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Do you want to get a PhD and become a scientist or are you more interested in a policy type position after your four year degree?
If scientist is your goal, you should take as much chemistry and physics as you can handle. I think environmental science anywhere is light weight on science and thus you may find yourself in a policy job afterwards. If you want to be a scientist, major in chemistry, or physics at CU, both are outstanding majors. You can minor in environmental science, if you wish. If you want to work in policy then by all means major in environment “science”. You can see my bent here. I am sure there are other opinions. With that, there is a LOT of climate change research at CU , NCAR and NOAA, all in Boulder that you can get involved with. Much of it is very mathematical and thus you may need a math degree or meteorology if you want to study climate. Computer science is also useful for studying climate. Environmental scientists sometimes get PhDs in say environmental chemistry and work for a variety of private or government labs. Just be sure to take a LOT of chemistry, more than the major requires, to be sure to get into graduate programs. Look right now at what most PhD programs require, and you will see its heavy on physics and chemistry.