Ice Ages Over Time
As discussed in Chapter 9: Climate, for much of Earth’s history there have not been persistent ice sheets in high latitudes. Any time that the world is cool enough to allow them to form is called an “ice age.” We are therefore living in an ice age right now! The current ice age began about 34 million years ago when ice sheets were first forming on Antarctica, followed by Greenland at least 18 million years ago, and finally on North America, which defined the beginning of the Quaternary period (about 2.6 million years ago). When most people use the phrase “the ice age,” however, they are referring to the last glacial maximum that saw much of North America and Europe covered in ice thousands of meters / feet thick, while many kinds of large, wooly mammals roamed the unfrozen portions of those continents.
Age of the Quaternary
In 2009, scientists at the International Commission on Stratigraphy voted to move the base of the Quaternary period to 2.6 million years ago, bumping it to 0.8 million years earlier than the previous date of 1.8 million years ago—a date set in 1985. They argued that the previous date was based on data that reflected climatic cooling that was only local to the region in Italy where it was first observed. On the other hand, the 2.6 million year mark shows a global drop in temperature, and it includes the entirety of North American and Eurasian glaciation, rather than dividing it between the Quaternary and the earlier Neogene period.
The Quaternary period is divided into two epochs. The earlier Pleistocene encompasses the time from 2.6 million to 11,700 years ago, including all of the Quaternary up until the most recent episode of glacial retreat. Most of the glacial features in the Midwest were created during the Pleistocene, because by the beginning of the Holocene 11,700 years ago, the glaciers had already retreated from much of the area.
Ice on a Schedule
The enormous continental glaciers that define an ice age are so large that their extent is most directly affected by global trends, while mountain glaciers are much more susceptible to local and short-term changes in climate. Continental ice sheets advance and retreat in cycles that last tens of thousands of years, controlled to a large extent by astronomic cycles.
Astronomic Cycles and Ice Sheets
The cyclical movements of ice sheets seem primarily to be caused by specific astronomic cycles called Milankovitch cycles, which change the amount of light the Earth receives, particularly when comparing the summer to the winter. The cycles, predicted through principles of physics a century ago, are related to the degree of tilt of the Earth, the Earth’s distance to the sun, and the point in the Earth’s revolution around the sun that the Northern Hemisphere experiences summer. When the cycles interact such that there are cool summers at high latitudes in the Northern Hemisphere (milder rather than extreme seasonality), glaciers can accumulate and thus advance. The cyclicity of glacial-interglacial advances was about 40,000 years from before the start of the Quaternary until about a million years ago. For reasons that aren’t clear, however, the cycles changed to about 100,000 years. If not for human-induced climate change, we might expect glaciers to cover the Midwest again in about 80,000 years!
Scientists continue to debate the particular causes of the onset of glaciation in North America over two million years ago. Movement of the Earth’s tectonic plates may have been a direct or indirect cause of the glaciation. As plates shifted, continents moved together and apart, changing the size and shape of the ocean basins. This, in turn, altered oceanic currents. Mountain building, which occurred when continents collided, erected obstacles to prevailing winds and changed moisture conditions. The freshly exposed rock from the rising of the Himalayas also combined with atmospheric carbon dioxide through chemical weathering; this consequent decrease in levels of atmospheric carbon dioxide was at least partially responsible for global cooling. Finally, the presence of continental landmasses over one pole and near the other was also a major factor enabling the development of continental glaciers.
While they have advanced over and retreated from the Midwest many times during the Quaternary, each advance, called a glacial period, scrapes away and reworks much of what was previously left behind, making it difficult to reconstruct the precise course of events. The two most recent glaciations, the Wisconsinian and the Illinoian stages respectively, are relatively well understood, while researchers believe there have been approximately 10 previous Midwestern glacial periods that are generally lumped together as “Pre-Illinoian.” After all that ice, it’s little wonder most of the Midwest has been worn nearly flat!
Seeking Detailed Records of Glacial-interglacial Cycles
When glaciers advance over the land, the historical rock records are largely erased with each glacial advance. Therefore, to investigate the details of any associated climate change we must seek environments that record climate change but are preserved. Since the 1970s, the international Deep Sea Drilling Project has provided a treasure trove of data on coincident changes in the ocean, preserved in sediments at the ocean bottom (Figure 6.7). In the 1980s, coring of ice sheets in Greenland and Antarctica provided similarly high- resolution data on atmospheric composition and temperature back nearly one million years (Figure 6.8). The data from these programs have revealed that the Earth, particularly the Midwest, experienced dozens of warming and cooling cycles over the course of the Quaternary period. Most of the earlier and less extensive Pleistocene glacial advances that occurred in the Midwest have been completely erased on land and so were unknown before records from deep- sea cores and ice cores revealed them.
Figure 6.7: Ocean bottom temperatures from 3.6 million years ago to present, based on chemical analyses of foraminifera shells. Notice how the amplitude of glacial-interglacial variations increases through time, and how the length of cycles changes.
Figure 6.8: Atmospheric temperature and carbon dioxide concentrations from an ice core taken in Vostok in Antarctica and CO2 data from several cores. Midwest glacial deposits are represented in gray at the bottom. Note that Kansan and Nebraskan deposits represent more than one glacial advance.
