Initial Publication Date: August 18, 2014

Mountains of Ice

3B: Why Do Ice Sheets Advance and Retreat?

Earth's climate has varied drastically in the past. It's hard to imagine that global climate can swing from being completely ice free to an Ice Age in a relatively short (geologically speaking) time frame. About 55 million years ago, in the span of just 20,000 years, worldwide temperatures rose 6° F. Many species of plants and animals became extinct, and camels and other mammals we associate with warm temperatures were able to thrive much closer to the Poles. Palm trees grew as far north as Alaska, crocodiles lived above the Arctic Circle, and forests of redwoods grew at 80° N latitude!

Today, Earth is currently somewhere in between the two temperature extremes. What causes such extreme climate change on our planet? Expedition 341 is studying sediments deposited within the last 6 million years to gain a better understanding of these long-term climate variations.

People across the globe have experienced extreme weather fluctuations recentlytornadoes, hurricanes, torrential floods, and drought all have taken a huge toll on plants and animals. Although weather the state of the atmosphere at a given location and time. It includes such variables as temperature, precipitation, cloudiness, wind speed and direction, and relative humidity. fluctuates from day to day, climate a region's long-term, average weather elements such as temperature and precipitation. The climate generally determines what kind of plants will grow in that region. is based on long-term precipitation and temperature averages. Driven by the sun, convection in the ocean and atmosphere plays the primary role in redistributing heat throughout our planet and maintaining global climate. This exchange of energy and matter between the atmosphere, ocean, and land is a primary driver of ocean and atmospheric circulation.

One of the most groundbreaking theories regarding the cause of long-term climate cycles was the life work of Serbian geophysicist, Milutin Milanković. Around the time of the First World War, he refined previous observations that the amount of solar radiation reaching Earth changes in a periodic way over time due to gravitational forces acting in our solar system. Milanković explained that there were three distinct long-term changes in Earth's orbit that could bring about changes in our climate.

Learn more about Milankovitch Cycles and climate

1. Make a table with the following columns: Name of Cycle, What's Changing? and Period of Cycle.

2. Watch the beginning of the following video with Richard Alley, and fill out the table for each of the three Milankovitch cycles. (You can stop the video at 07:16)

"Ice Age Meltdown" from National Geographic's Naked Science

3. For more information on Milankovitch Cycles and how Expedition 341 is using them to explain long-term changes in climate in Southern Alaska, read Milankovitch Cycles (Acrobat (PDF) 290kB Oct28 21) and answer the following questions.

Carefully examine the graphs below. The top graph shows changes in temperature from 5.5 million years ago to today. The bottom graph is the graph of the last 800,000 years of time

Stop and Think

7. How would you describe the temperature pattern shown in the top graph?

8. Over the past 800,000 years, what is the average time between glacial periods?