"I want to become a senior environmental scientist and also I want to become famous for my work. I believe that the skills and knowledge that I will gain as a part of my job responsibilities in this position will help me a lot in preparing myself for the future."
For answering this question go through the job description in detail and see what are the various components of the job and which one do you think hold the most importance and prepare your answer accordingly. You need to convince the interviewer that this job will act as a stepping stone in your career and will help you in learning the skills that you will need in order to achieve your goals.
Make sure that you are not bragging about what you want to accomplish or say something that you will not be able to achieve within this organization.