The Earth, our home, is a very special place. It is special because it is the only place in the universe where plants and animals can live. Scientists have found hundreds of other planets in our galaxy, but none have the right conditions for life. Over the past half billion years, millions of different species have evolved. Life makes our planet beautiful.

Life depends on key ingredients: water, air, food and warmth. The last ingredient, warmth is tricky. Too much heat, and life will burn up and too little and it will freeze. For hundreds of millions of years, life on Earth has been possible because temperatures have stayed within a narrow range: not too hot, not too cold. Some scientists call ours the Goldilocks Planet.

One of the main reasons that Earth stays such a nice temperature is because a blanket of gases called the atmosphere traps heat from the sun, and keeps it from escaping into space.

The average temperature across the whole planet today is about fifteen degrees (Celsius), the same as it is on a nice spring day. Without our atmosphere, average temperatures could be as low as –18 degrees.

Almost everything would be frozen, like Winnipeg in January. This is called the greenhouse effect, because the atmosphere keeps us warm the same way that the roof of a greenhouse keeps the flowers inside it warm.

The most important gases greenhouse gases are water vapour and carbon dioxide. On a warm day, you can see steam rising from oceans, lakes or even from the roof of a car. All that water in the air helps keep us warm.

Carbon dioxide, or CO2, is in the air you breathe out when you exhale. It comes from burning trees and other plants. It also comes from burning oil, gasoline and
coal. These fuels are called fossil fuels because they come from plants that died millions of years ago. All that CO2 also helps keep us warm. Several less common other gases like methane and nitrous oxide also have a greenhouse effect. The greenhouse effect is a good thing, because without it, we would freeze.

But remember the story of Goldilocks and the Three Bears… One planet is too hot, one planet is too cold, but this planet is just right.