Changes in Solar Brightness Too Weak to Explain Global Warming(Forwarded)

National Center for Atmospheric Research
Boulder, Colorado

Contacts:
David Hosansky, acting head of Media Relations
303-497-8611

Tom Wigley, NCAR Scientist
303-497-2690

September 13, 2006

Changes in Solar Brightness Too Weak to Explain Global Warming

BOULDER -- Changes in the Sun's brightness over the past millennium have
had only a small effect on Earth's climate, according to a review of
existing results and new calculations performed by researchers in the
United States, Switzerland, and Germany.

The review, led by Peter Foukal (Heliophysics, Inc.), appears in the
September 14 issue of Nature. Among the coauthors is Tom Wigley of the
National Center for Atmospheric Research. NCAR's primary sponsor is the
National Science Foundation.

"Our results imply that, over the past century, climate change due to
human influences must far outweigh the effects of changes in the Sun's
brightness," says Wigley.

Reconstructions of climate over the past millennium show a warming since
the 17th century, which has accelerated dramatically over the past 100
years. Many recent studies have attributed the bulk of 20th-century global
warming to an increase in greenhouse gas concentrations in the atmosphere.
Natural internal variability of Earth's climate system may also have
played a role. However, the discussion is complicated by a third
possibility: that the Sun's brightness could have increased.

The new review in Nature examines the factors observed by astronomers that
relate to solar brightness. It then analyzes how those factors have
changed along with global temperature over the last 1,000 years.

Brightness variations are the result of changes in the amount of the Sun's
surface covered by dark sunspots and by bright points called faculae. The
sunspots act as thermal plugs, diverting heat from the solar surface,
while the faculae act as thermal leaks, allowing heat from subsurface
layers to escape more readily. During times of high solar activity, both
the sunspots and faculae increase, but the effect of the faculae
dominates, leading to an overall increase in brightness.

The new study looked at observations of solar brightness since 1978 and at
indirect measures before then, in order to assess how sunspots and faculae
affect the Sun's brightness. Data collected from radiometers on U.S. and
European spacecraft show that the Sun is about 0.07 percent brighter in
years of peak sunspot activity, such as around 2000, than when spots are
rare (as they are now, at the low end of the 11-year solar cycle).
Variations of this magnitude are too small to have contributed appreciably
to the accelerated global warming observed since the mid-1970s, according
to the study, and there is no sign of a net increase in brightness over
the period.

To assess the period before 1978, the authors used historical records of
sunspot activity and examined radioisotopes produced in Earth's atmosphere
and recorded in the Greenland and Antarctic ice sheets. During periods of
high solar activity, the enhanced solar wind shields Earth from cosmic
rays that produce the isotopes, thus giving scientists a record of the
activity.

The authors used a blend of seven recent reconstructions of Northern
Hemisphere temperature over the past millennium to test the effects of
long-term changes in brightness. They then assessed how much the changes
in solar brightness produced by sunspots and faculae (as measured by the
sunspot and radioisotope data) might have affected temperature. Even
though sunspots and faculae have increased over the last 400 years, these
phenomena explain only a small fraction of global warming over the period,
according to the authors.

Indirect evidence has suggested that there may be changes in solar
brightness, over periods of centuries, beyond changes associated with
sunspot numbers. However, the authors conclude on theoretical grounds that
these additional low-frequency changes are unlikely.

"There is no plausible physical cause for long-term changes in solar
brightness other than changes caused by sunspots and faculae," says
Wigley.

Apart from solar brightness, more subtle influences on climate from cosmic
rays or the Sun's ultraviolet radiation cannot be excluded, say the
authors. However, these influences cannot be confirmed, they add, because
physical models for such effects are still too poorly developed.

About the article

Title: "Variations in solar luminosity and their effect on the Earth's
climate."
Authors: P. Foukal, C. Frohlich, H. Spruit, and T.M.L. Wigley
Publication: Nature, September 14, 2006

IMAGE CAPTION:
[ftp://ftp.ucar.edu/communications/activity-02-high.jpg (1.7MB)]
In this image from an active solar period in March 2001, colors are
shifted to highlight the contrast between sunspots (black and dark red)
and the faculae that surround them (bright yellow). During the peak of the
11-year solar cycle, the expansion of faculae outweighs the darkening from
increased sunspot activity. The result is a net increase in solar
brightness. (Image courtesy NASA.)