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.
'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 befPage: 1 2 3 Related medicine news :1
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