An Assessment of the Sun-Climate Relation on Time Scales of Decades to Centuries: The Possibility of Total Irradiance Variations

Abstract

Sunlight supplies most of the energy that drives the dynamics of the terrestrial climate. Speculation on the role of changes in the sun and their influence on changes in the earth’s climate has been ongoing for centuries. Riccioli (1651, 1653) considered the possible connection of sunspot activity with weather in Italy, e.g., the warm and dry weather during September of 1632 when there were no spots in contrast to the cold June of 1642 when there were many spots. Herschel (1801) reported, “I am now much inclined to believe that openings with great shallows, ridges, nodules and corrugations, instead of small indentations, may lead us to expect a copious emission of heat, and therefore mild seasons. And that on the contrary, pores, small indentations, the absence of ridges and nodules, and of large openings and shallows, will denote a spare emission of heat, may induce us to expect severe seasons” (see, e.g., Schove 1983 for a review of the history of the sun-climate connection).

Modern evidence on the possible influence of solar variability and terrestrial climate change has been accumulating as interest in the role of natural variability in climate change has increased. But there are two major stumbling blocks: the lack of (1) a complete description of solar variability; and (2) knowledge of the response of climate to changes in solar radiation. Modeling of that climate response must go beyond a mechanism driven purely radiatively; it must also include radiatively-induced dynamical changes as well (Palmer 1993; Holton 1994). But modeling efforts generally treat an increase in the solar constant input (a physically more correct prescription would be an increase of total irradiance) to the climate system to be radiatively equivalent to that from an increase in atmospheric concentration of trace anthropogenic greenhouse gases like carbon dioxide, methane, CFCs and nitrous oxide. The additional, regional impact of the anthropogenic sulfate aerosols has also been modeled in a similar way.

In addition, that assumption of the equivalence of input of solar and anthropogenic forcing ignores a key issue: Does the response of the climate system differ significantly for different forcings? If the answer to this question is affirmative, it should be possible to identify specific causes of climate change. As will be seen, the consequences of different kinds of forcings, including solar variability, are still difficult to disentangle.