A debate on the Sun’s role in global warming

Discoveries about how the Sun may affect the Earth’s
climate will be discussed on the Spanish island of Tenerife, 25-30
September 2000. The leading hypothesis in climate science is that
most of the warming during the 20th Century was due to manmade
gases, enhancing the natural greenhouse effect that reduces heat
loss into space. Behind the scenes, and especially among space
scientists, there is renewed attention to natural changes in climate
due to the Sun.

Experts will present their
findings and opinions at the
conference on “The Solar
Cycle and Terrestrial Climate”,
to be hosted in Santa Cruz de
Tenerife by the Instituto de
Astrofísica de Canarias. The
conference is co-sponsored
by the European Union.
Three scientists from the European Space Agency are helping to
organize the scientific programme, and ESA will publish the
proceedings at the end of this year.

“We look forward to very lively discussions,” says Manuel Vázquez,
the Spanish astrophysicist who is the chief organizer of the
conference. “We shall hear prominent experts from both a solar and
a greenhouse background. But we also welcome students to
Tenerife, and industrialists and environmentalists who are
interested in the climate issue.”

A link between sunspots and climate has been a popular idea for
200 years, but ten years ago measurements by spacecraft cast
doubt on it. They showed that the Sun is brighter when there are
many sunspots, at the climax of each sunspot cycle of roughly 11
years, but the variations seemed too small to have a major effect on
climate. These results encouraged the Intergovernmental Panel on
Climate Change (IPPC) in its preference for the greenhouse
hypothesis. The long-time leader of the IPCC’s scientific work, Sir
John Houghton, will be among the speakers at the Tenerife
conference.

During the past ten years astronomers and solar-terrestrial physicists
have reappraised the role of the Sun. Some American scientists,
who will also take part in the meeting, have argued that previous
changes in the Sun’s brightness were greater than those seen
during the short period of satellite measurements. In Europe, there
has been more emphasis on other ways in which the Sun may affect
the climate, either by invisible X-rays and ultraviolet radiation, which
peak when there are many sunspots, or by the solar wind, which
blows non-stop from the Sun and became more vigorous during the
20th Century.

For example, with the aid of results from the ESA-NASA solar
spacecraft Ulysses, Mike Lockwood of the Rutherford Appleton
Laboratory (UK) recently deduced that the magnetic field carried by
the solar wind has doubled in strength since 1900. And the climatic
record from weather satellites, showing changes in the Earth’s cloud
cover, suggests that cloud formation is affected by cosmic rays
arriving from the Galaxy. When the Sun is most active, there are
fewer cosmic rays and fewer clouds. Henrik Svensmark of the Danish
Space Research Institute will present new results on that apparent
link.

“ESA has made a big commitment to solar-terrestrial science, in the
Ulysses, SOHO and Cluster II projects,” comments Bernhard Fleck,
who is ESA’s project scientist for SOHO and one of the planners of
the Tenerife conference. “When we have established what effect
the Sun has on the Earth’s climate, the next task will be to
understand how the Sun’s behaviour in that respect may change in
the decades ahead.”