Overview and Advances in Radiometry for Solar Observations
Greg Kopp, University of Colorado, Boulder
Greg Kopp began by listing the potential climate-relevant changes in Earth's radiation budget
ranging from volcanic dust veils to heating from radioactive decay in Earth's interior. Solar irradiance is
the dominant energy source for Earth, four orders of magnitude greater than the next largest contributor,
radioactive decay. Although the Sun's irradiance can change significantly during the passage of features
on its surface, such changes are of sufficiently high frequency as to not affect climate. More relevant are
trends of a decade or longer. As Kopp explained it, the challenge is to delineate these longer-term solar
changes and to discriminate them from other causes of climate change. Statistical studies have suggested
that natural influences—including the Sun, effects from volcanic eruptions, and changes in Earth-
atmosphere-ocean coupling—together explain roughly 15 percent of the past century's temperature
anomaly. Although small, the contribution from the Sun must be determined with a high degree of
accuracy in order to reliably quantify the other contributors to climate change (Figure 2.1).
FIGURE 2.1 Global temperature and surface temperature components of Earth's climate. Combined ENSO,
volcanic aerosols, solar activity, and anthropogenic effects explain 85 percent of observed temperature variance.
SOURCE: G. Kopp and J.L. Lean, A new, lower value of total solar irradiance: Evidence and climate significance,
Geophysical Research Letters 38:L01706, 2011; Copyright 2011 American Geophysical Union, reproduced by
permission of American Geophysical Union.