Has an increasing trend in the Sun’s brightness contributed to global
warming over the last few decades? One study published recently says it has
but Judith Lean will tell a joint session of the UK/Ireland National
Astronomy Meeting and Solar Physics Meeting in Dublin that a different study
has come to the opposite conclusion when she tackles the controversial topic
of the relationship between our climate and the Sun on Tuesday 8 April.
Earth’s climate records feature many fluctuations apparently linked to solar
activity but the physical processes at work connecting the Sun and climate
are not yet properly understood. Satellites have measured how the Sun’s
brightness has changed in the past two decades, and these data can be
compared with high precision records of Earth’s temperatures over the same
period to throw light on the problem. But it is difficult separating solar
effects from other factors influencing our climate over different
time-scales, such as major volcanic eruptions and the increase in greenhouse
gases in the atmosphere due to human activities. For example, the most
recent increase in the Sun’s activity on its regular 11-year cycle
corresponded with a rise of 0.1 degree C. By comparison, the eruptions of El
Chichon and Pinatubo cooled Earth by 0.2 degree C for a short time.
A new study proposes that, on top of the change due to its 11-year cycle,
the Sun has brightened steadily during the past two decades. If true, the
suggested trend of 0.05% per decade would account for half or more of the
0.3 degrees warming currently attributed to greenhouse gas increases since
1980.
“This study is very controversial,” says Judith Lean. “It relies on splicing
together solar irradiance datasets made by different instruments flown on
various spacecraft. Because the datasets do not have the same absolute
scale, they must be cross-calibrated to construct the long-term record
needed for studying climate change. Drifts in the instrument sensitivities
must be properly clarified as well, to avoid mistaking spurious trends for
real solar brightness changes. For this purpose, the recent study used
observations previously reported to suffer from known instrumental effects
but did not take these effects into account.”
Dr Lean then cites another study, which brought together the various solar
brightness datasets in a different way and compensated for instrumental
drifts. It concluded that there was no general brightening of the Sun over
the past two decades. This result is consistent with what solar physicists
would expect from their understanding of the Sun’s magnetism. Sunspots and
faculae, both magnetic features on the Sun’s surface, respectively reduce
and enhance the Sun’s overall brightness and independent records of sunspots
and faculae show no underlying upward trends during past decades. The same
is true of numerous other indicators of the Sun’s behaviour that have been
closely monitored. This alternative study concluded that long-term solar
brightness changes are not a significant cause of recent global warming.
“Other claims in recent years have also exaggerated the role of the Sun in
climate change” warns Judith Lean. As an example, she quotes a study
published in 1991 that reported a close tie between the actual length of the
solar cycle (which averages 11 years but varies from 9 to 15 years) and
surface temperatures in the Northern Hemisphere throughout the entire
twentieth century. If true, it would mean that human influences, such as
increasing concentrations of greenhouse gases, have contributed little or
nothing to the approximately 0.8 degree C warming of Earth’s surface since
1885. But subsequent examination of additional data gathered over a longer
period of time, including the pre-industrial era, have made the real
connection between solar brightness and climate change clearer. “Temperatur=
e
changes in concert with solar activity are indeed apparent during the past
millennium,” reports Dr Lean, “but are typically of order 0.2 to 0.5
degrees C on time scales of hundreds of years. Since 1885, global warming in
response to changes in the Sun’s brightness is now thought to have been less
than 0.25 degrees C.”
“To really resolve the controversies, we need longer and more precise
monitoring of the solar brightness to determine whether or not there are
long-term trends,” she concludes. To that end, a new generation of solar
radiometers was launched into space in January 2003 on board the Solar
Radiation and Climate Experiment (SORCE).
CONTACT
Dr Judith Lean, E. O. Hulburt Center for Space Research
Naval Research Laboratory, Washington DC 20375
Phone: (+1) (202) 767-5116
Fax: (+1) (202) 404-7997
e-mail: lean@ssd5.nrl.navy.mil
Dr Lean will be at the NAM in Dublin from 7 to 11 April
NOTE
More information about the SORCE mission is available at
http://earthobservatory.nasa.gov/Library/SORCE/
and
