The Wrong Question

Open on continent-scale ice and exposed continental shelves, then make the first correction: technically, the Quaternary Ice Age continues. The investigation concerns escape from the last glacial period.

Define the Ending

Establish the Last Glacial Maximum, deglaciation beginning around 19–20 thousand years ago and the formal Holocene boundary at 11.7 ka. Regional event boundaries are not one synchronous global switch.

The Pacemaker

Orbital geometry redistributed seasonal sunlight. A modest increase in Northern Hemisphere spring and summer insolation acted on a vast unstable ice system; the Sun did not suddenly brighten worldwide.

The Feedback Cascade

Retreat exposed darker land and ocean, amplifying energy absorption. Ocean circulation and Southern Ocean carbon release helped spread and amplify warming; trigger and amplifier are different roles.

The Atlantic Switch

Heinrich Stadial 1 and the Bølling–Allerød show that Atlantic overturning can reorganise North Atlantic climate abruptly while global deglaciation continues.

The Younger Dryas Interruption

The 12.9–11.7 ka cold reversal was centred on the North Atlantic during an already ongoing deglaciation. AMOC was strongly reduced; freshwater involvement is plausible, but route and sufficiency remain unsettled.

The Alternatives Test

An eruption cluster is a credible emerging contributor; Laacher See fails the direct-trigger timing test. The impact hypothesis remains unconfirmed because chronology, replication, source specificity and crater age do not carry the larger claim.

What Actually Ended It?

Orbital redistribution paced retreat. Ice–albedo, ocean circulation and greenhouse feedbacks amplified and spread the transition. Abrupt reversals altered the route without reversing the destination.

What Remains Unresolved

Freshwater routes and sufficiency, volcanic interaction with AMOC and sea ice, changing carbon-source shares and the alignment of regional proxy chronologies remain active questions.