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In a greenhouse used for growing plants, sunlight comes in through the glass roof and walls and is turned into heat rays. Heat rays are shaped differently than light rays and cannot easily get back out through the glass. So, the inside of the greenhouse stays very warm. The "greenhouse effect", or global warming, is the gradual warming of the Earth because carbon dioxide and other gases in a layer around the Earth prevent heat from escaping to space.
The layer of gases is supposed to keep some heat close to Earth. The problem is that humans have increased the amount of carbon dioxide in the atmosphere by more than a third since the Industrial Revolution and so the layer of gases is holding in too much heat. Millions of cars, and power generation plants and factories all over the world, are burning fossil fuels (e.g. gasoline, coal), which puts more and more carbon dioxide into the atmosphere. At the same time, millions of trees that could have helped to absorb the carbon dioxide are being cut down.
During the past 100 years, the average global temperature has increased about 0.75 degrees Celsius. The ten hottest years on record have all occurred within the last fifteen years. A global temperature change of only a few degrees could make a big difference. The average world temperature during the last Ice Age was only 5 degrees C lower than it is now. Global warming of
1 to 5 degrees C could cause a major shift in weather patterns. We're already seeing more droughts and more intense tropical storms, warm areas becoming extremely hot, and melting of polar ice caps. If the current pattern continues, there will be a rise in sea level, flooding of low-lying areas (e.g. state of Florida, country of Bangladesh), and major habitat changes.
What can we do to prevent global warming? Part of the solution lies in reducing the burning of fossil fuels and making sure we have lots of trees. A growing tree can take in more than 20 kg of carbon dioxide a year.
Get a feel for how global warming works with a simple experiment. Place two jars on their sides, in the sunshine.
Put a piece of dark cloth into each jar. Place a thermometer on each cloth so that you can read the thermometer through the glass.
Put the lid on one jar. Turn both jars so that their open end faces away from the sun.
Watch the thermometers. Record the temperatures every minute. When a thermometer gets close to its highest temperature, stop the experiment (the thermometer may break).
In which jar does the temperature rise fastest? How much faster does it rise? Why? How is this like a greenhouse?
Try the same experiment on a cloudy day. How are the results different?
You can experience the greenhouse effect for yourself on a hot summer day. Get into a car that's been parked in the sun, with the windows closed. The car's interior should be very hot – even if the outside air temperature is just pleasantly warm. The closed car acts like a greenhouse.
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