Fourth Day of Light, 50th Day of Midsommar, 526 M.E. (Deneb): As the Atlantic Meridional Overturning Circulation (AMOC), the massive system of ocean currents that acts as a global conveyor belt, is on the verge of collapse, it's worth thinking about the Younger Dryas Event. About 12,800 years ago, something caused one of the most abrupt climate collapses in recent geological history. Global temperatures dropped several degrees within decades and megafauna across multiple continents went extinct. Then about 1,200 years later, conditions rebounded almost as fast.

The Younger Dryas was a long time ago, but doesn't predate human history. The oldest of the Giza pyramids were built around this time (more time had passed between construction of the Great Pyramid in Giza and the birth of Cleopatra than the birth of Cleopatra and now). The nomadic Clovis tribes in North America that hunted mammoths, mastodons, and caribou and lived in scattered, temporary settlements, traveling over long distances in search of game, had to switch to a more sedentary, agricultural lifestyle after the megafauna went extinct.  

The generally accepted explanation for the event is that meltwater from retreating glaciers disrupted Atlantic ocean circulation. If disruption of the Atlantic circulation system then could cause the catastrophic Younger Dryas, what would the collapse of the AMOC cause today?

The Stable Genius didn't think the issue was worth researching and his administration was set to remove the system of oceanic monitors and recording buoys in the Atlantic and Pacific that's being used to track changes in the AMOC and elsewhere. Fortunately, an outcry from the scientific community caused him to reconsider the decision, and the system will be left in place for now. But that's how governance under the Stable Genius goes these days - someone wonders "What does this switch do?" and turns it off, and then if the outcry seems alarmed enough, they'll turn it back on.

The specific mechanism that caused  disruption to the ocean currents during the Younger Dryas was thought to be freshwater outflow from the St. Lawrence River as the North American continental glaciers melted. However, salinity data doesn't support that hypothesis. The current thinking now has Canada's Mackenzie River providing the disruptive fresh-water flow. 

An alternative hypothesis to explain the Younger Dryas Event proposes that North America and other continents were subjected to some sort of extraterrestrial event, either a supernova shockwave, the impact or airburst from a meteor, comet, or some other object, or some combination thereof. That event supposedly caused the climate changes that define the onset of the Younger Dryas. More specifically, proponents claim that the proposed impact triggered an "impact winter" and the subsequent climate episode, biomass burning, megafaunal extinctions, and human cultural shifts and population declines.

It's ironic that as human-induced climate change warms the planet, that warming may eventually cause the same kind of disruption to the Atlantic circulatory system that caused the global cooling of the Younger Dryas. Collapse of the AMOC could provide a cooling effect to offset some of the warming, with the added benefit of killing off enough of the human population to stop with all the anthropomorphic GHG releases already.