Quote Originally Posted by GregortT View Post
We should keep in mind that the Earth's warmth is almost completely dependent upon the Sun (other than geo-thermal heating, which is not used very much, and the heat humanity itself creates, which is pretty small as well). If the Sun increased its output by only 1%, the Earth would (within the span of several months to a year or two) become completely uninhabitable. Luckily, the Sun is a very stable fusion engine - it's been going for billions of years, and it will continue to go for several billion more. However, all these temperature fluctuations on Earth could be due simply to the Sun changing its output by a fraction of a percent. That does not mean that humans are completely innocent (I'm sure all these greenhouse gasses don't help the situation any), and it doesn't imply that humans should throw up our hands and say that there's nothing we can do.
While it is very stable, by comparison to some variable stars, the Sun does have some variability of its own. The sunspot cycle runs 11 years from maximum to maximum, and their is a slight increase in luminosity at the peak of the cycle. The sunspot cycle has been shown to have an effect on global temperatures.

There is also a study here (http://www.space.com/scienceastronom...ut_030320.html) which indicates that the Sun's radiation has increased by .05% over the last decade. Though they've only been able to monitor it since the advent of satellite technology, there seems to be some evidence that the increase has been occurring over the last century and may be a significant factor in the current global warming scenarios.