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(soft ambient music)

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[Kristen] Why is the world warming up?

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Let's think of Earth for just[br]a moment as a greenhouse.

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Every second, a huge amount of energy

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smashes into our greenhouse.

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Our atmosphere, the glass roof and walls,

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reflect some of that energy,

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but most of it passes right[br]through and heats everything up.

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Much of that heat used to[br]escape back into space,

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keeping us at a stable, warm temperature.

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But then something changed.

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Humans began to spew enough carbon dioxide

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to fill billions of greenhouses each day.

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What does all that greenhouse gas do?

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It makes the glass of[br]our greenhouse thicker.

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Thicker glass means[br]heat escapes into space

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much more slowly.

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And don't forget, energy from the sun

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is still streaming in[br]just as quickly as before.

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The result is a planet with temperatures

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too hot for our liking[br]and, eventually, living.