Global warming is increasing the average temperature of the Earth's atmosphere because more of the sun's energy is being trapped by greenhouse gases. The Earth has always had a greenhouse effect that warms the planet and supports life. However, global warming is like a greenhouse with glass installed backwards, trapping too much heat. Evidence from past volcanic eruptions that cooled the planet shows what happens when the sun's warmth cannot reach the Earth - crops fail and people die. Now, too much heat is being trapped in the atmosphere, causing temperatures to rise faster than ever before in history.