High Latitude Insolation and Climate Response 23 April 2009 /// EAS 4803 Sean Miller

Introduction Milankovitch: high latitude NH summer insolation drives glacial-interglacial cycles Changes in precession (19-23 kyr cycle), obliquity (41 kyrs), eccentricity (100 kyrs) are important Marine benthic foraminifera δ18O used to track global ice volume changes Warm summers and cool winters are needed to melt Ice Sheets These change over geologic time Reflects ration of O18/O16 in seawater

Introduction Eccentricity: Orbit Shape Obliquity: Axial Tilt Precession: Direction of Axis All change slowly over time Eccentricity – 100 kyr cycle Obliquity – 41 kyr cycle Precession – 23 kyr cycle http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch

Data δ18O from Benthic Foraminifera 60ºN June Insolation δ18O : Oxygen Isotope Ratio Tracks global ice volume variability Lighter δ18O values suggest more precipitation over ice sheet Recorded every 1 kyr 780 ky of data 60ºN June Insolation Insolation received during NH high latitude summer Recorded every 1 kyr 1000 kyr of data Calcium Carbonate Shells / Reflects T,H2O where forams lived http://en.wikipedia.org/wiki/File:Benthic_foraminifera.jpg

Methods Create matrix of data Detrend data Fourier Power Spectrum plot periodograms Cross-Spectral Analysis cpsd Coherence mscohere

Time Series

Spectral Analysis Functions used: interp1, detrend, periodogram, xcov 2 3 2 1 3 4 Pictured: Significant peaks and 95% significance level. δ18O Signal of NH Ice Volume 1 – 87.0 kyr (Strongest) 2 – 39.1 kyr 3 – 23.0 kyr 4 – 18.6 kyr 60ºN June Insolation Changes 1 – 40.0 kyr 2 – 23.3 kyr (Strongest) 3 – 18.9 kyr

Cross-Spectral Analysis Functions used: detrend, cpsd, mscohere 1 – 97.7 kyr  eccentricity? 2 – 41.1 kyr  obliquity 3 – 23.0 kyr  precession The 23 year cycle has frequencies that show greatest variability among the two datasets. Variability is also noted at 41.1 and 97.7 kyr. All three peaks are coherent. 3 2 1

Results Proxy data have cycles with 87, 39, 21, and 19 kyr periodicities Insolation data have cycles with 40, 23, and 19 kyr periodicities Cross-Spectral Analysis shows 98, 41, and 23 kyr periodicities

Discussion Strongest signal varied mainly at 23 kyrs 41 kyr signal Suggests ice volume response to precession 41 kyr signal Suggests ice volume response to obliquity 90-100 kyr signal More complicated No peak in 60ºN insolation data Pronounced peak in paleoproxy record Possibly from internal feedbacks within the ice sheets themselves Ice sheets are non-linear systems

References Berger A. and Loutre M.F., “Insolation values for the climate of the last 10 million years.” Quaternary Sciences Review, Vol. 10 No. 4 pp. 297-317, 1991 Imbrie, J. Hays, J.D. Martinson, D.G., McIntyre, A., Mix, A. C, Morely, J.J, Pisias, N.G., Prell, W. L., and Shackleton, N. J., “The Orbital theory of Pleistocene climate: support from a revised chronology of the marine δ18O record. In Berger, A., Imbrie, J., Hays, J., Kulka, G., and Saltzman, B., (Eds.), Milonkovitch and Climate (Part 1)”. NATO ASI Ser. C, Math Phys. Sci., 126:269-305, 1984 Ruddiman, William F., Earth’s Climate Past and Future, New York: W.H. Freeman and Company, 2008 Ruddiman, W.F. Orbital insolation, ice volume, and greenhouse gases. Quaternary Science Reviews, 22(15), pp 1597-1629.