Montreal Protocol CFC refrigerants ozone

A global treaty sharply reduced chemicals that damaged the ozone layer

Scientists discovered that CFC refrigerants could damage the ozone layer, which helps block harmful ultraviolet radiation from the Sun. In 1987, countries adopted the Montreal Protocol to sharply reduce their use around the world.

Writing advanced when symbols began standing for sounds

A symbol that stood for one concrete thing could also be used for another word that sounded similar. This trick, called the rebus principle, made it possible to write words and ideas that were difficult to draw directly.

The Inca built an empire without wheels

The Inca built one of the largest empires in human history without the wheel, draft animals, iron or steel, or a writing system.

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Montreal Protocol CFC refrigerants ozone

A global treaty sharply reduced chemicals that damaged the ozone layer

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