Glaciers and Climate
Glaciers are sometimes called the “canary in the coal mine” when it comes to climate change. This is because alpine glaciers are highly sensitive to changes in climate. For instance, a glacier grows (advances) when it accumulates more ice than it loses from melting or calving. Advances tend to happen when cold, wet years dominate the local climate. On the other hand, a glacier will shrink (retreat) during warm, dry periods as it loses more ice than it gains each year.
As discussed in the chapter on 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.” Based on this definition, we are 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 on 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 during which much of North America and Europe covered in ice thousands of meters (feet) thick and 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 beginning of the Quaternary period to 2.6 million years ago, shifting it 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 start date was based on data that reflected climatic cooling that was only local to the region in Italy where it was first observed. In contrast, 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 having it divided 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—the beginning of the Holocene. During the Pleistocene, there were several dozen intervals of glaciation separated by warmer interglacial intervals characterized by glacial retreat. In North America, these cycles are known as the pre-Illinoian (1.8 million - 302,000 years ago), Illinoian (191,000 - 131,000 years ago), Sangamonian (131,000 - 85,000 years ago), and Wisconsinian (85,000 - 11,000 years ago). The Illinoian and Wisconsinian were cooler periods that saw glaciers advance, while the Sangamonian was a warm interglacial period.
The pre-Illinoian glaciation included many glacial and interglacial periods that were once subdivided into the Nebraskan, Aftonian, Kansan, and Yarmouthian ages. New data and numerical age dates suggest that the deposits are considerably more complicated; they are now lumped together into a single period. Most of the glacial features to the north in the Midwest were created during the Pleistocene, while glaciers that extended far enough to reach Kansas and Missouri only occurred during glaciations in the pre-Illinoian period.
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 and are controlled to a large extent by astronomic cycles.
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.
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 during which the Northern Hemisphere experiences summer. When the cycles interact such that there is milder seasonality (cooler summers and warmer winters) at high latitudes in the Northern Hemisphere, less snow melts in summer, which allows glaciers to grow. 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 approach Kansas and Missouri again in about 80,000 years!
Seeking Detailed Records of Glacial-Interglacial Cycles
While glaciers have advanced over central North America and retreated again dozens of times during the Quaternary, each advance scrapes away and reworks much of what was previously left behind, making it difficult to reconstruct the precise course of events.
Therefore, to investigate the details of any associated climate change we must seek environments that record climate change and are preserved in the geologic record. 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 6.11). In the 1980s, coring of ice sheets in Greenland and Antarctica provided similar high-resolution data on atmospheric composition and temperature back nearly one million years (Figure 6.12). The data from these programs have revealed that the Earth experienced dozens of warming and cooling cycles over the course of the Quaternary period. Traces of the earlier and less extensive Pleistocene glacial advances that must have occurred have been completely erased on land, so these advances were unknown before records from deep-sea cores and ice cores revealed them. These glaciations may have had indirect impacts on the South Central, particularly on sea level variations along the Gulf Coastal Plain, but they did not likely reach Missouri and Kansas.
Figure 6.11: 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.
