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Do sunspots affect the Earth's climate?

Temperatures on Earth have increased over the past century (`global warming'). So has the number of sunspots. Are these facts related? Can sunspots have caused the global warming? According to a review of existing results and new calculations performed by researchers in the United States, Switzerland, and the Max Planck Institute for Astrophysics the answer is no.

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).

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.

The Earth's temperature is determined mainly by the Sun's energy output: its its brightness. Sunspots are dark, they reduce the Sun's movie.gif 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 movie.gif 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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