Earth’s atmosphere resembles the glass in a greenhouse. It lets in sunlight but does not allow heat to escape. A few gases cause Earth’s greenhouse effect. Water vapor, ozone, and carbon dioxide are the main greenhouse gases. The air is made up of only a small fraction of these gases. Human activities are responsible for putting increased amounts of greenhouse gases into the atmosphere. Earth’s climate might change because of these gases. Each small increase in these gases causes our planet to become warmer.