In the Arctic, the summer ice is melting, making it hard for polar bears and their cubs to survive. Why is the world getting warmer? The heat of the sun is trapped by the “greenhouse” gases that surround Earth—carbon dioxide, methane, nitrous oxide, and water vapor. If there is just the right amount of these trapped gases, the air is warm enough for plants, animals, and people to thrive. But now there is too much greenhouse gas, especially carbon dioxide. Polar bears, and all of us, are in trouble. Robert E. Wells shows why so much carbon dioxide is going into the air and what we can do to help keep Earth cool.