The change to Europe, of cooling by 15 degrees, could happen almost instantly, at any time.
All it takes is enough fresh water melt off from Greenland, so submerge the Gulf Stream, and divert it.
Salt water is heavier than the fresh water melt off.
And as far as the rest of the world, there are climate accelerators that may be already starting to kick in, such as previously frozen methane hydrate, and simply water vapor. There are huge frozen methane deposits in tundra permafrost and deep oceans. Methane is 27 times more of a greenhouse gas than CO2. And once it gets a little warmer, then atmospheric water vapor increases, which is about 20 times more of a greenhouse gas than CO2.
The result could quickly become a runaway condition like Venus, where the surface temperature is that of molten lead.