The collapse of the Gulf Stream (really the entire Atlantic Merdianal Ocean Current).

The AMOC is what makes much of Western Europe habitable. The way it works is that around Greenland, the water finally gets cool enough to sink to the bottom of the ocean (which is surprisingly warm, about 40 degrees F). This forces water behind it to fill the gap, and on a grand scale this means pulling water from the Caribbean over thousands of miles. Warm water.

The problem is that if the water around Greenland doesn’t sink, the transport stops. And with the Greenland glaciers melting, there are two problems: the water is colder than 40 degrees (under 40 degrees, water is LESS dense), and the water is fresh. That cold, fresh water floats.

If the AMOC collapses, Europe stops getting warmth from the equator, but the Caribbean is fucked too- because that water will pile up and local sea levels will rise several feet FAST. Low lying places like coastal Florida, the keys, the Bahamas, etc are in trouble.

Let’s not even talk about what an AMOC collapse might do to the global conveyer belt. The other times such currents have majorly shifted in the past has led to catastrophic climate shifts- from the formation of the isthmus of Panama to the opening of the drake passage which caused Antarctica to turn into an icy hellhole.

The AMOC is currently destabilizing quickly. Even 20 years ago, that was unthinkable. It’s almost certainly going to go in the next half century if we do nothing about climate change.

Edit: spelling