Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 1
Overview
Solar irradiance, the flux of the Sun’s output directed toward Earth, is Earth’s main energy
source.1 The Sun itself varies on several timescales—over billions of years its luminosity increases as it
evolves on the main sequence toward becoming a red giant; about every 11 years its sunspot activity
cycles; and within just minutes flares can erupt and release massive amounts of energy. Most of the
fluctuations from tens to thousands of years are associated with changes in the solar magnetic field. The
focus of the National Research Council’s September 2011 workshop on solar variability and Earth’s
climate, and of this summary report, is mainly magnetically driven variability and its possible connection
with Earth’s climate variations in the past 10,000 years. Even small variations in the amount or
distribution of energy received at Earth can have a major influence on Earth’s climate when they persist
for decades. However, no satellite measurements have indicated that solar output and variability have
contributed in a significant way to the increase in global mean temperature in the past 50 years.2,3,4
Locally, however, correlations between solar activity and variations in average weather may stand out
beyond the global trend; such has been argued to be the case for the El Niño-Southern Oscillation, even in
the present day.
A key area of inquiry deals with establishing a unified record of the solar output and solar-
modified particles that extends from the present to the prescientific past. The workshop focused attention
on the need for a better understanding of the links between indices of solar activity such as cosmogenic
isotopes and solar irradiance. A number of presentations focused on the timescale of the solar cycle and
of the satellite record and on the problem of extending this record back in time. Highlights included a
report of progress on pyroheliometer calibration, leading to greater confidence in the time history and
future stability of total solar irradiance (TSI), and surprising results on changes in spectral irradiance over
the last solar cycle, which elicited spirited discussion. New perspectives on connections between features
of the quiet and active areas of the photosphere and variations in TSI were also presented, emphasizing
the importance of developing better understanding in order to extrapolate back in time using activity
indices. Workshop participants’ reviews highlighted difficulties as well as causes for optimism in current
understanding of the cosmogenic isotope record and the use of observed variability in Sun-like stars in
reconstructing variations in TSI occurring on lower frequencies than the sunspot cycle.
The workshop succeeded in bringing together informed, focused presentations on major drivers
of the Sun-climate connection. The importance of the solar cycle as a unique quasi-periodic probe of
climate responses on a timescale between the seasonal and Milankovitch cycles was recognized in several
presentations. The signal need only be detectable, not dominant, for it to play this role of a useful probe.
1
G. Kopp, LASP, University of Colorado, “Overview and Advances in Solar Radiometry for Climate Studies,”
presentation at the Workshop on the Effects of Solar Variability on Earth’s Climate, September 8, 2011.
2
S. Solomon, D. Qin, M. Manning, R.B. Alley, T. Berntsen, N.L. Bindoff, Z. Chen, A. Chidthaisong, J.M.
Gregory, G.C. Hegerl, M. Heimann, 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 (S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller, eds.),
Cambridge University Press, Cambridge, U.K., and New York, N.Y., 2007.
3
National Research Council, America’s Climate Choices, The National Academies Press, Washington, D.C.,
2011.
4
National Research Council, Advancing the Science of Climate Change, The National Academies Press,
Washington, D.C., 2010.
1
OCR for page 2
Some workshop participants also found encouraging progress in the “top-down” perspective, according to
which solar variability affects surface climate by first perturbing the stratosphere, which then forces the
troposphere and surface. This work is now informing and being informed by research on tropospheric
responses to the Antarctic ozone hole and volcanic aerosols. In contrast to the top-down perspective is
the “bottom-up” view that the interaction of solar energy with the ocean and surface leads to changes in
dynamics and temperature. During the discussion of how dynamical air-sea coupling in the tropical
Pacific and solar variability interact from a bottom-up perspective, several participants remarked on the
wealth of open research questions in the dynamics of the climatic response to TSI and spectral variability.
The discussion of the paleoclimate record emphasized that the link between solar variability and
Earth’s climate is multifaceted and that some components are understood better than others. According to
two presenters on paleoclimate, there is a need to study the idiosyncrasies of each key proxy record. Yet
they also emphasized that there may be an emerging pattern of paleoclimate change coincident with
periods of solar activity and inactivity, but only on long timescales of multiple decades to millennia.
Several speakers discussed the effects of particle events and cosmic-ray variability. These are all areas of
exciting fundamental research; however, they have not yet led to conclusive evidence for significant
related climate effects. The key problem of attribution of climate variability on the timescales of the
Little Ice Age and the Maunder Minimum were directly addressed in several presentations. Several
workshop participants remarked that the combination of solar, paleoclimatic, and climate modeling
research has the potential to dramatically improve the credibility of these attribution studies.
2
