

Warm salty water flows north in the Gulf Stream, and then sinks when it cools in the North Atlantic (because saltier water is denser). Once it sinks, it flows back south along the bottom of the ocean.
Cold fresh water entering the North Atlantic due to ice melt from Greenland disrupts that process. The faster Greenland’s ice cap melts, the slower the current gets.
If it stops entirely, heat stops getting transported by the Gulf Stream and northwestern Europe (especially Britain) starts looking more like Alaska.






It means that a passport only matters to the extent that the countries being traveled between care about it.