Coruscant on Earth

[Unorthodox]

The problem is that for every X number of people, you need Y acres of food production. 

This coupled with the problem that Cities were historically built around good farmland and now have grown to overcome that just aggravates the problem. 

I would concede that if we wanted ubercities we could theoretically relocate the populace to areas unsuited to farming and probably find ways to feed them. 

California is a good example.  Most the coast is great farmland, but sprawling cities have destroyed a lot of it for that purpose, and the increasing population is depleting the water reserves and destroying the water table for farming.  That's before we get into the pollution problem. 

As for the carbon sequestration, it's an attempt to stave off global warming which is just as bad for farming.  There's more than enough carbon around for biomass purposes even if we are able to sequester a sizeable portion. 

[Mart]

The problem is that for every X number of people, you need Y acres of food production. 

Yes, however this number is not "solid" or "constant" We can influence it.
And there is something, that already came to my mind some time ago:
Southern hemisphere has, approximately:
32 % of land area on Earth
10 % of Earth population

This where I would see in some near future possible agriculture, when considering where to put my simple crops.
If we come to a conclusion, that at present at least, we do not face serious shortages of nutrients, from carbon to let's say zinc and other like that (trace elements), then we still have room for producing more.
Food problems would be then due to economical, social, political, efficiency issues. Also, what is my personal view, some countries, that we consider developed, could have larger percentage of people working in agriculture. Here is some data:
http://data.worldbank.org/indicator/SL.AGR.EMPL.ZS
United Kingdom, 1%
Argentina, 1%
China is here estimated 35%

Even if right now economy is not such as more people need to work in agriculture, later that may change. What is my impression, is that our lifestyle puts limits here. We like subconsciously want certain state of things to last, e.g. we do not want that particular region to be changed to farmland, because we want to have Sahara there(?) we want to have that land free of migrants? or we like to live in northern hemisphere and not consider, that there is a lot of empty prairie to settle on, or in another words, we are sentimental to our ancestors' land.