Haveyouevergottenasunburn?Ifyouhave,you’vealreadylearne...

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Have you ever gotten a sunburn? If you have, you’ve already learned the hard way about the sun’s ultraviolet (紫外線的), or UV light. It can burn your skin so bad that it turns it red or even makes it peel off. The    51    helps you remember to wear sunscreen the next time.

Well, Earth has its own version of sunscreen, the ozone (臭氧) layer, which    52    us all from the vast majority of the UV light released by the sun. Without it, we wouldn’t just get a sunburn. Life on Earth would go    53    due to the hugely damaging power of all those UV rays.

The ozone layer    54    Earth’s stratosphere (同溫層), a part of the atmosphere that    55    from about 10 kilometers up to nearly 50 kilometers above the ground. Despite its name, the ozone layer isn’t just ozone gas. It contains relatively higher concentrations of ozone than the lower atmosphere, but that’s still a small amount    56    those of the main elements of the atmosphere. Even so, it    57    a lot of UV rays, preventing them from reaching the surface of Earth.

However, people weren’t fully aware of its importance until 1985, when a huge hole in the layer was found over Antarctica.

The    58    quickly pointed to a kind of chemical called chlorofluorocarbon, or CFC, which was widely used in refrigerators, air conditioners and hairsprays. CFCs are able to rise up to the stratosphere and cause reactions that destroy ozone. With a    59    ozone layer, people on Earth are more likely to develop skin cancer, cataracts (白內障) and other health problems due to too much UV light    60   .

As a result of this discovery, an international treaty (條約) called the Montreal Protocol was signed in 1987 to    61    the manufacture of CFC products so that the ozone layer could slowly recover and return to its natural state by 2050.

   62    , at a time when it seemed that everything was going back to    63    , earlier this month scientists detected four new man-made gases in Greenland and Australia that could causes new risks to the ozone layer.

Scientists haven’t identified the    64    of the gases, but “this highlights that ozone loss is not yet yesterday’s story,” said Professor Piers Forster from the University of Leeds, UK, to BBC. Scientists believe that there are more such gases out there, and they still have much work to do to “   65    the holes”.

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