Greenhouses work by trapping heat from the sun through glass panels that let in light but keep heat from escaping. The greenhouse effect occurs when greenhouse gases in the atmosphere absorb and re-emit infrared radiation from the earth, trapping heat and warming the atmosphere. The major greenhouse gases are water vapor, carbon dioxide, nitrous oxide, methane, and CFCs. While the greenhouse effect makes the earth warm enough for human life, increased greenhouse gases are causing global warming and climate change that threaten ecosystems and cause problems for plants and animals.