The gas that causes global warming is Carbon Dioxide (CO2). It is a colorless, odorless gas that is produced when we burn fossil fuels, such as coal, oil, and natural gas. These fuels are the primary source of energy for electricity, transportation, and heating, but the burning of these fuels releases large amounts of CO2 into the atmosphere. The CO2 builds up in the atmosphere and acts like a blanket, trapping heat from the sun and causing the earth's temperature to rise. This is known as the greenhouse effect, and it is leading to climate change and global warming.