Warren L Prell

Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000‐year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000‐year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000‐year cycle requires a source of climatic inertia having a time constant (∼15,000 years) much larger than the other cycles (∼5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity‐driven modulations in the 23,000‐year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher‐frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000‐year oscillations. Whether a glacier or ice sheet influences the climate depends very much on the scale…. The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence. Johannes Oerlemans [1991, p. 155]

Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind‐driven and density‐driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000‐ and 41,000‐year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end‐member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems. “Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step.” —Victor P. Starr

Paleoclimatic records adjacent to India and Africa show major variability that is related to large fluctuations in the wind and precipitation fields associated with monsoonal circulations. Much of the variability occurs at orbital periodicities, and all of the paleoclimatic time series show four monsoon maxima that occur during interglacial conditions and coincide with precession maxima and maxima of northern hemisphere summer radiation. During glacial conditions, paleoclimatic records are less distinct and show more individual variability. To identify the processes causing changes in monsoon circulation, we used 13 general circulation model (National Center for Atmospheric Research (NCAR) community climate model (CCM)) simulations that incorporate a large range of solar radiation and surface (modern to full glacial) boundary conditions. The spatial patterns of climate variables and their zonal and regional averages revealed that under interglacial conditions increased northern hemisphere solar radiation produced a larger land‐ocean pressure gradient, stronger winds, and greater precipitation over southern Asia and North Africa. Under glacial conditions, however, the monsoon is weakened in southern Asia (decreased winds and precipitation), but precipitation is increased in the equatorial west Indian Ocean and equatorial North Africa. Sensitivity coefficients are used to estimate the change in model‐simulated precipitation (Δ P ) relative to the changes in northern hemisphere summer radiation (Δ S ) and glacial age boundary conditions (ΔGBC); the coefficients are then used with time series of Δ S and ΔGBC to simulate past precipitation (Δ P ) for a specific area. Simulated records of Δ P for southern Asia and equatorial North Africa over the past 150,000 years show four monsoon maxima that are related to solar radiation maxima and are observed in the paleoclimatic data. The simulations also indicate that southern Asia is drier than today (weaker monsoon) for the period with extensive glacial boundary conditions, especially between 75,000 and 15,000 years ago. Conversely, equatorial North Africa is simulated to be wetter than today during glacial conditions. Both areas show stronger monsoons with increasing solar radiation during interglacial conditions. The agreement of simulated and observed paleoclimatic time series suggests that both orbitally produced solar radiation changes and glacial age boundary condition changes are necessary to explain the major regional features of monsoon climates at millenial or longer time scales. For southern Asia and equatorial North Africa the influence of these two factors is approximately additive.