In an era when the sun is dimmer than today, the earth is not covered by glaciers because of greenhouse gases, so life can be nurtured.
This is a big puzzle of climate science. In the first two billion years of the solar system, the intensity of sunlight was 20% to 30% weaker than today. If the atmosphere of the earth was the same as it is now, the earth would be frozen for centuries, and life would be difficult to maintain, let alone reproduce! However, there is conclusive evidence that this was not the case at that time. The Earth's surface is rich in liquid water, and life has appeared, showing a thriving scene in the sea.
In order to solve the "Dark Sun Paradox", people put forward several hypotheses. Today, the latest climate model can simulate the possible climate conditions of the early Earth, and can also explain this paradox: there were a lot of greenhouse gases in the atmosphere at that time.
Since the sunlight at that time was weaker, the evolution of life was slower. Living things on the earth can neither release a large amount of oxygen nor convert CO2 into carbonate. There is no vegetation on the land, so silicate rocks will not deteriorate because CO2 is absorbed by plant roots. The CO2 content in the atmosphere is 10 to 100 times higher than today! There is no oxygen in the air, so methane (CH4) can exist for a long time without being converted to CO2 (today, methane can be converted to CO2 within ten years). Therefore, the methane content in the air is much higher than today.
With such a large amount of greenhouse gases, even if there is a "cold" sun, the young Earth can maintain a warm climate similar to today's, and help maintain liquid water on the surface. Today, greenhouse gases can maintain the average surface temperature at about 15 ℃. Without them, the average surface temperature will drop to - 19 ℃.
Sunrise and sunset seem to record the rhythm of the sun's life. In fact, it's just the rhythm of the earth. The sun has its own experience. It has gone through childhood, and now it has completed half of its life journey.