As of now, the real big physical limit we have for resources are: land, water and air (maybe, sunlight in the future).
Assume a 10x increase in the human population (i.e. from the 7 billion, the human population reaches 70 billion) in the next 50 years (2070). The obvious questions are: where do we get the land for this population to live in? where is the land to produce food for this population? and where is the water for this population to consume?
I am going to assume that space travel (at-least to near Earth objects) is possible by then. However, this will still be early days of space travel - meaning that this is available for the adventurous types.
Food production can be done outside of earth - i.e. on moon, in space itself (like in ISS); or on Mars. The advantage with this approach is that most of the human population doesn't have to space travel. The adventurous types can grow food outside of Earth and bring it back to feed the population.
How do we find the water for the 70 billion humans and other animals? Even now, water is becoming scarce and it will not be long before there are skirmishes for water. However, since the entire food production has moved out of Earth, the water requirement for agriculture has reduced.
Also, by 2070, I expect there to be scientific advances in this area: Scientists need to figure out how to make water on Moon and Mars. Also, it is possible that there is progress on de-salinating water from the oceans and making it fit for human consumption.
This brings us to land for the population to live in. Perhaps if agriculture can be done on Moon/Mars, quite a bit of the agricultural land can be converted to housing. Not sure if this is enough.
I hope that we leave the forests alone. Also, I hope some of the farm land can be converted to forest land.
