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|Question 1: what causes ice ages (switch from "green house" to "ice house")?
much of earth's geological history is warm
evidence for 4 ice ages :
600-800 MYBP
420-480 MYBP
150-250 MYBP
2.5 MYBP: onset of most recent ice age; Cenozoic cooling began ~ 65MYBP
Question 2: what causes glacials and interglacials during ice ages?
isotope record shows large temperature oscillations throughout the most recent ice age
Changes in paleogeography
geographic location of continents
ice age 280 MYBP: Gondwana deposits
large land masses at high latitudes
increased albedo decreases temperature - positive feedback mechanism
Changes in atmospheric greenhouse gasses
carbon dioxide & methane trap outgoing longwave radiation
breakup of Pangaea associated with increased igneous activity and atmospheric CO2
chemical weathering of rocks removes CO2 from atmosphere
Changes in oceanic circulation
Antarctic Circumpolar current blocks warm water from reaching Antarctica
closure of the Panama Isthmus resulted in warm Gulf Stream water traveling farther north
Changes in atmospheric circulation
mountain building episodes, e.g. uplift of Himalayas
Changes in atmospheric greenhouse gasses
methane and carbon dioxide record shows clear variation with temperature
Orbital cycles (Milankovitch theory): eccentricity, obliquity, precession
glacials correlate with summer insolation at ~65N
Image credit: NOAA Paleoclimatology Program, Paleo Slide Set: The Ice Ages; adapted from Pisias and Imbrie (1986/1987) http://www.ncdc.noaa.gov/paleo/slides/slideset/11/eccentricity of earth's orbit (how close to circular the orbit is)
affects seasonal distribution of solar radiation
elliptical orbit produces stronger seasonal contrasts in one hemisphere than in the other
circular orbit produces more moderate seasons in both hemispheres
periodicity: ~ 100,000 years and ~400,000 years
eccentricity = 0; small change in distance of earth from sun throughout year
eccentricity = 0.5; larger change in distance of earth from sun throughout year
Image credits: Robert Simmon, NASA GSFC. NASA Earth Observatory. http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2.html
obliquity (tilt of earth's axis)
currently 23.5
ranges from 21.5 to 24.5
more tilt produces stronger seasonal contrasts
greatest impact at high latitudes
periodicity: ~41,000 years
Image credit: Robert Simmon, NASA GSFC. NASA Earth Observatory. http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2.html
precession of equinoxes
orientation of earth's spin axis changes
affects where earth is in its orbit at the solstices & equinoxes
if the earth is closest to the sun on the summer solstice, get hot summers compared to if the earth is farthest from the sun on the summer solstice
periodicity of 19,000 and 23,000 years
Image credit: NOAA Paleoclimatology Program, Paleo Slide Set: The Ice Ages; adapted from Pisias and Imbrie (1986/1987) http://www.ncdc.noaa.gov/paleo/slides/slideset/11/
Image credit: Robert Simmon, NASA GSFC. NASA Earth Observatory. http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2.html
ideal conditions for glacials: cool summers in the northern hemisphere
problems & current questions
Changes in ocean circulation
North Atlantic Deep Water (NADW) production & thermohaline circulation
Image credit: National Weather Service (2005) Jetstream: An Online School for Weather http://www.srh.weather.gov/srh/jetstream/ocean/circulation.htm
Younger Dryas (Greatlakean) substage: retreating Laurentide Ice Sheet allowed fresh-water outflow to northern Atlantic; shut down NADW
Dansgaard-Oeschger events, Heinrich events, & Bond cycles
Long term change - "green house" to "ice house"
Shorter term change - glacials and interglacials within "ice house"
