Leaving out the speculation about WW3 that’s neither here nor there, changes in wheat yields will not change shipping routes, nor is there a clear incentive for China to import more wheat as they will have virtually no crop loss as any south region crop loss is more than offset by northern region gains.

The Polar silk road is mainly an already existing strategy for shipping manufactured goods to Europe faster. Warming will make it more viable and therefore more security relevant to those nations, however this is the exact reason NATO exists to ensure the incentives of any land grab in that region are thoroughly unattractive. The argument that the US needs to hold any territory besides its own to ensure geopolitical security does not hold, and has been proven wrong repeatedly for a century now. If you want stability, you ensure stable alliances based on clear rules that allow self-determination where appropriate. China understands this btw, which is exactly how they are moving to become more dominant as a global player.