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Electrical

Deuterium Oxygen isotope conductivity Dust Calcium excess (%o) ratio, <5lsO {%») (ueq/l) (mg/l) («M) 5 10 -40 -35 -30 0 1 0 0.5 0 2 4

Electrical

Deuterium Oxygen isotope conductivity Dust Calcium excess (%o) ratio, <5lsO {%») (ueq/l) (mg/l) («M) 5 10 -40 -35 -30 0 1 0 0.5 0 2 4

Figure 4.2 Selected profiles from the GRIP ice core, Summit, central Greenland. (a) Oxygen isotope ratio and calcium ion concentration (200-year means). (b) Enlargement of shaded timeslice 1 in (a), with a dielectric profile added. This corresponds to the last interglacial (Eemian). Two sustained cold periods (5e2 and 5e4) separate three main warm periods (5e1, 5e3, and 5e5). Two extreme cooling events are indicated. (c) High-resolution profiles through 'Event 1'. (d) High-resolution profiles through 'Event 2'. Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Greenland Ice-Core Project [GRIP] Members 1993), copyright © 1993.

Figure 4.2 Selected profiles from the GRIP ice core, Summit, central Greenland. (a) Oxygen isotope ratio and calcium ion concentration (200-year means). (b) Enlargement of shaded timeslice 1 in (a), with a dielectric profile added. This corresponds to the last interglacial (Eemian). Two sustained cold periods (5e2 and 5e4) separate three main warm periods (5e1, 5e3, and 5e5). Two extreme cooling events are indicated. (c) High-resolution profiles through 'Event 1'. (d) High-resolution profiles through 'Event 2'. Source: Reprinted by permission from Macmillan Publishers Ltd: Nature (Greenland Ice-Core Project [GRIP] Members 1993), copyright © 1993.

forms amplitude modulation of global temperature into frequency modulation of global ice extent. This is fully consistent with observations (Rial 1999, 2004a) and with the well-known absence of spectral power at 413,000 years (Imbrie et al. 1993). The key assumption in the model is that global temperature modulates the carrying capacity (the maximum amount of ice that can accumulate), which happens to reproduce the observed frequency behaviour of the system. In addition, the results suggest that, although the third harmonic of the precessional forcing, and the precession itself, entrain the Dansgaard-Oeschger oscillations, the response is intermittent and occurs in relatively short intervals. It is only during these intervals that the LODE model neatly replicates the data, a fact congruent with the idea that climate is a complex system (Rind 1999) in which order can emerge spontaneously from chaos. Intriguingly, through frequency modulation of the Greenland ice cap's natural oscillation by the third harmonic of precessional forcing, LODE generates a persistent spectral sideband with period of about 1,500 years, which may account for the somewhat mysterious 1,470-year beat of the Dansgaard-Oeschger cycles. The cause of this beat may well lie outside the climate system (Rahmstorf 2003), and the LODE results accord with this idea, inasmuch as the mystery oscillation results from frequency modulation by an external driver. Nevertheless, the complexity of the climate system makes a strong contribution too, through its highly non-linear response, which causes synchronization, entrainment of high harmonics, and frequency modulation (Rial 2004b).

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