- Anywhere that has farmable soil is farmed. There isn't any north (or south) of sixty. It takes a long, long time to create by natural processes. (The ice melted ~10 kyears BP for most of Canada. Still lots of places in (for example) the Ottawa Valley that don't have soil yet. Even in a beaver pond in ideal (that is, there's lots of soil nearby) conditions you're looking at decades. We have no real idea what's in there because we can't culture most of it.
- We have no idea how to build a city without using fossil carbon energy technologies. (But stone masons! etc. First off, 12 quarrymen to a stonecutter; 12 stonecutters to a mason. There's a reason stone houses were swank for so long...) Critical things are water and sewage; building a water treatment plant is a bit more challenging than an aqueduct. What is the pipe made out of?
- We have no idea how to build any semiconductor, including solar panels, without fossil carbon.
- Every available indication we've got, including some Australian witness traditions, says "sea level rise is abrupt". We've got, at most, until the coastal cities drown. The old global economy goes with them. It takes a lot less rise than what we're going to get to break a city as a functioning machine.
- We have no idea how to get enough to eat without agriculture. Hunter-gathering in a depauperate ecosystem undergoing a mass extinction doesn't support enough (possibly "any") people. Pastoralism, well, maybe, some, but also "not enough".
- 4 C is the absolutely minimum predictable warming that can be considered to have any scientific credibility. It's nigh-certainly not an accurate prediction; the folks talking about Arctic amplification or the ocean feedback loops and albedo shifts aren't wrong about everything.
The Involuntary Human Extinction Project has succeeded.
We might be able to change that, but the appropriate level of effort is "full national mobilization and industrial rationalization", likely for a couple of generations. It takes absolutely ceasing to extract fossil carbon as soon as humanly possible. It takes replacing agriculture. It takes creating a local industrial toolkit -- trade is great, but you can't count on much in the uncertain future; sea level rise or fall has a history of rendering ports unusable, and we could get a century of "is it done yet?" -- that can be maintained at the local level. Try real hard to keep anesthetized dentistry while you're at it.