How to identify where a creaking/squeaking sound comes from?
As a bike mechanic on a college campus, I got this question a lot. From my experience, it usually is either the pedal or the bottom bracket. Considering the noise is happening when you are putting torque on your pedal, I would go with it being a bottom bracket problem. Check for play by pulling on the crank back and forth to see if there is some wiggle room, but even if there isn't, it could be that it's just dry in there and you're feeling the ball bearings more than usual. 

If you've never taken apart a bottom bracket, I would consider getting some help first. If you mis-align the threads screwing the cup back in, you could seriously damage your bike. I'm not sure what type of bottom bracket you have, but if they are loose ball bearings, I would give them a re-greasing, and then make sure to tighten it up so there's no play while also having it loose enough to turn with the greatest ease possible. Even just doing that could get rid of some noises. 

You could also take apart your pedal on the left side and check the ball bearings in there too, and give them a re-grease. 

If this is a bottom bracket problem like I predict, and you let it go, your ball bearings will eventually turn into misshapen balls and that will dent the cups in which they roll, which will just totally suck to ride because it will be hard to pedal and annoying to listen to the persistent noise. I doubt it would ever make you crash though. 

This is a personal picture I took of a bottom bracket with the same complaint as yours, and these are their ball bearings and cage. I'm not sure how he rode it for so long like that actually.


Hope this helps!