Final answer:
Carbon dioxide (CO2) is the gas emitted when brown coal is burned, and it is a significant contributor to the greenhouse effect that leads to global warming.
Step-by-step explanation:
The gas emitted when brown coal is burned is carbon dioxide (CO2). When coal is burned, it undergoes a reaction with oxygen that results in the emission of several gases, including carbon dioxide, which is a major greenhouse gas associated with global warming. In addition to other harmful pollutants like sulfur dioxide (SO2), nitrogen oxides (NOx), and particulates, the combustion of coal, particularly brown coal which has a higher moisture content, releases significant amounts of carbon dioxide. This has a profound impact on air quality, human health, and contributes to the enhancing of the greenhouse effect.