But 80% of the world is now unhabitable giving rise to food shortages.
Uhm, am I missing something from 5th? Where are you getting that number from?
It might not be an untrue guesstimation. There have been a LOT of ecological disasters like toxic spills and powerplanets going nuclear since the awakening. And on top of that nature has made one hell of a comeback with awaken critters and plants and wild spirits now roaming around the wilderness trying to keep "civilized" folks out of it.
Was thinking about this, and did a little research...
At current growth rates, population IRL will hit an estimated 9.2 billion by 2050; SR had a couple of pretty serious plagues, but generally it's accepted that 1988 Shadowrun had 5 billion inhabitants in 2050, and current SR has roughly 7 billion - in other words, SR's population sticks with current RL numbers for convenience's sake.
Earth's land surface is only about 29.2% of its surface area. Of that area ...
Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.
Therefore, approximately 11.68% of the current world is being used for cropland and pasture, with, I would guess, maybe another couple percent that's city and the rest - so let's be generous and call it 14%. That's already 85% of the world that's not lived on - and half of the land area (14.6% of the planet's total surface area) is either desert (14%), high mountains (27%), or other less suitable terrain. (Third paragraph of
http://en.wikipedia.org/wiki/Earth#Human_geography.) So really, we're already living on less than 20% of the earth's surface. If you're talking 80% of the land surface, though, you're going from living/farming/growing on 14% to living/farming/growing on 5.8%.
Considering the disasters, plagues, etc. - yeah, from 14% to 5.8% is within (the top end of) the realm of possibility. Considering how aquaculture and high-yield protein 'crops' (mycoprotein, soy, etc.) is so important in the SR world, and even middle-class people eat it with moderate regularity ... y'know, going from 50% to 20% of the land mass lived on isn't too far-fetched.