What causes most harm in the world? But is completely avoidable.

[invincibleIJIN]

excess population. to some extent, we won't able to fulfill the demands of the population. as more population, more use of resources more consumption of foods, more unemployment, land per person will be a less, more economic burden on the country, more pollution, more animals will die for only food, water crisis,

[cornivore]

Snapshot of a weed in the grass, it's one of those things that make me go hmm... so there's this green plant in the middle of brown grass, with flowers even, yet people decide that's a bad thing, and pull out those plants, only to dump all kinds of water or chemicals on the grass instead.

How dare that plant not be dependent on us for survival!

Oh yeah, that makes an expression "like watching grass grow" sound maniacal.

[brimstoneSalad]

excess population. to some extent, we won't able to fulfill the demands of the population. as more population, more use of resources more consumption of foods, more unemployment, land per person will be a less, more economic burden on the country, more pollution, more animals will die for only food, water crisis,

It's important to remember that replacement rate is economically important to avoid the problems of an ageing population with no younger generation to support them, although anything in excess of that is potentially avoidable sure. However, that's long term, and the challenges we face are immediate, so the most important things to look at are carbon footprint today.

With regards to population, the bigger issue is birth control so people can make the choice to stop having children if they want to when they've had a couple already, and a combination of economic security and low infant mortality so they can trust that their children will live and they'll be secure without having a whole bunch of them (as poorer people often have to do, to exploit child labor for their family's survival).