Roughly 12,900 years ago, as the last ice age loosened its grip and the Northern Hemisphere warmed, the climate abruptly reversed. Within decades, average temperatures over the North Atlantic plunged back toward glacial values and remained there for some twelve centuries before recovering just as suddenly. This episode, the Younger Dryas, has long served as the archetype of abrupt climate change, and for most of the past half-century a single mechanism has dominated explanations of it. That mechanism is the disruption of oceanic overturning. In the prevailing account, the melting Laurentide ice sheet had ponded an enormous volume of freshwater in a glacial lake; when an ice dam failed, the lake drained catastrophically into the North Atlantic. The influx of buoyant freshwater, being less dense than seawater, capped the ocean surface and suppressed the sinking of cold, salty water that drives the Atlantic's overturning circulation. Because that circulation ferries tropical heat northward, its slackening would have chilled the surrounding continents. The account is elegant, and it draws support from climate models in which imposed freshwater forcing reproduces cooling of the observed magnitude. Attractive as the meltwater hypothesis is, it has acquired an insistent rival. A group of researchers has proposed that the trigger was extraterrestrial: the airburst or impact of a fragmenting comet, which they argue ignited continental-scale wildfires, destabilized the ice sheet, and loaded the atmosphere with cooling debris. In support they cite a putative layer, laid down at the onset of the cooling, said to be enriched in microscopic diamonds, magnetic spherules, and other markers held to be diagnostic of impact. The impact proposal has met sustained resistance, and the grounds for skepticism are instructive. Independent laboratories have often failed to reproduce the reported markers, or have found that the same particles form through ordinary processes - wildfires, volcanism, even the slow settling of cosmic dust - that require no catastrophe. A crater of suitable age and size has never been identified. And the hypothesis must still explain the feature its rival handles naturally: the twelve-hundred-year persistence of the cold, which a brief impact, however violent, does not obviously produce, but which a reorganized ocean circulation does. None of this amounts to a refutation. The markers might yet be vindicated by more careful sampling, and advocates rightly observe that absence of a crater is not decisive for an airburst that need never have struck the ground. But the asymmetry in explanatory burden is real. The meltwater account must posit only a well-attested glacial lake and a physically understood consequence of freshening; the impact account must posit an unobserved body, an unlocated crater, and a chain of effects whose signatures remain disputed. Parsimony does not settle scientific questions, but where two hypotheses purport to explain the same event, the one demanding fewer unverified entities deserves provisional preference. On present evidence the ocean, not the sky, remains the better suspect.