The gas responsible for the greenhouse effect is **carbon dioxide (CO2)**. It is a naturally occurring gas in the Earth's atmosphere and is produced by various processes, including respiration, combustion, and volcanic activity. The greenhouse effect refers to the process by which greenhouse gases trap heat from the sun in the atmosphere, keeping the Earth warm enough to support life. However, human activities, particularly burning fossil fuels and deforestation, have significantly increased CO2 levels, contributing to climate change and global warming. Reducing CO2 emissions is crucial for addressing environmental challenges and promoting sustainability.