Mary: If we don't take good care of the earth now, our children and grandchildren will be born into a world with much pollution(污染). There will be fewer trees and more buildings, because there will be more people. 

Jack: I agree with Mary. Things will be worse. The weather in the future will be much hotter. Humans won't be able to live on the earth. We will have to look for another planet to live on. 

Linda: I think the life in the future will be better. Most people in the world will use "green cars", so there will be less pollution and people will live to be 200 years old. As for space travel, I would say, we will visit Mars in about fifty years.