As that heat rises, some of it is     1     by gases in the atmosphere and reflected back towards the planet’s     2    . Like the glass in a greenhouse     3     holds heat inside the building, gases in the air hold in heat around Earth. Scientists call this global heat-trapping     4     the greenhouse effect. The greenhouse effect keeps the temperature of our planet’s surface comfortably warm.

Several gases in the atmosphere     5     to global warming. The one you’ve     6     heard the most about is carbon dioxide. Carbon dioxide is a good heat-trapping gas—a greenhouse gas,     7     scientists say.     8     certain amount of carbon dioxide in the atmosphere is     9    . In fact, you add some     10     the air every time you breathe out.