The earth is 4.6 billion years old.
地球有46亿年的历史了。
We humans have lived on earth for only 35 000 years, but during this period we have changed our planet a lot in many ways.
人类只在地球上存在了3.5万年，但是在这段时间内，我们从很多方面改变了地球。
Some things we've done are very good for the earth while others are bad.
我们做的有些事对地球很有好处，有些事却对地球有害。
All over the world, people have cut down millions of trees.
全世界的人们砍伐了无数棵树木。
When it rains or when the wind blows, the earth is taken away.
刮风下雨时，地面的土壤都会被带走。
Forests have become deserts, so many kinds of animals and plants are disappearing.
森林变成了沙漠，很多动物和植物都在消失。
In big cities, cars and buses have polluted the air.
在一些大城市，轿车和汽车都在污染空气。
Now many people in cities have health problems.
现在城市里的很多人都有健康问题。
Factories have also polluted the land and the water.
工厂污染了土地和水源。
As a result, many rivers and lakes are now dead.
因此，很多河流和湖泊现在都没有了生气。
Around the earth, there's a special kind of oxygen call "ozone"(O3).
地球周围有一种很特别的氧，名为“臭氧”（O3）。
It is important to the earth.
它对地球很重要。
But now air pollution is destroying it and making a very big hole in the ozone layer.
但是现在空气污染正在破坏它，并且使臭氧层出现了空洞。
Too much harmful radiation from the sun passes through the hole and reaches the earth directly.
很多来自太阳的有害辐射都透过臭氧层空洞直抵地球。
This is very dangerous because this kind of radiation can cause cancer.
这很危险，因为这种辐射会致癌。
Carbon dioxide (CO2) in the air has increased a lot.
空气中的二氧化碳（CO2）增加了很多。
It mainly comes from burning oil, coal and wood.
这主要是因为燃烧石油、煤和木头。
This has formed a "blanket" around the earth.
这使得地球周围出现了一个“地毯”。
The heat from the sun can't escape so the temperature is rising.
来自太阳的热量无法散发，所以温度在上升。
This is called "the greenhouse effect".
这被称为“温室效应”。
It causes the level of the oceans to rise and the climate of the earth to change.
这导致海平面上升，地球气候发生改变。
All these problems are very serious, so we must do something now.
所有这些问题都很严重，所以我们必须采取行动。
