Study finds Atlantic Ocean circulation weakening

Driving the news: In a study published in Nature Geoscience on Thursday, researchers reported the AMOC — a system of ocean currents that includes the Florida Current and the Gulf Stream — is in its "weakest state in over a millennium."

• The AMOC is the oceans' thermohaline circulation, and it plays an important role in managing the global climate, including keeping temperatures in Europe warmer than they would otherwise be for its latitude.

The big picture: The AMOC has been called the "Achilles' heel" of the climate, causing drastic changes when it has shifted on and off during the Earth's history.

• It got a moment of public fame in the 2004 climate disaster film "The Day After Tomorrow," where it shut off and caused intense storms and weather to wreck major cities around the world.

Yes, but: Nothing remotely near that drastic is expected even if the current were to weaken far more, and researchers still don't have a direct grasp on the health of the AMOC, which is why they were forced to use proxy data in the study.

The bottom line: The biggest reason to worry about climate change — and act on it — isn't the most likely but survivable bad outcomes, but the less likely ones that could be truly existential.