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This page first made public: Jun 29, 2011
A Jigsaw Approach to the Weathering Thermostat Hypothesis
Topic: weathering and climate feedback loops
Course type: upper level university
This activity teaches Climate Literacy Essential Principle 2: Climate is regulated by complex interactions among components of the Earth system
This activity teaches: Concept F. Equilibrium and Feedback Loops - The interconnectedness of Earth's systems means that a significant change in any one component of the climate system can influence the equilibrium of the entire Earth system. Positive feedback loops can amplify these effects and trigger abrupt changes in the climate system. These complex interactions may result in climate change that is more rapid and on a larger scale than projected by current climate models.
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
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:
- What happens to the Ca2+, Si4+ and HCO3- ions?
- What effect would higher atmospheric CO2 levels have on silicate weathering? Why?
- What effect do warmer temperatures have on silicate weathering? Why?
- What effect do higher precipitation rates have on silicate weathering? Why?
- 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.
- Show the effect of the warming or cooling on temperature, precipitation, vegetation and rate of chemical weathering.
- How do these changes affect atmospheric CO2 levels?
- How do the changes in CO2 levels affect the original warming or cooling?
- 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 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.
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.