Now that we know that astronomical or orbital forcing of climate occurs, we need to understand how these changes combine to create changes in climate, namely, the Ice Ages.
Remember that insolation at a given latitude does not vary from year to year, but the amount received in a given season for a given latitude changes slowly over time because of changes in the geometry of the Earth's orbit around the Sun. That doesn't sound like enough of a change to create ice ages, does it? Not alone, but these changes step up conditions for positive feedback. Once cooling is set in motion, it begins to feed the process. Two important feedbacks have been identified that are part of the creation of ice age conditions: changes in albedo, and changes in the amount of greenhouse gases in the atmosphere. Here are four of the major players in the story:
- Orbital forcing: When the cycles of eccentricity, tilt, and precession line up, their combined interaction causes summers to be slightly cooler at a critical latitude (65 degrees) triggering the beginning of an ice age. Because summers are cooler, they provide insufficient solar energy to melt all the snow and ice that was formed during the winter. As a result, ice sheets grow. Examine the "Life Cycle of a Glacier" interactive to explore the journey of a snowflake.
- Once glaciers and sea ice begin to build up, they create positive feedback, acting as a reflective surface. Explore the "Earth's Albedo and Global Warming" interactive from PBS LearningMedia about the Earth's albedo. Make note of the role seasonality has on Earth’s albedo and how this changes over the course of a year.
- Water vapor is a powerful greenhouse gas. A great deal of the water vapor that is usually found in the atmosphere is locked up in continental ice masses. As ice grows, less water vapor is found in the atmosphere, and the atmospheric temperature cools. Learn more about greenhouse gases and their role in regulating the Earth's temperature by watching "Global Warming: The Physics of the Greenhouse Effect".
- Another source of positive feedback during the ice ages involves another greenhouse gas we are familiar with: CO2. To understand how large quantities of CO2 was removed from the atmosphere during the Ice Ages, we need to introduce the idea of the "biological CO2 pump". The biological, or CO2 pump, is responsible for pulling CO2 out of the atmosphere and sequestering it in the deep recesses of the ocean, where it is unavailable for greenhouse warming of the atmosphere.
Biological CO2 Pump Facts
In temperate and high latitudes, spring brings warming temperatures, increasing biological activity and surface plankton in the ocean. These plankton “bloom”, then die and sink, some decomposing and becoming nutrients for other plankton; others falling to the ocean floor and becoming buried in sediment for thousands of years. The carbon found in these living organisms falls with them to the ocean depths. When CO2 in surface water is reduced, more CO2 is drawn into the surface water from the atmosphere. Glacial maxima corresponds with decreased atmospheric CO2. But why would there be more biological activity during the Ice Ages? With less atmospheric CO2, cooling of the upper ocean by radiation from the ocean surface increases. Once again, we look to positive feedback: Increased cooling would then increase convection and a greater depth of the mixed layer, where nutrients are concentrated, and a greater mixed layer would encourage more plankton growth, priming the "CO2 pump."
Now that you have an understanding of the natural cycles of the Ice Ages, the orbital forcing factors that initiated the growth of ice sheets, and the positive feedbacks in the climate system that amplified changes in latitudinal and seasonal distribution of insolation, think about the question from the beginning of this module: "Can natural climate cycles be responsible for today's warming climate?"
Go back to the "Vostok Core & Milankovitch Cycles Climate Applet" and answer the following three questions to make your own determination.
Examine again the Vostok ice core data. Select the fields for "Eccentricity," "Precession," and "Tilt" and look at the magenta line. Describe the curve from about 10,000 to present (the last wiggle). What do you see?Answer
Orbital forcing shows that we are in a "warm phase" and the line is now on the upswing, suggesting that according to past data, orbital forcing is in the direction of a cooling trend.
Natural orbital forcing is believed to initiate the growth of continental ice, but it is not believed to be sufficient in and of itself to cause glaciations. What does the temperature graph tell you about the role greenhouse gases play?Answer
They have a positive feedback effect, enhancing the cooling begun by changes in the spatial and temporal distribution of solar energy resulting from orbital forcing.
How do you see these resources on orbital forcing being used with students in your classroom to help them understand the relationships between natural cycles and today’s warming climate?
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