[jessaustin]

That link performs a calculation based on some fairly back-of-the-envelope physics assumptions. Its conclusion:

Roughly one-half of the solar luminosity increase occurs during the last 2 billion years but there is no evidence for a parallel increase in the Earth's mean surface temperature. Indeed, isotopic studies of the Precambrian samples by Knauth and Epstein (ref. 12) indicate that the mean surface temperature has been decreasing during this time. Clearly, there is a need for further studies of the effects of crustal movements and volcanism, biological activity, etc. on the long-term evolution of the Earth's climate. At present, it appears that the effects of solar evolution are still buried in the "noise" due to other uncertainties in paleoclimatic models.

Is there anything since 1981 with more definitive conclusions? Is there any study that has attempted to measure sun output directly or indirectly, rather than simply calculating output based on the sun's age?