The main gas released by burning fossil fuels is carbon dioxide (CO2). Fossil fuels, which include coal, natural gas, and oil, are composed primarily of carbon. When they are burned for energy, the carbon combines with oxygen in the atmosphere, resulting in the production of CO2. This gas is a significant contributor to the greenhouse effect, leading to climate change and global warming. While CO2 is naturally present in the atmosphere and is also produced by other processes like respiration, the large quantities emitted from human activities, particularly the energy sector, have raised environmental concerns and prompted calls for more sustainable energy practices.