The changes in TSI over the solar cycle provide a good starting point for discussing these
challenges. Periodic, or quasi-periodic, forcing 17 provides invaluable information on climate dynamics.
Other than the seasonal variability on a yearly scale and the precession of the equinoxes (the change of
the season in which the minimum Sun-Earth distance occurs) with scales of 20,000 years, the only quasi-
periodic forcing term is the 11-year solar cycle. Based on the climate community's best estimates of
global climate sensitivity, the solar stimuli are much smaller than would be required to dominate the
temperature record on decadal timescales. 18 The search for the solar cycle signal in the temperature
record, albeit small, continues to motivate much of the climate research in this area, and so far two basic
mechanisms have been modeled. In the first, the 11-year cycle may affect the climate system via the
bottom-up total solar irradiance path through which solar cycle effects can manifest themselves at the
surface and its nearby environment. In general, this bottom-up driver is strongest in the tropics, where
there are feedbacks (from clouds, ocean currents, sea surface temperature, and so on) present in the
climate system that strengthen the effect and even show up at higher latitudes.
A second avenue of inquiry is the top-down mechanism that makes use of the modulated
absorption of ultraviolet radiation in the stratosphere. Top-down mechanisms operate through changes in
the more energetic, shorter-wavelength components of the solar spectrum that influence stratospheric
temperatures and winds directly and through absorption by stratospheric ozone. Early work by Karen
Labitzke and Harry Van Loon on interactions of the solar cycle and the Quasi-Biennial Oscillation (QBO)
of the equatorial stratosphere helped direct attention to the top-down pathway. 19,20 The modulation of
stratospheric temperatures is clear from observations. Climate models also take this modulation as input
and have demonstrated significant perturbations on tropospheric circulations. If borne out by future
studies and shown to be of sufficient magnitude, this mechanism could be an important pathway in the
Sun-climate connection, particularly in terms of regional impacts. However, it is important to realize that,
unlike the bottom-up mechanism, it can in itself contribute very little to global temperature variations.
The effects on climate of centennial timescale variations in TSI have been an even more difficult
and contentious issue. Since the work of Jack Eddy in 1976, 21 the claim that the lower temperatures of
the Little Ice Age from roughly 1600 to 1850 are connected to the secular changes in the Sun, as reflected
in paleoclimate data derived from cosmogenic isotopes in sediments and the observed record of sunspots,
remains an unresolved research topic (Figure 1.2). Recent findings that removal of small-scale
photospheric fields could dim the Sun more than previously expected increase the likelihood of such
variations in secular irradiance. 22 It remains to be shown whether or not the field decreased significantly
below levels observed during normal 11-year activity minima. Ongoing discussion of the role of solar
variations in the early 20th century has given rise to the unfounded conjecture that the observed increase
in temperature in the last half century could also be due to changes in TSI rather than to anthropogenic
17 Forcing, or radiative forcing, denotes an externally imposed perturbation in the radiative energy budget of
Earth's climate system. As defined in S. Solomon et al., Technical Summary 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 , 2007.
18 D.R. Marsh and R.R. Garcia, Attribution of decadal variability in lower-stratospheric tropical ozone,
Geophysical Research Letters, 34: L21807, 2007.
19 H. van Loon and Labitzke, The Southern Oscillation. Part V: The anomalies in the lower stratosphere of the
northern hemisphere in the winter and a comparison with the Quasi-Biennial Oscillation, Monthly Weather Review
20 K. Labitzkeand and H. van Loon, Associations between the 11-year solar cycle, the QBO and the atmosphere,
Part I: The troposphere and stratosphere in the northern hemisphere in winter, Journal Atmospheric and Solar-
Terrestrial Physics , 50:197-206, 1988.
21 J.A. Eddy, The Maunder Minimum, Science 192:1189-1202, 1976.
22 P. Foukal, A. Ortiz, and R. Schnerr, Dimming of the 17th Century Sun, The Astrophysical Journal Letters