1. If people don't stop dumping waste into rivers, pollution will increase. 

2. If the polar ice caps melt , huge landmasses will be under water.

3.  If the air in city is polluted, it can cause peole's respiratory problems.

4. If smog is frequently formed in the city, it will cause difficulty in breathing , headache and even lung cancer.

5. If carbon monoxide concentrates in great amounts, it will be harmful.

6. The climate will change if more trees are cut down for hardwood.

7. If we do not control pollution soon, it will be too late.

8. If we do not protect the frontier forests, we will suffer from many natural disasters.