An Introduction to Milankovitch Cycles

Transcription

1 An Introduction to Milankovitc Cycles Wat Causes Glacial Cycles? Ricard McGeee kiloyear bp temperature -1 Note te period of about 1 kyr. Seminar on te Matematics of Climate Cange Scool of Matematics April, 8 Can you spot a cycle of 41 kyr? ttp:// Wat Causes Glacial Cycles? Heat Balance Popular Hypotesis Te glacial cycles are driven by te variations in te Eart s orbit (), causing a variation in incoming solar radiation (insolation). Tis ypotesis is widely accepted, but also widely regarded as insufficient to explain te observations. Historical Overview of Climate Cange Science, IPCC AR4, p.96 ttp://ipcc-wg1.ucar.edu/wg1/report/ar4wg1_print_ch1.pdf Eccentricity Obliquity ttp:// ttp://upload.wikimedia.org/wikipedia/commons/6/61/axialtiltobliquity.png 1

2 Precession Eccentricity Note periods of about 1 kyr and 4 kyr. Recall tat glacial cycles ave a period of about 1 kyr. ttp://eartobservatory.nasa.gov/library/giants/milankovitc/milankovitc_.tml Science 9 (1), 687. Obliquity Precession Note period of about 41 kyr. Note period of about 3 kyr. Science 9 (1), 687. Science 9 (1), 687. ttp://en.wikipedia.org/wiki/milankovitc_cycles Petit, et al, Nature 399 (1999), p. 431

3 (Hays, et al) Climate Response, Hays, et al Hays, et al, Science 194 (1976), p. 115 Hays, et al, Science 194 (1976), p ) Tree indices of global climate ave been monitored in te record of te past 45, years in Soutern Hemispere ocean-floor sediments. )... climatic variance of tese records is concentrated in tree discrete spectral peaks at periods of 3,, 4,, and approximately 1, years. Tese peaks correspond to te dominant periods of te eart's solar orbit, and contain respectively about 1, 5, and 5 percent of te climatic variance. 3) Te 4,-year climatic component as te same period as variations in te obliquity of te eart's axis and retains a constant pase relationsip wit it. 4) Te 3,-year portion of te variance displays te same periods (about 3, and 19, years) as te quasiperiodic precession index. 5) Te dominant, 1,-year climatic component as an average period close to, and is in pase wit, orbital eccentricity. Unlike te correlations between climate and te iger-frequency orbital variations (wic can be explained on te assumption tat te climate system responds linearly to orbital forcing), an explanation of te correlation between climate and eccentricity probably requires an assumption of nonlinearity. 6) It is concluded tat canges in te eart's orbital geometry are te fundamental cause of te succession of Quaternary ice ages. 7) A model of future climate based on te observed orbital-climate relationsips, but ignoring antropogenic effects, predicts tat te long-term trend over te next seven tousand years is toward extensive Nortern Hemispere glaciation. ttp://en.wikipedia.org/wiki/milankovitc_cycles 3

4 Climate Response (Zacos, et al) Recent History A. Power spectrum of climate for te last 4.5 Myr. Note te peaks at 41Kyr and 1 Kyr. B. Power spectrum of climate for te period 5 Myr bp to.5 Myr bp. Note te new peak at 4 Kyr and te split peaks at 16Kyr and 95 Kyr. Zacos, et al, Science 9 (1), p. 689 Zacos, et al, Science 9 (1), p. 689 are Insufficient are Insufficient Rind, et al, used a global circulation model to try to predict te last ice age using te Milankovitc insolation data. It failed: Te results sow tat te model fails to maintain snow cover troug te summer at locations of suspected initiation of te major ice seets, despite te reduced summer and fall insolation. Te experiments indicate tere is a wide discrepancy between te model's response to Milankovitc perturbations and te geopysical evidence of ice seet initiation. Rind, continued. If te model results are correct, it indicates tat te growt of ice occurred in an extremely ablative environment, and tus demanded some complicated strategy, or else some oter climate forcing occurred in addition to te orbital variation influence (and CO reduction), wic would imply we do not really understand te cause of te ice ages and te Milankovitc connection. If te model is not nearly sensitive enoug to climate forcing, it could ave implications for projections of future climate cange. Rind, et al, J. Geopysical Researc 94 (1989), p Rind, et al, J. Geopysical Researc 94 (1989), p Wy focus on eccentricity? Solar intensity at distance r from te sun: Angular momentum: k1 Q() t = r() t Ω= r θ k 1θ Q() t = Ω Mean annual solar input ( T = one year): 1 T k T π k1 Q = Q() t dt = dt T θ = TΩ TΩ Wy focus on eccentricity? Total Energy: 1 1 v = r() t Kepler s Tird Law: 3 T = k Mean annual solar input: k3 Q = Ω More relevant question: How does Ω vary over geologic time? 4

5 Recall Budyko s model: ( ) ( 1 α( )( )) ( )( ) ( )( ) κt = Qs y T y I T y + H T y Te annual average insolation as a function of latitude q, were y = sin q, is Qs ( y) Even more relevant question: How does tis function vary over geologic time? 5