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 the city is polluted , it can cause people'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 don't control pollution soon , it will be too late
8. If we don't protect the frontier forests, we will suffer from many natural disasters