Solar Variability as a Source of Climate Change
- James E. Hansen and
- Taro Takahashi
Published Online: 19 MAR 2013
Copyright 1984 by the American Geophysical Union.
Climate Processes and Climate Sensitivity
How to Cite
Sofia, S. (1984) Solar Variability as a Source of Climate Change, in Climate Processes and Climate Sensitivity (eds J. E. Hansen and T. Takahashi), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM029p0202
- Published Online: 19 MAR 2013
- Published Print: 1 JAN 1984
Print ISBN: 9780875904047
Online ISBN: 9781118666036
- Ocean-atmosphere interaction—Congresses
The current understanding of the physical nature and magnitude of solar variability is reviewed with the purpose of assessing its role as a forcing mechanism for climate. In particular, the discussion is confined to variations of the total solar irradiance, which is directly significant on energetic considerations, as opposed, for example, to variations of the ultraviolet irradiance which, because of the small energy involved, could only affect climate as a trigger mechanism. There is, at the present time, well established evidence of solar variability at the 0.2–0.3 percent level on timescales from tens of days to ∼2 years. This variability can be almost fully explained in terms of a modulation of the total irradiance produced as the solar rotation brings into and out of view evolving active regions (i.e., spots and faculae). The most complete current modeling of the active regions modulation indicates that, during the past solar cycle, few months averages of the resulting irradiance did not vary by more than 3 parts in 104, and thus it was of little climate consequence. This same modeling, however, successfully produced the general irradiance decrease observed in 1981, which amounted to about 0.1 percent. Competing, simplified models of this modulation for the past cycle, however, lead to larger activity-related modulations, anti-correlated with solar activity. Although the final judgement of which modeling more realistically represents the solar behavior must await further observations currently being made, this simpler model, which predicts an increase of the total irradiance as the solar activity begins to decrease past the maximum early in 1980, does not produce the decline observed in 1981.
There is a second type of modulation of the total irradiance that has characteristic timescales of the order of tens of years, and potentially, larger amplitudes than the active regions modulation. This modulation arises as a consequence of structural variations of the Sun, which have manifested themselves in past changes of the solar radius, and more directly, as a long-term trend in the residuals of the observed values minus the active regions-modulated component under the assumption of a constant non-active photosphere. The physical origin of the structural modulation is the conversion of potential energy into radiative energy and vice versa when the solar outer envelope contracts or expands, probably as a consequence of variations in the dynamo magnetic field. By means of new measurements of the variations of the solar size to be carried out by instruments currently under design, and in conjunction with irradiance measurements to be made with existing instruments, a definitive resolution of the question of solar forcing of climate change is within grasp.