Interglacials, Milankovitch Cycles, Solar Activity, and Carbon Dioxide

The existing understanding of interglacial periods is that they are initiated by Milankovitch cycles enhanced by rising atmospheric carbon dioxide concentrations. During interglacials, global temperature is also believed to be primarily controlled by carbon dioxide concentrations, modulated by internal processes such as the Pacific Decadal Oscillation and the North Atlantic Oscillation. Recent work challenges the fundamental basis of these conceptions. 1. Introduction The history [1] of the role of carbon dioxide in climate begins with the work of Tyndall [2] in 1861 and later in 1896 by Arrhenius [3]. The conception that carbon dioxide controlled climate fell into disfavor for a variety of reasons until it was revived by Callendar [4] in 1938. It came into full favor after the work of Plass in the mid-1950s. Unlike what was believed then, it is known today that, for Earth’s present climate, water vapor is the principal greenhouse gas with carbon dioxide playing a secondary role. Climate models nevertheless use carbon dioxide as the principal variable while water vapor is treated as a feedback. This is consistent with, but not mandated by, the assumption that—except for internal processes—the temperature during interglacials is dependent on atmospheric carbon dioxide concentrations. It now appears that this is not the case: interglacials can have far higher global temperatures than at present with no increase in the concentration of this gas. 2. Glacial Terminations and Carbon Dioxide Even a casual perusal of the data from the Vostok ice core shown in Figure 1 gives an appreciation of how temperature and carbon dioxide concentration change synchronously. (Temperature is given in terms of relative deviations of a ratio of oxygen isotopes from a standard. δ18O means δ18O = / , measured in parts per thousand ( ). See R. S. Bradley, Paleoclimatology (Harcourt Academic Press, New York, 1999).) The role of carbon dioxide concentration in the initiation of interglacials, during the transition to an interglacial, and its control of temperature during the interglacial is not yet entirely clear. Figure 1: Time series from the Vostok ice core showing CO 2 concentration, temperature, , and mid-June insolation at 85°N in . Based on Figure？？3 of Petit et al. [ 22]. Between glacial and interglacial periods, the concentration of atmospheric carbon dioxide varies between about 200 and 280？p.p.m.v., being at ~280？p.p.m.v. during interglacials. The details of the source of these variations is still somewhat controversial, but it is clear that carbon dioxide concentrations