It shows that solar luminosity was significantly lower in the past, which counters the denier argument that "well if CO2 was 6000 ppm back then, why wasn't there immense warming?") But I do not understand this graph. Why is temperature so constant? The Sun is the perfect example of a blackbody -- it absorbs all radiation incident upon it. (And everything else, too.) So why doesn't it follow the Stefan-Boltzmann law, where
luminosity = sigma*T4 ?
I'm confused. Can anyone help me?