What causes the temperatures in the lower troposphere to be warmer?

Temperatures in the lower troposphere are warmer primarily due to the Earth's surface absorbing solar radiation and then re-emitting it as infrared radiation. This heat is trapped by greenhouse gases, such as carbon dioxide and water vapor, which enhance the greenhouse effect. Additionally, the density of air in the lower troposphere allows it to retain heat more effectively compared to the upper layers of the atmosphere. Consequently, this results in a warmer lower troposphere compared to higher altitudes.