all these threads together in a now-famous paper of 1976. Examining the historical record, he argued that the current patterns of solar regularity and reliability -- an 11-year solar cycle and a constant level of radiation -- may likely be ephemeral phenomena amid a longer-term record of solar variability. Records of ¹⁴C and of naked-eye sightings of auroras, sunspots, and the solar corona all point to earlier minimums and at least one maximum of solar activity in the past 2000 years. These maxima and minima, Eddy claimed, coincide with periods of climatic extreme -- the Little Ice Age of the sixteenth and seventeenth centuries and the Medieval Climatic Optimum of the eleventh through thirteenth centuries, for example [Eddy, 1976].

Eddy's claims met with considerable scepticism, but evidence for solar variability and its influence on climate has become firmer over the last two decades. While Vladimir K–ppen had attempted a determination of global temperature in the early decades of this century, the first reliable estimates appeared in the early 1960s. In the mid-1980s P.D. Jones, T.M.L. Wigley and others made available the first global syntheses of surface-temperature measurements over both land and oceans. [Jones, et al., 1986; T.M.L. Wigley, 1986] These last estimates all agreed in showing global warming from the late nineteenth century to around 1940, a cooling to the mid-1960s, and substantial warming since then.

Anxiety over the contribution of greenhouse gases to global warming had motivated these calculations of global temperature. But Eddy's line of argument suggested that part of the temperature change might result from solar variation. By the beginning of our own decade satellite measurements offered continuous, longer-term datasets than had been earlier available. The variations in solar luminosity they reported are sufficient to account for significant climate change, especially if linked to solar cycles on 100-year time scales [Reid, 1988 and 1992; Friis-Christiansen and K. Lassen, 1992; Ardanuy, 1992]. Studies of sun-like stars also point to eras of cyclic magnetic activity punctuated by periods, like our Maunder Minimum, of magnetic quiet. [Baliunas, 1992.]

Many readers of this article have witnessed or contributed to the developments sketched in the last several paragraphs. We invite you to visit "Solar Variability and Climate Change": whether your orientation is terrestrial, solar, or extra-solar; whether you examine ¹⁴C, ¹⁰Be, tree-ring, other proxy data, estimate global temperature, model terrestrial climate or solar change or study variations in solar-type stars -- or whether you are merely an "innocent bystander." You will find the site at http://www.agu.org/history/SV.shtml. Enjoy reading what is there, and help create history online by submitting your thoughts, recollections, and documentation.

Theodore S. Feldman
PSDI, Bedford, Massachusetts

References

Ardanuy, P.E., D V Hoyt, and H. Lee Kyle. "Evidence of sun-climate-greenhouse gas connections over the last decade." Eos, 73:14 supplement, 245, 1992.

Baliunas, S.L. et al. "Long-term variability of solar total irradiance: studies of solar-type stars." Eos, 73:14 supplement, 245, 1992.

Cliver, E. W. and Bierly E. W. History on the Web: an Experiment in the Geosciences. EOS 78:46 Nov. 18, 1997, 521-2.

Eddy, J. The Maunder Minimum, Science, 192, 1189-1202, 1976.

Feldman, T.S., The ancient climate in the eighteenth and early nineteenth century, in Michael Shortland, ed., Science and Nature. Essays in the History of the Environmental Sciences, British Society for the History of Science, 1993.

Fleming, J.R., Meteorology in America, 1800-1870, Johns Hopkins University Press, 1990.

Friis-Christiansen and K. Lassen, "Solar variability and temperature changes." Eos, 73:14 supplement, 245, 1992.

Gooday, G. Cosmos, Climate, and Culture: Balfour Stewart and the Technologies of Universal Meteorology, presented at History of Science Society Annual Meeting, New Orleans, November 1994.

Hoyt, D. and K. Schatten, The role of the sun in climate change, Oxford University Press, 1997.

Hufbauer, K., Exploring the Sun. Solar Science since Galileo, Johns Hopkins University Press, 1991. ^{1   2   3}

Jones, P.D., T.M.L. Wigley, & P.B. Wright, "Global temperature variations between 1861 and 1984." Nature, 322, 430-434 1986.

Reid, George. "Solar variability and climate change on the time scale of decades to centuries," Eos, 69:18, 567, 1988.

Reid, George. "Solar total irradiance variability and global ocean temperature variations." Eos, 73:14 supplement, 244, 1992.

Stuiver, M. J. Geophys. Res., 66, 273 1961.

Webb, G. "Solar Physics and the Origins of Dendrochronology." Isis, 77, 292-301, 1986.

White, O.R., ed. The Solar Output and its Variation, Colorado Associated University Press, 1977.

T.M.L. Wigley, P.D. Jones, and P.M. Kelly, "Warm world scenarios and the detection of climatic change induced by radiatively active gases." Bolin, Bert, Bo R. D–s, Jill J”ger, and Richard A. Warrick, eds. The greenhouse effect, climatic change, and ecosystems. John Wiley, 1986, 271-322.