Why is the land no longer farmed? Could it be there isn't enough topsoil left to be fertile? Could it be that there are new crops of McMansions and ever further commutes as the core of our cities is abandoned?
If population has gone up, so food needs have gone up, and total land under agriculture has gone down, and the planet has greened... that means that farming efficiency has increased by so more than enough to feed the planet's people that it's not profitable to keep so much land under agriculture, and most of the land that is no longer under agriculture didn't get paved over -- it's forest now.
