Is it just the sun?
Is it possible, as some have hopefully proposed, that changes in
the sun account for global heating?
There has been no overall increase in the
energy put out by the sun. It is not a variable star (will not become
so until it moves toward its red giant stage in about 5 billion
years). It is, over human time-scales, remarkably constant, as shown
by 31 years of measurement.
But could sunspots provide a cause of heating,
in some more complex way? A Danish space scientist, Eigil Friis-Christensen,
published in 1991 a graph showing a close correlation between global
temperature and variations in the length of the roughly 11-year
sunspot cycle. He and Henrik Svensmark proposed a complex mechanism,
involving cosmic rays and cloud cover, by which the sunspot cycle
could have this effect. This "natural cause" is much used, usually
without understanding its complexity, by deniers of human-caused
heating. Others showed that the data were flawed in several ways,
and it is now "undisputed" among scientists in this field, including
Friis-Christensen, that sunspot-cycle-length and global temperature
have actually diverged gone in different directions on the