You have a lot of time to think about this. As some examples, both daughters changed their majors after arriving at university, and at the point that I graduated from university (as a math major) I still had no idea what I wanted to do with my career. We each figure it out over time.
I think that we can each suggest some ideas to think about from our own experience, but it is perfectly reasonable and to be expected that you will take multiple years to figure this out. Also, once you get to university you will have opportunities to try a bit of this and a bit of that, which might help you to decide.
Medical school is indeed a very long path that requires a great deal of time, effort, and commitment. The closest we come to this in my immediate family is a daughter who is solidly on track to get her DVM in May. I have said that the most important of the many skills needed to get there is a strong determination that this is the right path. An MD adds some number of years of residency to the path (after getting the MD) which makes it an even longer path. I think that you need to be very strongly drawn to this career to want to pursue it. On the other hand, you can major in almost anything for your bachelor’s degree and still be on track for medical school as long as you complete the premed requirements. One problem here is that the combination of engineering requirements plus premed requirements is probably close to surpassing the abilities of the most dedicated and talented human possible. At some point each student needs to choose (but this point is definitely not when still in high school, and probably not freshman year of university).
Improving the environment can come in a few large steps, but is more likely to come from a huge number of small steps. A few years back we had our septic system fail, and needed to get a civil engineer to design a new one. They took soil samples in various locations (including digging holes that you probably could have hidden a car inside), and then picked a spot, and then had a “discussion” with the local conservation commission regarding the location. After a lot of discussion and a few more holes (with room to bury a few more cars) they all agreed on a spot that allegedly was a bit better environmentally compared to the first spot. Civil engineers do get to do some work that is related to minimizing the environmental impact of various projects.
PhD’s are usually fully funded in North America. However, “fully funded” means that the student spends a lot of time supporting various professor’s research projects, and the university pays for tuition and fees and hopefully health care insurance, and gives the student a stipend which is sort of barely enough to live on. 6 or 7 years of this is indeed a hit on one’s long term financial situation. I think that this is more likely to be tolerable if the parents can help a bit financially. As one example, apparently the stipend counts as income for contributing to an IRA, but the stipend is going to be small enough that the student is not going to be able to both survive financially and afford to contribute to the IRA without parent support, and a 6 or 7 year postponement of the beginning of funding an IRA will make a big difference 40 or 45 years later when the student reaches retirement age. Of course 6 or 7 years of very low income also impacts things like saving to purchase a first home. Getting a PhD I think again falls into the “you have to be drawn to do it” category. Once you get a PhD, the pay is generally not sufficient to justify the amount of time and effort that it took to get there.
I have two wild thoughts that are environment related but I have no clue whether either of them is even remotely practical.
One is that I keep hearing about plastic pollution in the oceans. I wonder if someone is working to come up with alternate types of plastics that will completely degrade over time. Of course I do not want my plastic milk jug to degrade while there is still milk in it, so I have no idea whether this is even possible.
Good luck to them! Apparently someone has thought of this. It would be huge if they are successful. Of course with research there is always a chance that you will fail, and a chance that someone else will succeed before you succeed (the second airplane to fly did not get much press, nor did the second person to invent the telephone).
My other thought vaguely relates to what I did for a living. I was a math major. I got involved at some point in modeling dynamic distributed systems that evolve over time. I see people where I live deploying electrical generation that fluctuates over time (such as solar panels and wind panels). Where I live you can go on-line and see the instantaneous electrical demand and the current mix of how it is being satisfied. I have wondered to what extent the utilities or the government or others have run detailed dynamic simulations to determine whether or not various forms of generation that are being deployed actually help to reduce overall emissions, and how this impacts the total cost of the overall system, and how much and in what way this could all be impacted by for example modest increased storage capabilities. This seems like an area where there would at least in principle be an opportunity to use some combination of advanced mathematical techniques, dynamic algorithms, and/or networking algorithms to analyze what is possible and what is pragmatic. I have no idea to what extent this is being done, or to what extent the quantitative reality of how various solutions can work impacts the political reality of what gets funded.
As one example, I know several former engineering majors who work as lawyers (having gone to law school after getting their engineering degrees, and in some cases after working as an engineer for a few years). They do work on legal issues that have something to do with engineering.
I agree with other responses that there are quite a few different types of engineering jobs, and that engineering is a major that is a lot easier to transfer out of rather than to transfer into.
There are a lot of options, and a lot to think about. Fortunately you have time to consider multiple options and to get to university and try a bit of this and a bit of that before you pick a career path. Also career paths will often evolve over time.
And best wishes.