CAUSES OF CLIMATE CHANGE

A. Introduction

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 :

700 MYBP

450 MYBP

280 MYBP

2.5 MYBP: onset of most recent ice age; Cenozoic cooling began ~ 65MYBP

Question 2: what leads to glacials and interglacials during ice ages?

isotope record shows large temperature oscillations throughout the most recent ice age

B. Long Term Global Oscillations

1. 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

2. Changes in atmospheric greenhouse gasses

carbon dioxide & methane trap outgoing longwave radiation

breakup of Pangaea associated with increased igneous activity

igneous activity increases atmospheric CO2

chemical weathering of rocks

3. Changes in oceanic circulation

Antarctic Circumpolar Current blocks warm water from reaching Antarctica

closure of the Panama Isthmus resulted in warm Gulf Stream traveling farther north

4. Changes in atmospheric circulation

mountain building episodes, e.g. uplift of Himalayas

C. Oscillations Within the Last Ice Age

1. Changes in atmospheric greenhouse gasses

methane and carbon dioxide record shows clear variation with temperature

2. Astronomical (Milankovitch) theory

glacials correlate with summer insolation at ~65N

insolation affected by eccentricity, obliquity, and precession

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 isclosest 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

3. 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: retreating Laurentide Ice Sheet allowed fresh-water outflow to northern Atlantic; shut down NADW

Bond cycles, Heinrich events, & Dansgaard-Oeschger events

D. Summary

Long term change - "green house" to "ice house"

Shorter term change - glacials and interglacials within "ice house"