Atmospheric Carbon: Can We Offset the Increase?

Joceline Boucher, Corning School of Ocean Studies, Maine Maritime Academy
Author Profile


This activity lets students discover first hand how the big issues in climate change -- here, the increase in atmospheric CO2 and the utility of carbon offsets -- can be understood through measurement, application of simple concepts, and "back of the envelope" estimates. It also provides students both personal and global context for understanding the magnitude of current greenhouse gas emissions.

Used this activity? Share your experiences and modifications



I use the activity in the classroom with a non-majors, Chemistry In Context course, and for homework in an elective course, Changing Oceans, intended for marine science and marine biology majors. The activity would be ideally suited to an environmental chemistry course or a science-based course in climate change.

Skills and concepts that students must have mastered

Students need a rudimentary knowledge of chemistry (mole concept; ideal gas law) to do this activity.

How the activity is situated in the course

The activity serves as a "prequel" to a unit on ocean acidification, and could be used to introduce carbon sequestration or the concept of carbon footprints. It could also be used after a unit on greenhouse gases.


Content/concepts goals for this activity

This activity builds familiarity with climate change vocabulary and enables students to demonstrate, from simple calculations, that other sinks beside the atmosphere must exist for anthropogenic carbon emissions.

Higher order thinking skills goals for this activity

  • Problem solving
  • Analysis of graphs
  • Reflection on societal concerns

Other skills goals for this activity

This activity will help to build numeracy skills.

Description and Teaching Materials

The activity can be done in small, in-class groups, following a unit on greenhouse gases and the enhanced greenhouse effect. It might also be assigned for homework prior to a unit on ocean acidification.

Teaching Notes and Tips

Most students will have forgotten the formula for the volume of a sphere, but usually one or two will remember and can be encouraged to share.

Because my students are mostly from Maine, the worksheet uses this location as an example where reforestation could occur. It's not a very good example, since Maine is already about 90% forested. Instructors may wish to substitute another place in this part of the exercise. Nowak et al. (2013; see link below) provide carbon uptake estimates for urban tree planting; the IPCC report (link below) provides uptake ranges for boreal, temperate, and tropical forests.

After the activity, I like to show a website offering carbon offsets through tree planting (find these by searching for "buy carbon offset".)

Besides showing that reforestation cannot offset anthropogenic emissions of CO2, the activity could also prompt a class discussion on the climatic feedbacks associated with tree planting and changes in land cover (see Caldeira et al. link below).


I usually grade this activity as check/check-plus/check-minus.

References and Resources

Caldeira, K., Gibbard, S., Bala, G., Wickett, M. E., & Phillips, T. J. (2005, December). Will growing forests make the global warming problem better or worse?. In AGU Fall Meeting Abstracts (Vol. 1, p. 03). Online:

Nowak, D. J., Greenfield, E. J., Hoehn, R. E., & Lapoint, E. (2013). Carbon storage and sequestration by trees in urban and community areas of the United States. Environmental Pollution, 178, 229-236. Online:

Watson, R. T. (2000). Land use, land-use change, and forestry: a special report of the intergovernmental panel on climate change. Cambridge University Press. Online:

To update graphics: