Ice Ages and Interglacials

The existence of past ice ages was discovered by several 19th century geologists from scratch marks on rocks, erratic boulders, moraines, and other physical observations. As early as 1920, Chamberlain provided a map of the North American and Greenland ice sheets at the last glacial maximum that remain quite accurate even today. Two massive ice sheets dominated the northern hemisphere. Nearly a quarter of the earth’s surface lay under the weight of a mountain of ice. The Laurentide ice sheet is believed to have reached a height of 12,500 ft. Ice covered nearly 5 million square miles of North America. As the glaciers grew, they drew so much water that the ocean levels dropped more than 100 m. The expansion of the glaciers dramatically affected the distribution and composition of vegetation. Global flora was impacted, by both CO₂ starvation than cold. Deserts expanded and wind-blown dust became prevalent at the last glacial maximum.

The overall heat balance of the surface of the Earth is dictated by a number of factors. Three important elements are: There is evidence that over hundreds of millions of years, the Earth has undergone wide variations in climate. In this book, however, we concentrate on the occurrence of Ice Ages and Interglacials over the past ~800,000 years. These quasi-cyclic climate variations apparently began about 3,000,000 years ago (ya), but the duration and depth of the cold periods increased as time progressed. We refer to these cold periods as “Ice Ages.”

The ice cores, representing accumulated past snowfall in the polar caps and ice sheets provides a basis for paleoclimate reconstruction. Paleo-climatic information derived from ice cores is obtained from four principal mechanisms: (1) analysis of stable isotopes of hydrogen and atmospheric oxygen which provide data on past temperatures; (2) analysis of other gases in the air bubbles in the ice which provide evidence of past trace gases, particularly CO₂; (3) analysis of dissolved and particulate matter in the ice core that provide past data on dust accumulation; and (4) analysis of other physical properties such as thickness of the layers which provides evidence on past precipitation. Ice cores have been taken at multiple sites in Antarctica (age up to 800,000 years) and in Greenland (age up to 130,000 years). The mechanism by which stable isotopes of oxygen and hydrogen carry a temperature signal is described. Dating the layers in an ice core is a difficult matter, and a number of techniques have been employed. Counting annual layers visually is the most straightforward, but this method is limited to more recent times. Other, more sophisticated methods are described in the text. Ultimately, ice cores from Antarctica provide a wealth of data on past temperatures, carbon dioxide concentration, and levels of particulate matter.

The locations of various ice core drilling sites at Greenland and Antarctica are described. Temperature data from Greenland and Antarctica are given in detail. It is shown that data from several sites at Antarctica provide similar data, which indicates that the data represent regional climate data. A comparison of Greenland and Antarctica ice core records provides some interesting relationships that are not fully understood. The Greenland data show large numbers of sharp, relatively sudden changes in climate. A number of theories have evolved that partly explain these changes. High elevation ice sheets, particularly in Tibet, have also provided data on past climates. It was not until the processing of ice cores in the 1990s that it was discovered that the CO₂ concentration dropped to extremely low levels (less than 200 ppm) during the latter phases of Ice Ages. With this discovery, many scientists have attempted to explain why this occurred, but only with limited success. There is ample evidence that the CO₂ concentration reaches roughly 280 ppm during Interglacials, but drops off during the progress of an Ice Age, typically over 70,000–90,000 years. During the last 10,000 years of an Ice Age (“glacial maximum”) the CO₂ concentration typically drops to about 190 ppm. Quite a number of studies have attempted to explain this CO₂ cycle but none are entirely satisfactory. The ice cores have also generated quite a bit of data on dust entrained in the ice. Chapter 11 expands on this topic.

A steady rain of shells from small, surface-dwelling animals falls continually, eventually building up hundreds of meters of sediment at the bottoms of the oceans. These sediments preserve the shells of these small animals for millions of years. The most important of these animals, foraminifera (or forams for short), construct their tiny shells from a form of calcium carbonate (CaCO₃). The carbonate, originally dissolved in the oceans contains oxygen, whose atoms exist in two naturally occurring stable isotopes, ¹⁸O and ¹⁶O. The ratio of these two isotopes is dependent on past temperatures. When the carbonate solidifies to form a shell, δ¹⁸O varies slightly, depending on the temperature of the surrounding water. However, there are complications. Paleoclimate reconstruction from the study of forams has resulted from basically three types of analysis: (1) oxygen isotope composition of calcium carbonate, (2) relative abundance of warm- and cold-water species, and (3) morphological variations in particular species resulting from environmental factors. Most studies have focused on oxygen isotope composition. Planktic forams represent a proxy for past sea surface temperatures. One of the most remarkable proxies is the ratio of oxygen isotopes in benthic (bottom dwelling) forams. This ratio, in ancient sediments, is believed to reflect the total amount of ice that existed on the Earth at the time the sea beds were formed. This ratio is therefore interpreted as a proxy for global ice. The assumption that benthic δ¹⁸O represents the ice volume is complicated by the fact that benthic δ¹⁸O is also affected by deep water temperature change. Generating a robust age model for benthic δ¹⁸O or ice volume without the assumptions of orbital tuning remains an important, unsolved problem. The dating of most ocean sediment cores is difficult, so many studies have resorted to “tuning”. This involves comparing the time line of ocean sediment data to various in solar input to high latitudes (for which accurate dating is known) and assigning dates to the ocean data based on variability of the solar data. This leads to circular reasoning in trying to develop a solar theory of Ice Ages. The ocean sediment data compare well to the ice core data, but again, use of “tuning” in both cases might make the comparison partly artificial.

A variety of other sources provide data on past climates, but most of these are relatively minor compared to ice cores and ocean sediments. Devil’s Hole is an open fault zone adjacent to a major ground-water discharge area in south-central Nevada. This open fissure is lined with a dense calcite that has precipitated continuously from calcite-supersaturated groundwater over a time span of more than the past 500,000 years. The isotopic variations in atmospheric precipitation are believed to reflect changes in average winter-spring land surface temperature, the season during which recharge is most likely to have occurred. Dating was accomplished radiometrically with high precision. The results conflicted with those of ice cores and ocean sediments. This led to some rather bitter controversies. Speleothems are secondary mineral deposits in caves which may span tens of thousands of years. Other sources of data include rock magnetism, pollen records, coral terraces, mountain glaciers and Red Sea sediments.

A number of models have been proposed to explain the alternating cycles of glaciation and interglacial cycles as follows:

The astronomical theory is built on the fact that the solar input to high northern latitudes varies over many years due to “wobbles” in the Earth’s orbit, and these variations act as a controlling factor regarding the extent of northern ice sheets. The Earth’s orbit depends on the obliquity, the eccentricity and the precession of the equinoxes. As a result, the solar input to high altitudes goes through periodic oscillations every ~22,000 years, and the amplitude varies from cycle to cycle. Simple models have been derived that aim to show how this solar input leads to the observed cycles from Ice Ages to Interglacials, but these models are highly approximate because the 22,000-year solar oscillations differ from the observed pattern of ice volume versus year.

A comparison of the solar input to high northern latitudes to the plot of ice volume [v(t)] over the past 800,000 years is not very revealing. There is no obvious relationship except that on those occasions when the ice volume drops precipitously, the solar input is on the rise. However, when one compares the solar input to the SPECMAP dv/dt, there is an apparently clear relationship. It is found that dv/dt varies inversely with the solar input. This demonstrates that the solar input is a pacemaker for the Ice Age—Interglacial cycles. However, the SPECMAP data were tuned to the solar data in the first place, so this demonstration involves circular reasoning. A comparison of the HW04 dv/dt (which does not use tuning) to the solar curve is not as convincing. But the astronomical theory (in itself) cannot predict when Ice Ages will terminate, nor why terminations are so rapid. Spectral analysis of the patterns of ice core temperature versus time or of the ocean sediment data, clearly reveal frequencies corresponding to solar variation, which again supports the conclusion that solar input to high northern latitudes is the pacemaker for Ice Age—Interglacial cycles.

One of the immediate difficulties is how to define when an Interglacial occurred in the past? In a number of cases, the long-term pattern of temperature versus time shows a long Ice Age followed by a relatively sudden termination, which in turn is followed by a period of warmth we call an Interglacial. In other cases, there are short upward temperature excursions during an ice age, and it has been debated whether to call these Interglacials, or not. An Interglacial can be defined as a unique interruption in an Ice Age, that occurs in a very mature Ice Age after a rapid termination, leading to a warm period of 5500 years that might be extended another 20,000 years in some cases. The Interglacial begins the transition to an incipient new Ice Age over a range of time that can be defined in several ways. The duration of an Interglacial depends on the measure used to define it. As an Interglacial matures, sea level remains high. Ice may start accumulating around 65°N latitude after 5500 years, but the initial impact on sea level is small. If the duration of an Interglacial is defined by sea level, it will certainly be much longer than 5500 years. If an Interglacial is defined as the period until the new ice sheets start to form, it will be quite a bit shorter than the duration measured by sea level. Past Interglacials typically had CO₂ concentrations similar to the current Interglacial, but some were warmer than the current Interglacial. Due to the rise in CO₂ and the large amount of dust and soot deposited on northern ice and snow, it seems likely that the current Interglacial will persist and a new Ice Age will not develop.

It is observed that over the past several hundred thousand years, Ice Ages sometimes come to an abrupt end, with the gigantic ice sheets terminating in a mere ~5500 years. While there is ample evidence that the variation of solar input to high northern latitudes acts as a pacemaker for long-term variations in the ice volume of far northern ice sheets, there is nothing in the so-called Milankovitch theory that predicts when a termination will occur, and how it occurs. The ups and downs of solar input due to the precession cycle produce ups and downs in the ice volume but occasionally, a solar up-lobe produces a termination. But many up-lobes do not produce terminations. Evidently, an additional X-factor is necessary at that juncture to produce a termination on an up-lobe in the solar precession curve. Over the past 800,000 years, the natural state of the Earth was that of what we call an “Ice Age”. Apparently, Ice Ages occurred because the energy balance of the Earth in pre-industrial times favored production of ice sheets in the North. So, what we had was not, as one might tend to perhaps assume, unusual Ice Ages that interfered with natural periods of relative warmth. Instead, we had persistent Ice Ages that were intermittently terminated when the X-factor(s) arose, as an exception to the rule, rather than as a state of normalcy. Therefore, the search for the holy grail of Ice Ages is essentially the search for the X-factor(s) that causes terminations, along with rising solar input to the NH. All terminations occur on an up-lobe of the solar input to the NH. But many up-lobes do not produce terminations. Some up-lobes that do not produce terminations are stronger than some that do produce terminations. Therefore, an up-lobe in solar input to the NH appears to be a necessary adjunct to termination, but is not sufficient to cause termination of an Ice Age in itself. An X-factor(s) is needed to add to, or enhance the solar input to drive it over some threshold to originate runaway erosion of the ice sheets. Ellis and Palmer (2016) proposed that as an Ice Age reaches its greatest maturity, large amounts of dust from arid, CO₂-deprived areas are transported by winds to the ice sheets, which in combination with rising solar input to high northern latitudes, produces much greater absorption of solar energy, thus initiating a termination. In support of this thesis, they presented convincing evidence. The X-factor appears to be dust deposited on the ice sheets. This chapter goes into the greatest detail to present evidence to support the proposal of Ellis and Palmer.

There are some important things about Ice Ages that we don’t understand. These include: We do understand how the solar power input to high northern latitudes changes with time over long time-periods. This pattern is dominated by the 22,000-year period due to precession, and its amplitude is governed by obliquity and eccentricity. We have good data to show that as Ice Ages evolve, the rate of growth of the ice sheets increases when the solar input is low, and vice versa. The solar input acts as a pacemaker to regulate the pattern of ice volume versus time during an Ice Age. But the Ice Ages persist through the ups and downs of several 22,000-year precession cycles; the ice sheets relentlessly advance. Over the last 800,000 years, we observe that at 9 instances, spaced by many tens of thousands of years, the gradual (but sometimes bumpy) expansion of the ice sheets came to an abrupt halt (a termination); the ice sheets disappeared (or were greatly reduced) in a relatively short time (~5500 years). After a warm period with minimal ice that we call an Interglacial, an incipient new Ice Age begins, and the pattern reproduces itself. The evidence seems to suggest that during this 800,000-year period, prior to large-scale human activity, the natural state of the Earth was an Ice Age. A perennial Ice Age would have persisted for the entire 800,000 years except for the fact that in nine instances, something occurred that caused the ice sheets to greatly diminish, but those events were temporary; the ice sheets began rebuilding perhaps ~10,000 years later. For some reason unexplainable, the community of scientists studying Ice Ages mainly did not concern itself with terminations. We note the following facts: These facts suggest that an up-lobe in the solar input as governed by precession is necessary for a termination to occur, but not sufficient. Only when the up-lobe is preceded by large amounts of dust deposition, does a termination occur.