Voilà pour le Soleil (désolé, pas le temps de traduire, et pas très envie non plus vu la flemme de certains commentateurs) :

"Study acquits sun of climate change, blames humans.

By Alister Doyle, Environment Correspondent Wed Sep 13, 2:49 PM ET

OSLO (Reuters) - The sun’s energy output has barely varied over the past 1,000 years, raising chances that global warming has human rather than celestial causes, a study showed on Wednesday.

Researchers from Germany, Switzerland and the United States found that the sun’s brightness varied by only 0.07 percent over 11-year sunspot cycles, far too little to account for the rise in temperatures since the Industrial Revolution.

« Our 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, » said Tom Wigley of the U.S. National Center for Atmospheric Research.

Most experts say emissions of greenhouse gases, mainly from burning fossil fuels in power plants, factories and cars, are the main cause of a 0.6 Celsius (1.1 F) rise in temperatures over the past century.

A dwindling group of scientists says that the dominant cause of warming is a natural variation in the climate system, or a gradual rise in the sun’s energy output. « The solar contribution to warming over the past 30 years is negligible, » the researchers wrote in the journal Nature of evidence about the sun from satellite observations since 1978.

They also found little sign of solar warming or cooling when they checked telescope observations of sunspots against temperature records going back to the 17th century. They then checked more ancient evidence of rare isotopes and temperatures trapped in sea sediments and Greenland and Antarctic ice and also found no dramatic shifts in solar energy output for at least the past millennium.

« This basically rules out the sun as the cause of global warming, » Henk Spruit, a co-author of the report from the Max Planck Institute in Germany, told Reuters. Many scientists say greenhouse gases might push up world temperatures by perhaps another 3 Celsius by 2100, causing more droughts, floods, disease and rising global sea levels.

Spruit said a « Little Ice Age » around the 17th century, when London’s Thames River froze, seemed limited mainly to western Europe and so was not a planet-wide cooling that might have implied a dimmer sun.

And global Ice Ages, like the last one which ended about 10,000 years ago, seem linked to cyclical shifts in the earth’s orbit around the sun rather than to changes in solar output.

« Overall, we can find no evidence for solar luminosity variations of sufficient amplitude to drive significant climate variations on centennial, millennial or even million-year timescales, » the report said.

Solar activity is now around a low on the 11-year cycle after a 2000 peak, when bright spots called faculae emit more heat and outweigh the heat-plugging effect of dark sunspots. Both faculae and dark sunspots are most common at the peaks.

Still, the report also said there could be other, more subtle solar effects on the climate, such as from cosmic rays or ultraviolet radiation. It said they would be hard to detect."

"Changes in Solar Brightness Too Weak to Explain Global Warming

September 13, 2006 BOULDER

Changes in the Sun’s brightness over the past millennium have had only a small effect on Earth’s climate, according to a review of existing results and new calculations performed by researchers in the United States, Switzerland, and Germany.

The review, led by Peter Foukal (Heliophysics, Inc.), appears in the September 14 issue of Nature. Among the coauthors is Tom Wigley of the National Center for Atmospheric Research. NCAR’s primary sponsor is the National Science Foundation.

In this image from an active solar period in March 2001, colors are shifted to highlight the contrast between sunspots (black and dark red) and the faculae that surround them (bright yellow). During the peak of the 11-year solar cycle, the expansion of faculae outweighs the darkening from increased sunspot activity. The result is a net increase in solar brightness. Click here or on the image to enlarge. (Image courtesy NASA.)

“Our 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,” says Wigley. Reconstructions of climate over the past millennium show a warming since the 17th century, which has accelerated dramatically over the past 100 years. Many recent studies have attributed the bulk of 20th-century global warming to an increase in greenhouse gas concentrations in the atmosphere. Natural internal variability of Earth’s climate system may also have played a role. However, the discussion is complicated by a third possibility : that the Sun’s brightness could have increased.

The new review in Nature examines the factors observed by astronomers that relate to solar brightness. It then analyzes how those factors have changed along with global temperature over the last 1,000 years.

Brightness variations are the result of changes in the amount of the Sun’s surface covered by dark sunspots and by bright points called faculae. The sunspots act as thermal plugs, diverting heat from the solar surface, while the faculae act as thermal leaks, allowing heat from subsurface layers to escape more readily. During times of high solar activity, both the sunspots and faculae increase, but the effect of the faculae dominates, leading to an overall increase in brightness.

The new study looked at observations of solar brightness since 1978 and at indirect measures before then, in order to assess how sunspots and faculae affect the Sun’s brightness. Data collected from radiometers on U.S. and European spacecraft show that the Sun is about 0.07 percent brighter in years of peak sunspot activity, such as around 2000, than when spots are rare (as they are now, at the low end of the 11-year solar cycle). Variations of this magnitude are too small to have contributed appreciably to the accelerated global warming observed since the mid-1970s, according to the study, and there is no sign of a net increase in brightness over the period. To assess the period before 1978, the authors used historical records of sunspot activity and examined radioisotopes produced in Earth’s atmosphere and recorded in the Greenland and Antarctic ice sheets. During periods of high solar activity, the enhanced solar wind shields Earth from cosmic rays that produce the isotopes, thus giving scientists a record of the activity.

The authors used a blend of seven recent reconstructions of Northern Hemisphere temperature over the past millennium to test the effects of long-term changes in brightness. They then assessed how much the changes in solar brightness produced by sunspots and faculae (as measured by the sunspot and radioisotope data) might have affected temperature. Even though sunspots and faculae have increased over the last 400 years, these phenomena explain only a small fraction of global warming over the period, according to the authors.

Indirect evidence has suggested that there may be changes in solar brightness, over periods of centuries, beyond changes associated with sunspot numbers. However, the authors conclude on theoretical grounds that these additional low-frequency changes are unlikely.

“There is no plausible physical cause for long-term changes in solar brightness other than changes caused by sunspots and faculae,” says Wigley.

Apart from solar brightness, more subtle influences on climate from cosmic rays or the Sun’s ultraviolet radiation cannot be excluded, say the authors. However, these influences cannot be confirmed, they add, because physical models for such effects are still too poorly developed."

About the article Title : « Variations in solar luminosity and their effect on the Earth’s climate. » Authors : P. Foukal, C. Frohlich, H. Spruit, and T.M.L. Wigley Publication : Nature, September 14, 2006"