The sun is about 40% than it was 4.5 billion years ago, but the Earth's surface is cooler. What has changed over time? Mainly, the amount of greenhouse gases in our atmosphere has fallen greatly. It is estimated that, if the atmosphere still had the same composition that it had 4.5 billion years ago (very high in greenhouse gases), the Earth's surface temperature would be about 300 C. 

To put this more simply, if the sun alone determines Earth's surface temperature, why has a much hotter sun actually resulted in an cooler surface temperature?