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Modeling Early Earth Climate with GEEBITT

Cindy Shellito
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University of Northern Colorado
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Summary

This activity requires students to use a simple spreadsheet model to consider the roles of solar luminosity, planetary albedo, and greenhouse gases in controlling surface temperature in the present and during the Archean and Proterozoic.

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Context

Audience

I have used this activity in an undergraduate (junior level) Paleoclimatology course, required for Environmental Studies majors and Earth Science Secondary Education majors. The only prerequisite for the course is one introductory level meteorology course.

Skills and concepts that students must have mastered

Students should be somewhat familiar with current understanding of the early evolution of the atmosphere, the role of greenhouse gases in the 'Faint-Young Sun' Paradox, and evidence for Proterozoic glaciation. Students should also have had an introduction to concepts in atmospheric radiation: relationship between emitted radiation and temperature (qualitative understanding of Stefan-Boltzmann Law), black-body temperature, shortwave vs. longwave radiation, albedo, and the role of greenhouse gases in absorbing longwave radiation. It is also helpful if students have some understanding of the roles of models (of varying levels of complexity) in climate research.

How the activity is situated in the course

This activity is a stand-alone exercise, introduced after discussions of Early Earth climate. The activity requires a full 50-minute class period, including some introduction to the model in a previous class period.

Goals

Content/concepts goals for this activity

Explain how albedo regulates surface temperature.
Describe how an Energy Balance Model works.
Describe the roles and limitations of parameterization in a climate model.
Compare climate of the Archean and Proterozoic with modern climate.
Provide an explanation for the 'Faint-Young Sun Paradox'.

Higher order thinking skills goals for this activity

Collect and analyze data from a simple model.
Develop a hypothesis regarding factors that have the greatest effect on climate.
Develop an understanding regarding the limitations of using a model to understand past (and present) climate.

Other skills goals for this activity

Students work in groups to collect and evaluate data, then share their results in a class discussion.
Students become more familiar with operating a spreadsheet, such as Excel.

Description of the activity/assignment

In preparation for this activity, students read articles on the climate of the Archean and the Neoproterozoic 'snowball' episodes (in Scientific American: When Methane Made Climate, by J. Kasting (July 2004) and Snowball Earth, by Hoffman and Schrag (January 2000)). In class, students work in groups of two or three using a simple spreadsheet climate model which allows them to calculate global mean surface temperature based on solar luminosity, albedo, and an atmospheric greenhouse gas loading parameter. They are asked to consider how each of these parameters must change from that of present day climate in order to (1) produce an Archean climate favorable to methanogens and (2) produce a 'snowball' earth. The activity requires students to collect data from the model. They acquire practice using a spreadsheet and working with others to decide how to conduct their model 'experiments'. While becoming more familiar with the physical processes that made the early earth climate so different from that of today, they also acquire first-hand experience with a limitation in modeling, specifically, parameterization of critical processes.

Determining whether students have met the goals

(1) Students are evaluated informally in class discussion. After completing the in-class portion of the exercise, students what they feel were their most surprising/interesting results.
(2) Three homework questions at the end of the in-class activity require students to analyze their model results, evaluate the effectiveness of the EBM, and make suggestions for improving the model. Students have met the goals of the assignment if they can present their data clearly, and discuss, based on their data, the importance of solar luminosity, albedo, and greenhouse gases in affecting climate. They must also clearly discuss the role and limitations of the Greenhouse Factor parameter, and the implications of this in their results.

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Supporting references/URLs

The GEEBITT Model at NASA GISS:
http://icp.giss.nasa.gov/education/geebitt/

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