SCIENCE OBSERVER
Living in Sunny Times
David Schneider
What forces, human or otherwise, are now warming the Earth? Debate
over this highly politicized question continues to simmer. And a
publication in Nature last October by solar physicist Sami
K. Solanki of the Max-Planck-Institut für Sonnensystemforschung
and four of his colleagues is bound to intensify the arguments.
Solanki and coworkers attempted to estimate "sunspot
numbers," a general barometer of solar activity, for times long
before the beginning of the observational record, which starts four
centuries ago. Their main result is expressed in the title of their
paper: "Unusual activity of the Sun during recent decades
compared to the previous 11,000 years."
They came to this conclusion after comparing historical observations
of sunspots with the amount of carbon-14 and beryllium-10 stored,
respectively, in tree rings and ice cores. Carbon-14 and
beryllium-10 are "cosmogenic nuclides," created when
cosmic rays (energetic particles arriving from outside the solar
system) strike Earth's atmosphere. The solar magnetic field
partially shields the Earth from cosmic rays—more so when the
Sun is highly active; less so during quiet periods. Thus, after
correction for various confounding factors (including the additional
shielding that the Earth's magnetic field provides and the ongoing
radioactive decay of carbon-14), the tree-ring and ice-core archives
of these cosmogenic nuclides can serve to document solar activity.
Solanki's recent work suggests that sunspot numbers (averaged over
the 11-year cycle in solar activity) have mostly remained below 50,
often far below. In the 15th century, for example, average sunspot
numbers were typically well under 20. But as the observational
record clearly shows, average sunspot numbers have been increasing
in fits and starts since, rising sharply over the 20th century to
the modern level of about 75.
Judith Lean, a solar-terrestrial physicist at the Naval Research
Laboratory, says that "this is startling news—I'm just
not sure that it's right." Solanki fully accepts that his
11,000-year record might yet require refinement: "I'm too much
of a scientist to make the claim, 'Yup, this is it.'"
Why are sunspot numbers such a hot topic? Intuition suggests that
lots of dark spots on the face of the Sun should diminish solar
irradiance, but as Solanki explains, "the Sun is actually a
little bit brighter when there are many dark spots." The reason
is that bright areas called "faculae" tend to accompany
sunspots. Although not easily discerned in telescopic views of the
Sun, these features are brilliant enough that the overall effect of
the combined spots and faculae is to boost irradiance levels. The
question is by how much.
Direct satellite measurements of changes in the Sun's output (which
have only been available since 1978) show that solar irradiance
indeed moves in step with sunspot numbers, but the effect is quite
small—about a tenth of a percent. "If you plug that into
a climate model, you get zip," explains Peter Foukal, an
investigator who runs Heliophysics, a research company in Nahant,
Massachusetts. Yet it appears that changes in the Sun have
influenced climate. Gerard Bond, a geologist at Columbia
University's Lamont-Doherty Earth Observatory, and nine coworkers
demonstrated this long-debated link particularly well in 2001 when
they compared dramatic swings in the climatological state of the
North Atlantic over the past 12 thousand years with matching swings
in the activity of the Sun. Presumably, these ancient changes in
solar irradiance were on par with the 0.1-percent variation observed
over the last two of the Sun's 11-year cycles. How such a tiny
change can affect climate is puzzling. "One needs an
amplifier," says Solanki's coauthor Ilya G. Usoskin, a
physicist at a the University of Oulu in Finland.
Two amplifying mechanisms are being actively considered. The first
invokes changes in the stratosphere. These changes include variation
in the high-level winds that circle the Arctic in winter, which are
observed to speed up and slow down in pace with the 11-year solar
cycle. Solar ultraviolet radiation is thought to be the culprit.
Although UV constitutes only a small fraction of the Sun's total
energy, this portion of the solar spectrum varies much more than
does total irradiance. So there is a significance change in the
heating that goes on in the stratosphere when high-level ozone
absorbs the Sun's UV light. Also, increases in the UV output of the
Sun tend to create more stratospheric ozone than normal, further
amplifying the warming that goes on there. The curious thing is that
certain climate models indicate that this rather subtle
high-altitude effect might be felt near the surface, by influencing
the way the underlying atmosphere distributes its own heat. Drew
Shindell, of NASA's Goddard Institute for Space Studies, Lean and
three other colleagues demonstrated this connection with the surface
through modeling studies in 1999.
The second possible amplifier involves cosmic rays. The argument
goes like this: When cosmic rays hit the atmosphere, they not only
form the cosmogenic nuclides, they also form ions, which give rise
(through complicated and as yet poorly known ways) to
cloud-condensation nuclei. That is, cosmic rays may spur the growth
of clouds. An active Sun partially shields the Earth from the normal
barrage of cosmic rays, leading, presumably, to fewer clouds. Clear
skies in turn let in more light and heat the surface more intensely
than normal.
Although the physics and chemistry of the cosmic ray-cloud link are
murky, there is evidence that cloudiness varies in synch with the
11-year solar cycle, at least in some places. Usoskin, for example,
along with four colleagues presented evidence last August in the
journal Geophysical Research Letters that variations in
cosmic-ray induced ionization affect cloudiness in the lower
atmosphere at middle latitudes. Michael E. Schlesinger, a climate
modeler at the University of Illinois at Urbana-Champaign, says he's
"inclined to think that this influence on clouds is
chimerical," but he admits "that doesn't mean it's not
possible." Solanki isn't so dismissive, pointing out that
"the mechanism is just too beautiful to ignore."
Further work may help determine whether cosmic rays indeed affect
cloudiness and, if not, how it is that tiny changes in the Sun can
manage to alter Earth's climate. But perhaps more pressing than the
question of how is the question of how much.
Solanki and colleague Natalie A. Krivova attempted to answer that
question in 2003, when they published a paper in the Journal of
Geophysical Research titled "Can solar variability
explain global warming since 1970?" Their reckoning revealed
that the Sun is responsible for less than 30 percent of the rise in
surface temperature experienced since that time. Although other
investigators have made similar estimates, the jury is still out,
and gauging the role of the Sun in global climate change remains a
key area of research.
One reason to want to know exactly what the Sun contributes is
obvious from Solanki's long history of sunspot numbers: If the Sun
continues to follow the pattern seen in that record, it should move
out of its high-activity state within the next decade or two. This
change might thus help to combat some of the forces that have
lately been pushing climate toward a hotter world.
Investigations of the Sun's influence on climate are relevant for
other reasons as well. For one, this work should help to sort
out what portion of recent climate change is natural. In addition,
it will improve the ability of climate modelers to assess Earth's
sensitivity to increasing levels of carbon dioxide. (Right now it's
anyone's guess whether a doubling of atmospheric carbon dioxide
could reasonably be expected to heat the Earth by 2 degrees or by
5.) As Schlesinger aptly notes, "It's important to sort this out."