What is the faint sun paradox?

The “faint sun paradox” has to do with the relationship between the sun’s size and Earth’s climate.

Astronomers believe the sun is about 15% larger in radius now than it was very early in its history, about 4.5 billion years ago. The sun’s surface is thought to have expanded, and, with a larger surface, the sun can emit more heat and light. 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. How could a smaller, less luminous sun create a warmer planet Earth? 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. Carbon dioxide and methane are “greenhouse gases.” That means they trap heat near Earth’s surface.

Over billions of years, scientists say, both geological and biological processes have removed most of the carbon dioxide and methane from the atmosphere. Now we have mostly an oxygen-nitrogen-water atmosphere. So Earth’s surface has cooled from the first billion years or so of its history, even though the sun is thought to be hotter now than it was then.