During the Pleistocene epoch, major climatic fluctuations, including the advance and retreat of continental glaciers, were influenced by small variations in the amount of solar energy reaching high latitudes in the summer. Determining the details of this influence has major implications for scientists' ability to predict future climate change.

Characterized by repeated cycles of glacial cold and interglacial warmth, Pleistocene climate was always changing. Scientists believe that the timing of these changes was controlled by periodic variations in the orbit of the Earth. They are still trying to figure out how the small changes produced by these variations were amplified into dramatic climatic shifts. Of particular interest is the question of if and when another glacial advance will occur.

Geologists divide up the 4.6 billion years of Earth's history, usually based on the planet's fossil record. The planet's history is divided into eras, which are divided into periods, which are divided into epochs, which are divided into ages. Eras are neatly separated by major mass extinctions, and most of the other divisions are also reasonably clear. However, the division between two period/epoch pairs has proven to be more difficult: The base of the Quaternary period and the base of the Pleistocene epoch are either at the top or the bottom of the Gelasian age.

The ages within both the Pliocene and the Pleistocene epochs are determined by the marine fossils from those epochs, which are not difficult to date. A problem arises, however, from an attempt to put the beginning of the ice ages at the right age boundary. In 1948, at a meeting of the International Union of Geological Sciences, the top of the Gelasian age (1.81 million years ago) was chosen as the advent of the ice ages. Since then, a tremendous amount of climate-related research has suggested that the bottom of the Gelasian, 2.59 million years ago, might have been a better choice. As often happens, both dates are used. The discrepancy is not significant in the context of climate issues. Because the climate began to behave differently a little before 2.59 million years ago and it did not change much 1.81 million years ago, a description of Pleistocene climate will also apply to Gelasian climate, whether one considers the Gelasian age to be part of the Pleistocene or part of the Pliocene epoch.

Around 2.6 million years ago, the climate began to oscillate between glacial and interglacial states. When only land-based data on glacial advances were available, only a few advances were described, but as the ability to interpret isotope data from ocean cores developed, it was determined that there have been at least fifty advance/retreat cycles in the last 2.6 million years. The shift from continental rocks to ocean sediment cores as the source of definitive data was important in providing this additional data.

The surfaces of the continents, particularly when being scraped repeatedly by large glaciers, do not keep very good archives. In some places, glacial deposits lie on top of other glacial deposits, and those earlier deposits often have complete soil profiles. Geologists thus knew that there had been more than one glacial period, but they had little detailed knowledge about the succession of such periods. Each new advance usually removed any former glacial deposits, obscuring the record. In the deep oceans, however, remains of dead organisms accumulate, and there is little to remove them. As isotope geochemistry developed, it became apparent that the ratio of oxygen 18 (O18) to oxygen 16 (O16) preserved in these remains changed systematically in all of the world's oceans during the last three million years or so. Evaporation removes more O16, and during glaciations will sequester much of this isotope in the ice sheets, thereby enriching the oceans in O18. Fluctuations in the isotope ratio can indicate whether the Earth was in a glacial (high O18/O16 level) or interglacial (low O18/O16 level) state.

As cores of sediments were retrieved from the ocean floor, changes in the O18/O16 ratio as a function of time were observed and analyzed. Shifts were numbered, starting with 1 for the present interglacial and using odd numbers for interglacials and even numbers for glacials. The time period represented by each number is called a marine isotope stage (MIS). Glacial cycles already known from continental evidence correlated with some of the MIS cycles, but there were many more MIS cycles, they were global in nature, and their dates were known more accurately. Terms such as "Illinoian" and "Moscovian Dnieper" were replaced by designations such as "MIS 6 (Macdougall, 2004)."

Using Fourier analysis and other methods, scientists found that some periods of time were very strongly represented in the marine isotope record. These peaks in information seemed to correspond to natural cycles. (Imagine recording the sounds from a neighborhood continuously for a decade and then analyzing them. One might expect to see information peaks at intervals corresponding to the cycles governing human activity: every twenty-four hours, every week, and possibly every year.) The peaks in the sediment cores occurred every forty-one thousand years, about every twenty-two thousand years, and every hundred thousand years.

A glacier will grow if snow accumulates over periods of years, as snow that fell during the previous winter does not melt completely during the summer (Imbrie, 1979). Thus, summer temperatures are the limiting factor in glaciation. If summers are not warm enough to melt all of winter's ice accumulation, the glaciers will advance. Winter temperatures do not matter to this process, so long as they remain below freezing. Temperatures on Earth are determined primarily by the Sun, and its influence is modulated by periodic changes in the tilt of the planet's rotational axis, the precession of that axis, and the eccentricity of Earth's orbit. These changes were known by the middle of the nineteenth century, and James Croll suggested in 1864 that they might be responsible for the advance and retreat of continental glaciers.

Data, computations, and dating techniques were not yet sufficient to support the theory, however, and interest in it waned until Milutin Milankovic resurrected the idea in 1913. He worked on the problem for decades, publishing his completed book in 1941; it was translated into English in 1969.

Milankovic was able to calculate how much solar energy would reach a latitude of 65œ north during the summer. He obtained this result by combining the effects of a 100,000-year cycle in the eccentricity of the Earth's orbit, a 41,000-year cycle in the inclination of the Earth's axis, and a roughly 22,000- year cycle in climatic precession. Thus, the peaks evident in ocean sediment cores matched precisely Milankovi6's cycles.

When the ocean core analyses were complete, there was no question that glacial advances and retreats followed the timing of the Milankovic cycles. As a result, serious concerns were raised for a time about a coming glacial advance. Some papers written in the 1970's--as well as meetings, symposia, and water cooler discussions--concerned the global cooling that Milankovi6's work seemed to predict. In part, this was due to a decade-long cooling period in the Northern Hemisphere, but it also stemmed from the recognition that, according to Milankovic, the current interglacial should end soon.

Trying to guess when it will end, scientists examined the record. Between 3 million and 0.8 million years ago, the cycles were dominated by the 41,000- year cycle. Afterward, however, a period began that is often called the Mid-Pleistocene Transition, and the cycles became dominated by the 100,000-year cycle. Although many theories have been put forward to explain this transition, it is still not well understood. In addition, it is unknown whether the current interglacial period has been extended by the effects of agriculture and other human activities over the last eight thousand years or whether it would have been longer than average regardless of human behavior.

Estimates made from isostatic rebound studies suggest that the continental glaciers at their height during Pleistocene glaciations were 1-2 kilometers thick. The ice moved from north to south, reaching a latitude of 40œ north, near the northern boundary of the state of Pennsylvania. Even when the glacier stopped advancing, the ice within it continued to move from north to south, grinding away at the material beneath it. At its southern edge, melting converted the ice to liquid water.

Where the glaciers terminated, the ice rose steeply, with large accumulations of ice adjacent to large lakes and hills. In the summer, this ice must have caused wind and precipitation patterns quite different from those experienced during interglacial periods. All year long, the presence of the glacier is likely to have altered global wind patterns. It was white, cold, and added considerably to the elevation. With so much water tied up in ice, sea level fell by about 100 meters, exposing huge expanses of the continental shelves to weathering and erosion.

If Earth's climate continues to behave cyclically, as it has for the past 2.6 million years, then another glacial advance will occur in the future, perhaps within a millenium or two. Alternatively, anthropogenic inputs of greenhouse gases may have warmed the planet so much that another glacial advance will only occur after humans stop burning fossil fuels and the resulting emissions have left the atmosphere. A third possibility is that anthropogenic outputs have altered the land so much that continental glaciations are no longer possible. Clearly, these alternatives represent very different climate change scenarios. Although scientists are confident that Milankovi6 cycles were the pacemakers for Pleistocene climate change, there is little agreement on the mechanisms connecting these pacemakers to the various drivers and amplifiers of the climate system. Nor is there agreement as to why the dominant cycle shifted from one of 41,000 years to one of 100,000 years in the middle of the Pleistocene epoch. As these and other issues become better understood, it may become possible to predict the future with greater confidence.

References

1. Imbrie, J., and K. P. Imbrie. Ice Ages: Solving the Mystery. Short Hills, N.J.: Enslow, 1979.

2. Macdougall, J. D. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press, 2004.

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