The Younger Dryas was a period of abrupt cooling that occurred approximately 12,900 years ago. Two scientists propose different hypotheses regarding the primary trigger of this cooling event.
Scientist 1
The Younger Dryas cooling was triggered by the sudden release of a massive volume of freshwater from Lake Agassiz into the North Atlantic Ocean. This freshwater influx reduced the salinity and density of the surface waters, disrupting the Atlantic Meridional Overturning Circulation (AMOC). Because the AMOC transports warm tropical water northward, its slowdown immediately cooled the North Atlantic region, initiating global climate feedbacks.
Scientist 2
The Younger Dryas cooling was triggered by the impact or airburst of a disintegrating comet over North America. This impact event ignited widespread wildfires, releasing immense quantities of soot, ash, and dust into the atmosphere. This atmospheric shroud blocked incoming solar radiation, causing immediate global cooling (an 'impact winter'). The physical disruption also destabilized ice sheets, leading to freshwater runoff, which was a secondary effect rather than the primary cause of the cooling.
Based on the passage, match each concept on the left with the corresponding hypothesis or description on the right.
- Primary trigger of cooling according to Scientist 1Disruption of the Atlantic Meridional Overturning Circulation due to freshwater influx
- Primary trigger of cooling according to Scientist 2Atmospheric shroud of soot and dust blocking sunlight following a cometary impact
- Role of freshwater runoff according to Scientist 2A secondary consequence of ice sheet destabilization rather than the primary cause