The human population today is approximately seven billion. Many parts of the Earth (like Siberia, Australia, Northern Canada, the Amazon) are still...

I would have to say that the answer to this question is that these areas do not offer relief to the world population growth mentioned in this scenario.  There is a reason that these areas of the world are on populated:  they are not ideal for human habitation.  If they are geographically habitable, they are being used for purposes that support the population, especially agriculture (like the areas in the middle of the United States.)  I have attached a map to your question as a reference for the rest of the answer.

You can tell by the map that humans like to live near water sources.  The majority of the dark areas are near water.  This is because urban areas are located near bodies of water for transportation and all of the economic benefits that water offers.  These areas will most likely need to absorb the additional population in the coming centuries.

The areas that are light on the map are difficult environments to live.  They are dense jungles or harsh deserts.  The areas to the far north and south are cold. While it is possible for people to live in these environments (and some do,) the regions cannot support large population density.  They are not attractive places for people to locate business and industry.  This means that jobs would not be available for large populations.  Additionally, they are so unattractive as a place for people to live that most would choose to live in crowded cities and suburbs than to relocate to these sparsely populated areas.