A Jigsaw Approach to the Weathering Thermostat Hypothesis

This activity is part of the community collection of teaching materials on climate and energy topics.

These materials were submitted by faculty as part of the CLEAN Climate Workshop, held in June, 2011 and are not yet part of the CLEAN collection of reviewed resources.

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Goals

Students should be able to do the following:

• explain how the weathering of silicates is affected by environmental variables, such as temperature and precipitation rates
  
• draw and explain a negative feedback loop involving silicate weathering and atmospheric CO₂
• explain what a negative feedback loop is and how it tends to promote equilibrium

Description

Instructor provides an introduction to the weathering cycle and connection to ocean chemistry. We consider the following question as a group before splitting up for the Jigsaw portion of the exercise:

If we take the chemistry of wollastonite (CaSiO₃) to represent continental rocks, what is the chemical equation of weathering with carbonic acid (H₂CO₃)?

Students conduct research and develop expertise in one aspect of the weathering-CO₂ cycle. Each student produces a 1-2 page description of their area of expertise. Students studying the same aspect then meet to deepen understanding and identify and clear up any misconceptions. Groups check in with instructor or teaching assistant.

These are the aspects of the weathering-climate cycle the students will consider:

1. What happens to the Ca²⁺, Si⁴⁺ and HCO₃^- ions?
2. What effect would higher atmospheric CO₂ levels have on silicate weathering? Why?
3. What effect do warmer temperatures have on silicate weathering? Why?
4. What effect do higher precipitation rates have on silicate weathering? Why?
5. What effect does increased vegetation have on silicate weathering? Why?

Students are then redistributed into mixed groups and learn about the entire cycle through peer teaching. Each of the mixed groups is given an external forcing of either "Climate becomes warmer" or "Climate becomes cooler". They need to answer the following questions and draw a figure representing the feedback cycle.

1. Show the effect of the warming or cooling on temperature, precipitation, vegetation and rate of chemical weathering.
2. How do these changes affect atmospheric CO₂ levels?
3. How do the changes in CO₂ levels affect the original warming or cooling?
4. Draw the connections as a loop or cycle and explain how equilibrium in the system is promoted by the weathering cycle.

Each group produces a poster explaining the negative feedback cycle. Class time is spent visiting the posters, providing peer feedback and a final group discussion.

During the final group discussion, we emphasize that both final scenarios involve "negative" feedback. In each case, there is a reduction of the initial forcing, whether it is a warming or a cooling.

Assessment

Assessment is based on the 1-2 page summaries, the group posters and a portion of the midterm/exam.

Connections to other Activities

I used a CLEAN resource called Understanding the Carbon Cycle: A Jigsaw Approach by David Hastings of Eckerd College as a template for this exercise. It would be a good introduction to the carbon cycle and the jigsaw method and this activity could follow.

References

I use "Earth's Climate: Past and Future" by William F. Ruddiman as a text for this exercise. Students would need to use a variety of reference materials for the initial research part of the activity.