In what could be the simplest explanation for one
component of global warming, a new study shows the Sun's
radiation has increased by .05 percent per decade since
the late 1970s.
The increase would only be significant to Earth's
climate if it has been going on for a century or more,
said study leader Richard Willson, a Columbia University
researcher also affiliated with NASA's Goddard Institute
for Space Studies.
The Sun's increasing output has only been monitored
with precision since satellite technology allowed
necessary observations. Willson is not sure if the trend
extends further back in time, but other studies suggest
"This trend is important because, if sustained over
many decades, it could cause significant climate
change," Willson said.
In a NASA-funded study recently published in
Geophysical Research Letters, Willson and his
colleagues speculate on the possible history of the
trend based on data collected in the pre-satellite era.
"Solar activity has apparently been going upward for
a century or more," Willson told SPACE.com
Further satellite observations may eventually show
the trend to be short-term. But if the change has indeed
persisted at the present rate through the
20th Century, "it would have provided a
significant component of the global warming the
Intergovernmental Panel on Climate Change reports to
have occurred over the past 100 years," he said.
That does not mean industrial pollution has not been
a significant factor, Willson cautioned.
Scientists, industry leaders and environmentalists
have argued for years whether humans have contributed to
global warming, and to what extent. The average surface
temperature around the globe has risen by about 1 degree
Fahrenheit since 1880. Some scientists say the increase
could be part of natural climate cycles. Others argue
that greenhouse gases produced by automobiles and
industry are largely to blame.
Willson said the Sun's possible influence has been
largely ignored because it is so difficult to quantify
over long periods.
efforts to determine the Sun's role is the fact that its
energy output waxes and wanes every 11 years. This solar
cycle, as it is called, reached maximum in the middle of 2000
and achieved a second
2002. It is now ramping down toward a solar minimum that
will arrive in about three years.
Changes in the
solar cycle -- and solar output -- are known to cause short-term
climate change on Earth. At solar max, Earth's thin upper
atmosphere can see a doubling of temperature. It swells,
and denser air can puff up to the region of space where
the International Space Station orbits, causing
increased drag on the ship and forcing more frequent
boosts from space shuttles.
1996, near the last solar minimum, the Sun is
nearly featureless. By 1999, approaching
maximum, it is dotted by sunspots and fiery hot
gas trapped in magnetic loops.
|SOURCE: ESA/NASA/SOHO/US Naval Research
Cams: See the Sun Now
Long-term: A previous study showed that
changes in the Sun's output appear to be related
to temperatures on Earth, based on studies of
tree rings, sunspots and other
Solar max has also been tied to a 2 percent increase
in clouds over much of the United States.
It might seem logical to assume tie climate to solar
output, but firm connections are few. Other studies
looking further back in time have suggested a connection
between longer variations in solar activity and
temperatures on Earth.
Examinations of ancient tree rings and other data
show temperatures declined starting in the
13th Century, bottomed out at 2 degrees below
the long-term average during the 17th
Century, and did not climb back to previous levels until
the late 19th Century. Separate records of
sunspots, auroral activity (the Northern Lights) and
terrestrial deposits of certain substances generated in
atmospheric reactions triggered by solar output, suggest
the Sun was persistently active prior to the onset of
this Little Ice Age, as scientists call the event.
Solar activity was lowest during the 17th
Century, when Earth was most frigid.
Large-scale ocean and climate variations on Earth can
also mask long-term trends and can make it difficult to
sort out what is normal, what is unusual, and which
effects might or might not result from shifts in solar
To get above all this, scientists rely on
measurements of total solar energy, at all wavelengths,
outside Earth's atmosphere. The figure they derive is
called Total Solar Irradiance (TSI).
The new study shows that the TSI has increased by
about 0.1 percent over 24 years. That is not enough to
cause notable climate change, Willson and his colleagues
say, unless the rate of change were maintained for a
century or more.
On time scales as short as several days, the TSI can
vary by 0.2 percent due to the number and size of
sunspots crossing the face of the Sun. That shift, said
to be insignificant to weather, is however equal to the
total amount of energy used by humans, globally, for a
year, the researchers estimate.
The study analyzed data from six satellites orbiting
Earth at different times over the 24 years. Willson
ferreted out errors in one of the datasets that had
prevented previous studies from discovering the
A separate recent study of Sun-induced magnetic
activity near Earth, going back to 1868, provides
compelling evidence that the Sun's current increase in
output goes back more than a century, Willson said.
He said firm conclusions about whether the present
changes involve a long-term trend or a relatively brief
aberration should come with continued monitoring into
the next solar minimum, expected around 2006.