(If there was no risk of failure, it wouldn't be research.) Instead, what we're interested in seeing is if you have a clear handle on the process and structure of research as it's practiced by our discipline.
If you can present a clear and reasonable thesis idea, if you can clearly relate it to other relevant literature, if you can justify its significance, if you can describe a method for investigating it, and if you can decompose it into a sequence of steps that lead toward a reasonable conclusion, then the thesis proposal is a success regardless of whether you modify or even scrap the actual idea down the line and start off in a different direction.
What a successful thesis proposal demonstrates is that, regardless of the eventual idea you pursue, you know the steps involved in turning it into a thesis.
Your thesis proposal should have the following elements in this order.
Of course, the state of the individual projects at the end of the fall will vary, and therefore also the format of the elements discussed below. However, it is difficult to write an abstract until you know your most important results.
Sometimes, it is possible to write the introduction first.
If you enjoy science, you should be able to talk non-stop about it. You need to be sure to understand everything clearly when you choose an essay topic.
Do not hesitate to ask questions if there are some unclear points.
A secondary purpose of the proposal is to train you in the art of proposal writing.
Any future career in Environmental Sciences, whether it be in industry or academia will require these skills in some form.