Ice ages and Global Warming EAS 4610 David

Ice ages and Global Warming EAS 4610 David Damm Greg Mc. Cormick

Outline • • • Earth Temperature History Climate System Forcing and Response Ice sheet response Other feedbacks and responses Future climate

3 Million yrs

Past 400 k yrs

Past 10 k yrs - Interglacial

Present Temperatures

Climate system forcing and response

Astronomical Forcing

Continental Ice Sheets • Accumulation vs. ablation: control extent, thickness, and rate • of growth (shrinkage) Topography: flow control, altering of terrain, continental depression, glacial deposits

Continental Ice Sheet Equilibrium • Accumulation: dependent on temperature, precipitation, moisture content of atmosphere • Ablation: (melting) strong temperature dependence Slow growth; Rapid decay Glacial cycle characteristic: long, slow period of glacial advance, punctuated by short interglacial period with rapid de-glaciation

Orbital forcing and ice sheet response insolation – solar energy trapped by earth Ice sheets advance: Obliquity (41 k) and precession (23 k) combine for minimum insolation Ice sheets retreat: maximum insolation during summer months

Orbital forcing and ice sheet response

Continental Ice Sheet Equilibrium

Continental Ice Sheet Cycle A No ice sheet (interglacial) B Insolation drops (orbital), equilibrium line shifting south ice sheet grows C Insolation rises, equilibrium line shifting north, (glacial maximum) North 1000’s km South D Equilibrium line far north rapid decay of ice sheet (deglaciation)

Ice Age Theories • Spectral analysis yields dominant frequency of temperature history • What is the cause of the 100 k cycles? • Do CO 2 and CH 4 lead or lag ice volume and temperature? other feedbacks? • How well can ice age models reproduce the history of the last ~3 Myr?

Ice Age Theories • Adhemar (1842) proposed precessional forcing and Milankovitch (1941) included obliquity and eccentricity • Greenhouse gas based theories (1800’s): Tyndall, Arrhenius, Chamberlin insolation (alone) and CO 2/CH 4 (alone) cannot explain the ice age cycles of the last ~3 Myr [2] Current research efforts focus on better understanding of the interaction between external forcing (insolation) and internal forcing/feedbacks such as: GHG’s, albedo, vegetation, ocean/atmosphere circulation, dust, etc. [2] Paillard, D. (2006) “What drives the ice age cycle”, Science, 313, 455 -456

Ice Age Theories Ruddiman (2006): Insolation forcing with GHG forcing at 22 k signal, and GHG/albedo feedback at 41 k. 100 k signal is due to uniquely coincident forcing and feedback. – 22 k insolation drives monsoon flooding of tropical wetlands producing fast (leading) CH 4 response and subsequent forcing on ice volume – 44 k insolation drives ice volume response which drives lagging GHG response for positive feedback Zeng (2007): Central process is burial and preservation of organic carbon by icesheets; after prolonged glaciation, subglacial transport becomes sufficient to release buried carbon to atmosphere; CO 2 is dominant factor in 5°C cooler glacial climate with contributions from insolation and albedo

Ice Age Theories Huybers (2007): Obliquity at 40 k period dominates glacial cycles; deglaciations skip one or two beats for 80 k or 120 k cycles resulting in “apparent 100 k variability”; integrated insolation modeling parameter predicts 33 of 36 deglaciations Johnston, et al. , (2006): Role of dust and other northern forcing of CO 2 changes Courtillot, et al. , (2007): Earth’s magnetic field effects on climate

Future Climate Change What does the past tell us about the future?

Concluding Remarks • Ice age cycles over the last 3 Myr have resulted from complex interactions between external forcing (insolation) and internal forcing/feedbacks such as: – greenhouse gases (CO 2, CH 4) – albedo changes (due to changes in ice, vegetation, and clouds) – changes in global ocean/atmospheric circulation (resulting in major and rapid regional climate patterns) • • Ice sheet response to astronomical forcing helps mediate global temperature Anthropogenic GHG emissions have caused a fundamental shift away from natural glacial cycles and towards a warmer climate not seen for millions of years. References and resources used [1] Ruddiman, W. F. (2001) Earth’s Climate Past and Future, http: //www. whfreeman. com/ruddiman/ (figures used here) [2] Paillard, D. (2006) “What drives the ice age cycle”, Science, 313 455 -456 [3] Ruddiman, W. F. (2006) “Orbital changes and climate”, Quaternary Science Reviews, 25 3092 -3112 [4] Zeng, N. (2007) “Quasi-100 ky glacial-interglacial cycles triggered by subglacial burial carbon release”, Climate of the Past, 3 135 -153 [5] Huybers, P. (2007) “Glacial variability over the last two million years…”, Quaternary Science Reviews, 26 37 -55 [6] Johnston, T. C. , Alley, R. B. , (2006) “Possible role for dust or other northern forcing …”, Quaternary Science Reviews, 25 3198 -3206 [7] Courtillot, V. , et al. , (2007) “Are there connections between the Earth’s magnetic…”, Earth and Planetary Science Letters 253 328 -339 [8] Petit, J. R. , et al. , (1999) “Climate and atmospheric history of the past 420, 000 years…”, Nature 399 429 -436 [9] Muller, R. A. , Mac. Donald, G. J. , (2000) Ice Ages and Astronomical Causes, Praxis Publishing Ltd.