What are we planning on doing, digging up a trillion tonnes of the formerly fertile soil from previous areas and transporting it? I very much doubt that Antarctica and northern Russia have soils that are as amenable to agriculture after a few millennia frozen solid. Even if the area were enough, which it isn't, yields would likely plummet.
Civilisations collapsed in the distant past, migrated and fell, yet there was a glut of available land everywhere. So hardly surprising here we still are -- we were nowhere near limit. Somewhere else fertile and usable could be found. Borders weren't controlled.
To give a rough example, the UK currently has a population around 68,000,000. The black death in the 1350s led to the population falling from around 3m to 1-1.5m. London's population fell 60% from 120,000. I don't know the population during the Roman civilisation period, but well under a million. 200k perhaps? That's a hell of a lot more area per person, and a huge amount of scope to relocate come catastrophe.
p.s. The West has more forest than a century ago because most of Europe's forest was clear cut centuries ago. What's been planted particularly in the 19th and first half of the 20th century is monoculture forestry, completely unrelated to the former temperate rain forests and other mature native forests that once thrived, long ago. It's certainly nothing to crow about.