What environmental pressures do you think may affect humans as earth becomes more populated with humans in the future

Scarcity of natural resources.
Scarcity of land as it is taken up by agriculture and expanding housing development.
Not sure if this is what you're looking for, but something along these lines. As population increases (and it is, exponentially) we are loosing both space and non-renewable resources. 
An interesting topic to research, for certain.