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 first began forming on Antarctica, followed by their appearance 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.
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 to 302,000 years ago), Illinoian (191,000 - 131,000 years ago), Sangamonian (131,000 - 85,000 years ago), and Wisconsinan (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.
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 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 in the Northwest Central were created in the Pleistocene, during the Wisconsinian glaciation.
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.
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!
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.
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 6.14). 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 6.15). 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.
Figure 6.14: 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 6.15: Ice core atmospheric temperature and carbon dioxide concentrations from an ice core taken in Vostok in Antarctica along with CO2 data from several cores in Greenland give a record of glacial advances over the past 800,000 years. Note that Kansan and Nebraskan deposits represent more than one glacial advance.
A large proportion of glacier and climate research involves making regular inventories of existing glaciers and their characteristics to determine how they are impacted by global, regional, and local climate changes. Equally important is determining the impact of changing glaciers on seasonal streamflow. Glaciers act as water reservoirs where winter snowfall is released as meltwater during summer, when precipitation is low. This characteristic is particularly important to farms and fisheries in areas downslope from glaciated mountains like the Northern Rockies.
In addition to investigating present-day glacier behavior, researchers use clues from the landscape to reconstruct ancient glaciers. This information, along with climate evidence from tree rings and lake sediments, provides a long view of climate change that has done much to improve our understanding of how climate systems work, and what the future might have in store for us.
As the last Pleistocene ice age came to a close, the Laurentide Ice Sheet and alpine ice caps throughout the Rockies retreated, leaving behind rugged mountain ranges, deep glacial valleys, and plains covered with thick deposits of glacial sediment. The time from the end of the Pleistocene to now is regarded as an interglacial period (a warm spell with diminished glaciers), but it has not been without its minor ice ages. The most recent of these, the Little Ice Age, began somewhere between 1300 and 1500 CE and ended by the late 19th century. Presently, the continental ice sheets and ice caps of the Pleistocene are gone, but some 150,000 alpine glaciers remain worldwide, and the impact of the ancient ice sheets and caps can be seen in nearly every region of the Northwest Central States.
Today’s warming climate is having a profound impact on the glaciers that still exist in the Northwest Central. For example, Glacier National Park in Montana contained 150 named glaciers in 1850, but thanks to the effects of climate change, today only 26 of these glaciers remain. Scientists estimate that all of the park’s glaciers will have vanished by 2030 (Figure 6.16).
Figure 6.16: An example of glacial recession in Glacier National Park: Grinnell Glacier, as seen in 1940 and 2006.
