Will there be enough room for everyone? Will there be enough food and water if Earth's current population of 7 billion doubles, as some prognoses regarding demographic growth predict?
During the time of the building of the Great Pyramids, Earth's total population was no more than 30 million people. 4000 years later, when Big Ben was built in London, this number rose to an impressive 1 billion.
The 1st thing the world will need is more, and larger, residential buildings. Modern-day American skyscrapers will seem like miniature models compared to the projects for the billions upon billions of people, since the only practical direction for expansion will be up. Construction crews would need to build 200-story skyscrapers.
But even this won't be enough. Millions will be living in public places such as malls; tent cities will spring up nearly everywhere.
Housing won't be the populace's biggest problem. Food and water will be limited. To be able to feed Earth's doubled population would require an agricultural area the size of the US. This would in turn require chopping down thousands of square yards of forest.
Once wheat and dairy product reserves are depleted, billions around the world will begin to starve. Food prices will rise drastically - a trip to the supermarket may cost you $700.
The US is among the largest grain producers in the world and the scarcity will be felt all too late, with all of the rest of the countries in the world quickly using up their wheat and corn reserves.
Even in the most developed nations, freeways won't be able to handle the growing traffic, thousands will die in car accidents. Gradually, transportation with cars will be prohibited.
The enormous crowds will present their own danger, in which you might suffocate while walking amongst.
Over time, even tap water will become unfit to drink, as more and more people produce sewage and thus pollute our environment beyond repair.
Lack of clean water will lead to a rise in diseases such as dysentery and diarrhea. There will be trash everywhere and the more people, the more rats we'll see. We will witness a reemergence of rare diseases - cholera, typhoid fever.
Only 3% of Earth's water is suitable for use. Shortage of it means mandatory water restrictions - it will be stopped at a certain time and each household will have an exact quantity available for use.
We will need to build new desalination plants which will in turn fill the air with toxic clouds, gradually blocking out the Sun and eventually turning lethal.