Your question is supposed to combine the scientific belief that our environment is changing dramatically and very quickly and put these ideas in essay form. That means that first, you must decide what your view is of this statement. Do YOU believe the environment is changing now and then think ahead for 50 years of change. If you believe this, your thesis statement should reflect your belief. An example might be, "In my opinion, our environment will change dramatically because of many environmental pressures which exist today." Then you can decide if you want to use atmospheric pollution CAUSED BY WHAT, water pollution CAUSED BY WHAT and the constant decrease in the ozone layer which is CAUSING WHAT TO HAPPEN. You have plenty of material available because pollution and global warming are hot topics. Be sure to be specific and use examples to prove what you say which prove your thesis. With your conclusion, you might want to suggest what people who read your essay might do to help the situation now.