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Abb. 1:An image of the solar photosphere (the surface of the solar disk seen
in visible light), showing the structures responsible for the
total solar irradiance (TSI) variations. The granules, covering most of
the area, are the convective flows carrying energy from the interior.
They contribute the steady component of the irradiance received by the Earth.
Magnetic structures are dark (sunspots) or bright (the small bright points
called faculae) and contribute a component that varies with the sunspot cycle.
Faculae show up especially towards the limb of the solar disk (to upper right
in this image). Their contribution dominates over the dark spots, so that the
Sun is slightly brighter at sunspot maximum. Length of the bar is 1000 km
(Copyright of image: Swedish 1-m Solar Telescope/B. de Pontieu).
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Abb. 2:Variation of the Sun's brightness, as measured by radiometers on
spacecraft since 1978. The total solar irradiance (TSI) increases around
the maxima of sunspot number that occurred near 1980, 1990 and 2001. The
rapid variations are caused by the changing projected areas of spots and
faculae on the solar disk as the Sun rotates on its axis in approximately
27 days.
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The Earth's temperature is determined mainly by the Sun's energy
output: its its brightness. Sunspots are dark, they reduce the Sun's
 brightness (MPEG movie, 7.5MB). .
If sunspots were the only kind of blemishes on the Sun's surface, the
increased spottiness over the past centuries would caused the climate
to become cooler not warmer, so the answer would have been a simple
no. But in addition to spots there are also bright patches on the Sun
called faculae. They are quite small but there are very many of
them (MPEG movie, 7.5MB) .
(Fig. 1).
Their number is largest at times when there are many sunspots
(around the years 1991 and 2002 for example).
The Sun's brightness has been measured accurately since 1978. It turns
out to be about 0.07% higher at times of sunspot maximum than at
minimum (Fig. 2). This is because the faculae, though less obvious
because of their small size, actually have a bigger net effect than
the dark spots. Are this kind of brightness changes enough to explain
historical variations in the Earth's climate such as the `global
warming'?
Sufficiently accurate measurements of the Sun's energy output exist
only for the past 30 years, but observations of sunspot activity for
the past 300 years can be used to extend these data. A theory for the
connection between spot activity and brightness is needed to do
this. The structure of the Sun is known from well-tested theory. This
theory makes a simple statement: apart from the brightness changes due
to spots and faculae, there are no additional, `hidden' brightness
changes. In this way the brightness of the Sun can be reconstructed
since the 17th century. With these brightness variations as input,
computer simulations of the Earth's climate can be made.
With such simulations of the climate the researchers could show that
the effects of spots and faculae is about 4 times to low to explain
the observed climate variations. The 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.
P.V. Foukal, C. Fröhlich, H.C. Spruit, T. Wigley
Original publication:
P.V. Foukal, C. Föhlich, H.C. Spruit, T. Wigley:
Variations in solar luminosity and its effect on the Earth's climate,
Nature (14. September 2006)
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