1. Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, UCB 449, Boulder, Colorado 80303, USA
2. Institute of Behavioral Science, University of Colorado at Boulder, 483 UCB, Boulder, Colorado 80309, USA
3. Science Education Resource Center, Carleton College, One North College St., Northfield, Minnesota 55057, USA
4. Department of Geological Science, University of Colorado at Boulder, UCB 399, Boulder, Colorado 80309, USA