The growing need for timber means that more forests are being cut down and that's bad because trees breath in Carbon Dioxide and breath out Oxygen and since trees are being cut down that means less Oxygen is being released in the Atmosphere.
Things that make population go up.
These days people have better homes, health, technology and better doctors. Rather then 100 years ago if you were often left to die or they just couldn't find out what was wrong with you. The houses these days are very different to what they used to be.
War.
Although war is horrible it's a pretty good way of getting rid of people but maybe there will be another baby boom? We just don't know.