California State University Long Beach 

Dr. Rick Behl 

The Earth System has stepped between stable glacial or interglacial states

• The Earth is not an analog system!
• Reorganizes in sudden steps or jumps
• Abrupt change in state from last ice age to present interglacial
• 18-20,000 ka (ka = thousands of years ago)
• Last glacial maximum (LGM)
• 14,000 ka
• Rapid warming of sea surface temperatures
• Rapid retreat of North American ice sheets
• Rapid meltback of alpine glaciers
• Northern and southern hemisphere
• Rapid decrease in dust from LGM to Holocene
• Shown in Antarctic and Greenland ice cores
• 11,000 to 10,000 years ago
• Younger Dryas cold episode
• Rapid cooling to near glacial conditions
• Rapid warming at end
• Both occurred in a few years to a few decades!
• 1300 to 1800 A.D.
• Little Ice Age
• Global glacial advance
• Minor cooling compared to Younger Dryas

Reorganization of the ocean-atmosphere system

• Switching on/off of global conveyor belt
• Changes latitudinal heat distribution
• Modern (interglacial) conveyor circulation provides enormous heat to North Atlantic
• Glacial mode circulation did not
• Ocean is the major reservoir of CO₂ in the surficial system
• 60 times that of the atmosphere
• Biological pump of CO₂
• From surface water to deep water
• Presently (interglacial) CO₂ is recycled by upwelling
• Glacial mode slowed circulation
• Pumps CO₂ to deep ocean
• Depleted surface waters draw down atmospheric levels
• Icehouse atmospheric concentration of CO₂
• 2/3 of preindustrial level

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Younger Dryas cold episode

• Retreat of ice sheet shifted drainage of meltwater Lake Agassiz
• From the Mississippi to the St. Lawrence
• Flooded North Atlantic with fresh water
• Low density water won’t sink
• Shut down or slowed conveyor belt circulation
• Strongly reduced transport of heat from equator to high latitudes
• How do we know?
• Lake shore levels and ages
• Oxygen isotopes in Gulf of Mexico
• Lake Agassiz also responsible for huge Scabland floods

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