We talk about what's called the faint sun paradox. Because the sun used to be smaller, astronomers say that the sun was less luminous billions of years ago. But here's the paradox. The fossil record indicates that Earth had a warmer surface billions of years ago than it has now. The answer lies in the composition of Earth's atmosphere over time.
Scientists believe that our atmosphere used to consist mainly of carbon dioxide, methane and water.
That means they trap heat near Earth's surface. For today, that's our program. With thanks to the National Science Foundation, I'm Deborah Byrd with Joel Block for Earth and Sky.