Hi. I'm at a different Ivy though I think our grading is comparable to Columbia's. In my experience, science classes tend to give out lots and lots of Bs and relatively few As. If I were to give a speculative average distribution of grades in the major intro/mid level science classes based on my own and other's final grades and percentiles on exams, I would say that about 10% get As, 10% get A-s, 15% get B+s, 35% get Bs, 10-15% get B-s, and the rest get somewhere in the C range. Obviously this is just what I've been able to surmise from taking classes and comparing scores/looking at histograms, but I think it's fairly accurate. What this means is you have a lot of people clustered in the B range and only the top receiving As. This means your science gpa is going to average somewhere around a 3.3 or 3.4 if you're an average student taking premed science courses. However, if have your ish together, you can probably pull a B+/A- average in your science courses without tooo much difficulty, as long as you figure out a way to study that works for you and you're disciplined. However, if you try and slide through, you're going to end up with a lot of Bs which will hurt your sGPA. Take it for what it's worth. IMO, it's easier to get a B at an Ivy/similar than at a state school, but harder to get an A because the competition at the top is very stiff. You're competing with some of the smartest and most motivated students in the country.
However, worry about getting into college first. Even if you are valedictorian with a 2400, you might not get into an Ivy or similar caliber school.