Background - The Effects of Solar Variability on Earth's Climate - page 2

astronomical work on Sun-like stars, whether they exhibit quiet periods, and whether their brightness is
diminished during quiet periods. 5
The observed sunspot number has been demonstrated to be negatively correlated with the cosmic-
ray flux. The cosmic-ray flux reaching Earth's surface is modulated by the strength of the solar wind. It
is now understood that this decrease in cosmic rays is due to changes in the magnetic field geometry in
the heliosphere, the bubble blown in the interstellar medium by the solar wind. 6 Higher levels of solar
activity lead to a decrease in the cosmic-ray flux at Earth. Cosmic rays are potentially implicated in
climate change on Earth because as they penetrate Earth's atmosphere they leave behind an ionized path
that could serve as a source of condensation centers that in turn affect cloudiness and Earth's albedo
(reflectivity of solar radiation). 7 Cosmic rays may also have an effect on the global electric circuit of
Earth's atmosphere that is caused by thunderstorms separating charge from the surface to the
troposphere. 8 Research is being conducted on these potential mechanisms and their possible relevance as
a climate-forcing agent. Furthermore, solar energetic particle (SEP) events, created at the shock front of
coronal mass ejections (CMEs), for example, can influence the composition of the upper atmosphere.
During a SEP event, solar protons and electrons follow Earth's magnetic field lines toward the poles. The
higher-energy particles can penetrate well into the stratosphere where they ionize the atmosphere,
producing nitrogen oxides, whereas lower-energy particles can create nitrogen oxides in the lower
thermosphere and mesosphere that then descend into the polar stratosphere. These nitrogen oxides can
destroy ozone, thus altering not only the chemistry but also the radiative balance of that region.
THE MEASUREMENT RECORD FROM SPACE
The TSI and the SSI (spectral solar irradiance) are the two most important measurements of the
Sun's output as it impacts climate. The continuous 33-year record of total solar irradiance from space-
based observations is shown in Figure 1.1. This data record is the result of overlapping measurements
from several instruments flown on different missions. Measurements made by individual radiometers
providing the data shown in Figure 1.1 exhibit a spread of nearly 1 percent that is of instrument rather
than solar origin (the upper panel) and far exceeds the 11-year or rotational solar variability. These
radiometers' measurements have a reproducibility of roughly 0.03 percent over decadal timescales. 9 A
2005 workshop conducted at the National Institute of Standards and Technology (NIST) in Gaithersburg,
Maryland, 10 sparked investigations into the effects of diffraction, scattered light, and aperture area
measurements on the differences between instrument results.
Evident in this combined, recalibrated record is an 11-year cycle with peak-to-peak amplitude of
approximately 0.07 percent and variations greater by a factor of two to three that are associated with
short-term transits of sunspots due to solar rotation (the lower panel of Figure 1.1). Measurement
continuity has enabled successive radiometric time series obtained from different space missions to be
intercalibrated to produce a 33-year-long composite TSI record. The need for such intercalibration makes
5 P.G. Judge, and S.H. Saar, The outer solar atmosphere during the Maunder Minimum: A stellar perspective,
The Astrophysical Journal 663:643, 2007.
6 L.A. Fisk, K.P. Wenzel, A. Balough, R.A. Burger, A.C. Cummings, P. Evenson, B. Heber, J.R. Jokipii, M.B.
Krainev, and J. Kota, et al., Global processes that determine cosmic ray modulation. Space Science Review 83:179-
214, 1998.
7 R. Harrison, The global atmospheric electric circuit and climate, Surveys in Geophysics 25:441-484, 2004.
8 L.I. Dorman and I.V. Dorman, Possible influence of cosmic rays on climate through thunderstorm clouds,
Advances in Space Research 35(3):476-483, 2005.
9 M. Fligge and S.K. Solanki, The solar spectral irradiance since 1700, Geophysical Research Letters 27:2157,
2000.
10 J. Butler, B.C. Johnson, J.P. Rice, E.L. Shirley, and R.A. Barnes, Sources of differences in on-orbital total
solar irradiance measurements and description of a proposed laboratory intercomparison, Journal of Research of
National Institute of Standards and Technology 113:187-203, 2008.