In the last 100 years or so, over population has become a major problem that people do not wish to address. It is a difficult one to address because you are potentially talking about people's lives.
Over the last 100 years, as the population increased, Green house gasses increased, poverty increased, food shortages increased, education declined (in some places), and so on. All these things continue to grow.
Over population is a problem. It's the biggest problem. Earth can only healthily support about half the people that are currently on this planet. I'm not saying that population should be regulated, but we should know the problem at least.
Sure, population makes for less resources but even that could be resolved if not for greed.
Unfortunately, greed is part of the human condition. I am not sure if it is a natural trait, learned behavior or even both but it is at the root of just about every problem this world faces. When I say "greed" I am not just talking about money but other things that people want more of as well like power.
Money is probably the main thing because with enough of it, we can either get or feel we can get the rest. Look at the majority of prisoners. Most of them committed crimes including murder because of greed. Look at the majority of homeless or poor. They are also victims of other peoples greed. At one time I heard a statistic but I am not sure how accurate it is, at least not today, but it said that about 90% of the worlds wealth is in the hands of about 10% of the population. There are billion and trillionaires who would think nothing of spending more on one meal than a family of four may spend on the weeks groceries, yet we have people that starve to death. When people are poor and see others flaunt their money, they want that life style so much that some may literally kill for a piece of it.
If greed did not exist, there would be plenty to go around. Even though the population of every country rises every day, they all still have enough space that we could all have enough food, water, shelter, medical care and so on. Crime would also be a rare thing because everyone would have everything they need and nobody would have an unfair amount more that draw envy.
Clearly, population would take a back seat to greed as far as problem in the world.