Greenhouse gases (GHGs) are gases in Earth’s atmosphere that trap heat. GHGs drive the greenhouse effect - the heat-trapping effect - which is important to keep the Earth warm enough to sustain life on our planet. Without the greenhouse effect, the temperature of Earth would be similar to conditions experienced on the moon - cold and inhospitable.
But the amount of heat our planet can endure has a limit. As the concentration of GHGs increases, the atmosphere traps more heat, and Earth grows warmer.
How are humans responsible? For decades the global population has increased and we have continued to rely on burning fossil fuels, which generates GHGs. While some GHGs like carbon dioxide occur naturally in the atmosphere, our activities have drastically increased the amount in the atmosphere so that there exists far greater than can be naturally pulled out. Carbon is stored in vast amounts in plants and underground deposits (like coal & natural gas). By burning these storage centers at a global industrial scale, we've converted harmless carbon into atmospheric carbon dioxide in amounts so great that it is actually changing the entire planet’s climate. That should shock you.
What’s more, we've even been adding greenhouse gases to the earth's atmosphere that are NOT naturally occurring! Man-made gases include the chlorofluorocarbons, hydrofluorocarbons, and perfluorocarbons, as well as sulfur hexafluoride. Atmospheric concentrations of both the natural and man-made gases have been rising over the last few centuries due worldwide industrialization. Parallel to the increase of all of these GHGs, the temperature of the planet has dangerously increased.
So while GHGs play a natural role in sustaining life on Earth, humans have drastically increased the concentration of gases that drive atmospheric warming beyond what the planet can handle.