In modern times we humans face changing conditions.

For humans to live,we need clean air and clean water.

Pollutions is now a growing problem around the world.

Pollutions poisons the air and water that we depend on.

Polluted air makes people sick and afraid to go outside.

Polluted water poisons our food supply.

As a result,we never know which foods are safe to eat.

Human beings cannot live in a poisoned environment.

Therefore,pollution is a major threat to our existence.

Humans need temperatures to be in a comfortable range.

To be in a comfortable range means to be neither too hot nor too cold.

With global warming,global tempertures are rising.

As temperatures rise,the polar icecaps will melt.

As the polar icecaps melt ,ocean levels will rise.

Areas of some countries will soon be under water.

People will be forced to relocate from flooded areas.

In nature, even small changes can sometimes have large effects.

It's difficult to predict（预测） what's going to happen.

The entire ecosystem（生态系统） that we depend on is changing.

some of these changes are irreversible（不可逆转的）.

Irreversible changes cannot be undone.

Let's hope the humans are smart enough to understand how the world is changing.

With more understanding ,we can make better choices about what to do.

We can face the challenges of pollution and global warming.

We need to do this before it's too late.

This planet Earth is our only home and we need to protect it.

we need to protect environment that supports us.