"The material released by fossil-fuel burning is grossly dominated by CO2. At first, CO2 may not seem like an especially alarming by-product—it is essential for life, after all, and is around us all the time. Yet, as the early physician Paracelsus stated in the sixteenth century, 'All things are poison and nothing is without poison, only the dose permits something not to be poisonous.' This is often simplified to 'the dose makes the poison.' If you were to chat with someone who barely escaped drowning and then with someone who has faced the desert without water, you would surely learn that too much as well as too little of something can be bad." Richard Alley in the companion ETOM book.

Read more: Cleaning Up After Ourselves - The Scientist - Magazine of the Life Sciences

Segment 6

How do we know that today’s levels of CO2 are caused by humans burning fossil fuels, and not by some natural process, such as volcanic outgassing? In a closely-argued, step-by-step explanation, Richard shows that by using physics and chemistry we can analyze the “flavors” (isotopes) of carbon in Earth’s atmosphere to detect the clear signature of humans burning fossil fuels rather than the increase being the result of purely natural causes.

Annotated Script
Interactive version: panels 55-60

• Students will understand that carbon atoms come in three different isotopic forms.
• Students will learn that scientists can tell where the carbon dioxide in the atmosphere is coming from based on analysis of the carbon atoms in the CO2 molecule.
• Students will understand the evidence that demonstrates that the increase of carbon dioxide in Earth’s atmosphere relates to humans burning plant-based fossil fuels.

Anthropogenic, carbon dioxide, climate, fossil fuels, ice sheet, oxygen, radioactive decay, sea level rise

One of the most common disagreements between “climate contrarians” and most mainstream climate scientists about what’s happening to Earth’s climate is whether the changes we are seeing are simply “natural variation” or the result of human actions. (The preceding CO2 & THE ATMOSPHERE segment shows how Earth’s orbit around the Sun shifts sunlight around the planet, triggering changes that release or capture CO2, and—in a feedback effect—then change temperature.) There’s no disagreement that Earth has been much hotter in the past, and that levels of carbon dioxide have been much higher at times when there were no industrialized human civilizations around to affect the climate. So, to put the question provocatively, are we humans “guilty” of causing today’s levels of CO2, or are we “innocent”? And does science provide an objective way to find out?

This video segment lays the foundation for a set of activities centered on “anthropogenic” (or human-caused) drivers of climate change. Begin this unit by asking students what they already know (prior knowledge) about causes of global climate change. Accept and list all answers, including misconceptions and contradictory responses. Then brainstorm what students might need to know (what they want to find out) to verify their understandings.

For this somewhat controversial topic, you may want to have a class discussion about people’s “beliefs” versus scientists’ conclusions, which are based on evidence rather than feelings. Perhaps ask them “Do you believe Earth’s temperature is warming?” “Do you believe in climate change?” See also the Interview with Rear Admiral David Titley at this website in which he lists the multiple observations—not beliefs—which underpin the Pentagon’s assessment of the reality of climate change.


After undertaking a few of the following activities, be sure to return to what students have learned about evidence for a human role in the escalating CO2 levels, in order to have them reflect on how their initial ideas may have changed.

One suggested sequence is to introduce The Atom, then play the “It’s Us” video clip, and then undertake one or more of the remaining activities. The first promotes the use of math as a tool and the second incorporates hands-on investigations. The third is more complex, requiring more time, materials and a prudent assessment of student safety.

In order to appreciate the differences between the “flavors” of carbon referred to in the clip, students can benefit from an introduction to atomic structure and the definition of isotopes.  This short activity provides a quick but useful overview for the carbon discussion in the video clip.

1) CONCEPT:  Individual atoms of carbon may contain different numbers of electrons.


This is an interactive, online module that teaches students about basic atomic structure. Appropriate for middle school and up, it contains a more robust background essay that provides additional information.

In the interest of time you may want to engage your whole class together by projecting this activity on a screen or whiteboard. After you have run through the tutorial, which takes only a few minutes, be sure to have the students read and discuss the essay, which contains the description of isotopes. You will need computer access and connectivity. High school teachers can probably skip this activity.

2) Concept: Humans’ use of energy from fossil fuels contributes to higher CO2 levels in the air.

ACTIVITY: How Much Do You Spew?

Students analyze the energy consumption of a hypothetical household to determine the amount of carbon dioxide they are adding to the atmosphere each year.

Fictitious families’ energy use is examined by teams in this grades 6-10 activity. You will need calculators for the math involved. Allow adequate time for reflection.

3) CONCEPT: Students experiment with carbon dioxide and so gain hands-on experience with what is a colorless, odor-less gas.

ACTIVITY: Global Climate Change: The Effects of Global Warming

Students conduct an experiment to learn about CO2 levels found in four different gases: the air, our breath, car exhaust*, and the product of a chemical reaction between baking soda and vinegar. *PLEASE NOTE THAT YOUR SCHOOL AND DISTRICT MAY VERY WELL HAVE RESTRICTIONS ON SAMPLING SUCH GASES, ESPECIALLY CAR EXHAUST. You, not ETOM, are solely responsible for ensuring students comply with all prudent and mandated safety requirement.

Students then reflect on CO2 production on a global scale. They also look at evidence of global warming in our environment, and consider their own role in contributing to global warming, especially from their use of automobiles.

Note: This activity was adapted with permission from an early draft version of "Sampling Carbon Dioxide," in Chapter 5, pages 44-47, of the book Climate Change in the Global Systems Science (GSS) project series. Copyright 2004 by the Regents of the University of California. The latest version can be found at Global Systems Science . The GSS project materials are produced by the Lawrence Hall of Science, University of California, Berkeley.

This activity targets grades 9-12 and suggests computer access for each pair of students. It is richly media-infused and will take several periods to complete. Hands-on materials are inexpensive and locally available, however you will need to source bromthymol blue in advance. Questions which check for understanding are included. Again, please note the safety disclaimer placed above.

4) CONCEPT: Carbon atoms cycle through various locations over the expanse of geological time: what matters is where the carbon resides: buried in fossil fuels, in the atmosphere, or in dinosaur breath!

ACTIVITY: Dinosaur Breath—Learning about the Carbon Cycle

This activity illustrates the carbon cycle using an age-appropriate hook, and includes thorough discussion and hands-on experimentation. Students learn about the geological (ancient) carbon cycle, they investigate the role of dinosaurs in the carbon cycle, and the eventual storage of carbon in the form of chalk. Students discover how the carbon cycle has been occurring for millions of years and is necessary for life on Earth. Finally, they may extend their knowledge to the concept of global warming and how engineers are working to understand the carbon cycle and reduce harmful carbon dioxide emissions. The activity takes one 50 minute period, implementing the extensions takes more time. Additional materials are necessary.

It’s important that students feel they can be part of climate change solutions for the future. As an extension you may want to challenge your students to come up with one method their family can use to reduce their carbon footprint and to share it with the class.  There are many online sites that offer students information and opportunities to become involved. 

One example you can follow up with is the EPA Kids Site: http://www.epa.gov/climatechange/kids/solutions/index.html

Grades 6-8
ESS Core Idea 4:
Human activities are constrained by and, in turn, affect all other processes at Earth’s surface.
How do human activities alter Earth?
How do human activities alter Earth’s climate?

ET Core Idea 3:
People are surrounded and supported by technological systems.
Effectively using and improving these systems is essential to long-term survival and prosperity.
How do systems relate to larger and smaller systems?

Grades 9-12
ESS Core Idea 3:
Earth’s surface continually changes from the cycling of water and rock driven by sunlight and gravity.
How does climate change over space and time in response to both natural and man-made causes?



1 2 3 4 5 6 7 8 9 10


1 2 3 4 5 6 7 8 9


1 2 3 4 5 6 7 8