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The Laurentide Ice Sheet was a mass of ice that covered most of Canada and part of the United States over two million years ago. Learn about its physical features and explore how its past decline can help to understand present ice sheets’ response to future climate warming.

Formation of Ice Sheets

We tend to think of ice in nature as something that only covers our roads for a little while in winter. But some ice covers large amounts of land that starts in winter as snow and lasts throughout the summer. Over millions of years this snow turns into thick ice and forms ice sheets.An ice sheet is permanent ice that extends for more than 50,000 square kilometers (20,000 square miles). These ice sheets are constantly in motion and flow more readily downhill.

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Near the coasts they can lose their mass to the sea, but they continue to thrive because new snow replaces what is lost. This difference is called the surface mass balance (SMB), which calculates the amount of ice mass that has accumulated and the amount that has melted.For example, ice in your drink will melt eventually. But if you keep adding new ice, you can hit a balance and never run out of ice. The SMB may change depending on the conditions of the climate, but equilibrium is satisfied when the amount of gain and loss is approximately equal.

An example of an ice sheet in Greenland.
ice sheet

Laurentide Ice Sheet

You’ve probably heard about how North America used to be under ice in the last ice age. Well, during what is called the Last Glacial Maximum (LGM) or about 21,000 years ago, North America was covered by an ice sheet called the Laurentide Ice Sheet that was approximately four kilometers (about 2.5 miles) thick and 13 million sq kilometers wide (5 million sq miles).It covered most of Canada and parts of the United States forming as far back as 2.6 million years and beginning to decline at about 11,600 years ago.

This time period is known as the Pleistocene Epoch. In some areas the ice sheet reached a maximum of 4 km in thickness and was centered over the Hudson Bay.

Collapse of Laurentide Ice Sheet

Obviously there’s no ice sheet over all of North America now. So what happened? Well, deglaciation occurred from 11,600 – 9,000 years ago because of a shift in the climate. This had to do with radiative forcing, a change in the amount of energy going into the Earth versus that escaping out into space.The sunlight reaching the surface of the Earth and carbon dioxide levels were high during deglaciation.

The higher sunlight levels were due to orbital changes of the Earth that causes hotter summers every 11,000 years.They were almost comparable to today’s climate. Though high sunlight levels reaching the surface were high before, the combination with high carbon dioxide levels caused a kick to the climate, enough to cause melting.

The carbon dioxide levels increased from about 200 parts per million to about 250 parts per million (today, these levels are at 400ppm). The resulting increase in temperature of the ice sheet increased by 6-7 degrees Celsius.This shift caused a positive SBM, whereby there was more melting than accumulation of ice. This shows just how sensitive our climate system is to small changes in the amount of sunlight reaching the surface of the Earth or the amount of greenhouse gases such as carbon dioxide.

The warming that occurred at this time is similar to what is expected in the year 2100. Thus, knowledge of such a collapse in this ice sheet can tell us something about how the present ice sheets of Greenland and Antarctica will behave with climate warming.

Melting of an ice sheet in Antarctica.
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