Teacher Notes for Food, the Carbon Cycle and Global Warming

Teacher Notes for “Food, the Carbon Cycle and Global Warming

– How can we feed a growing world population without increasing global warming?”[1]

In the first section of this analysis and discussion activity, students analyze information about global warming and greenhouse gases. Students learn that correlation does not necessarily imply causation and analyze the types of evidence that establish causal relationships. In the next two sections, students analyze carbon cycles, the effects of food production on greenhouse gas emissions, and the reasons why the production of different types of food result in very different amounts of greenhouse gas emissions. The last section engages students in proposing and researching ways to feed the world’s growing population without increasing global warming.

Before beginning this activity, students should understand photosynthesis, cellular respiration, and trophic pyramids. For this purpose, I recommend these learning activities:

• “Photosynthesis and Cellular Respiration” (
• “Food Webs, Energy Flow, Carbon Cycle, and Trophic Pyramids” (

Internet access is desirable for some parts of this activity.

• Question 6ain the Student Handout has the URL for an animated graph for students to view. If Internet access is a problem, a possible alternative is suggested on page6of these Teacher Notes.
• To answer question 14c, ideally, students will access the information at the URL given in that question. An alternative is suggested on page12of these Teacher Notes.
• To answer question17, students will conduct Internet research using the sources listed on pages 9-10 of the Student Handout. Alternatively, you could provide printed copies of these sources.

If you are using the recommended Internet sources, you may want toensure that your students will be able to access these websites through your school Internet service.

I estimate that this activity will require at least three 50-minute class periods.

Table of Contents

Learning Goals – pages 2-3

Additional Information and Instructional Suggestions

General instructional suggestions – page 3

I. Global Warming and Greenhouse Gases – pages 3-8

II. CO2 and the Carbon Cycle – pages 8-9

III. Food Production and Greenhouse Gases – pages 10-13

IV. How can we feed a growing world population without increasing global warming? – pages 13-15

Sources for Student Handout Figures – page 16

Appendix – Grams of Protein per Kilogram of Greenhouse Gas Emissions for Various Foods – pages 17-18

Learning Goals

Given the nature of the topic, both the learning goals and the general approach are interdisciplinary. In accord with the Next Generation Science Standards[2]:

• Students will increase their understanding of Disciplinary Core Ideas:

• LS2.B: “… At each link upward in a food web, only a small fraction of the matter consumed at the lower level is transferred upward, to produce growth and release energy in cellular respiration at the higher level. … Some matter reacts to release energy for life functions, some matter is stored in newly made structures, and much is discarded. The chemical elements that make up the molecules of organisms pass through food webs and into and out of the atmosphere and soil, and they are combined and recombined in different ways. … Photosynthesis and cellular respiration are important components of the carbon cycle, in which carbon is exchanged among the biosphere, atmosphere, oceans and geosphere through chemical, physical, geological, and biological processes”
• ESS2.D: “The foundation for Earth’s global climate system is the electromagnetic radiation from the sun, as well as its reflection, absorption, storage and redistribution among the atmosphere, ocean and land systems, and this energy’s re-radiation into space.” “Changes in the atmosphere due to human activity have increased carbon dioxide concentrations and thus affect climate”
• ESS3.D: “Human activities, such as the release of greenhouse gases from burning fossil fuels, are major factors in the current rise in Earth’s mean surface temperature (global warming). Reducing the level of climate change… depends on the understanding of climate science, engineering capabilities, and other kinds of knowledge, such as understanding of human behavior and on applying that knowledge wisely in decisions and activities.”
• Students will engage in several Science and Engineering Practices.

• Constructing Explanations and Designing Solutions: “Apply scientific ideas, principles and/or evidence to provide an explanation of phenomena and solve design problems, taking into account possible unanticipated effects.” “Design, evaluate, and/or refine a solution to a complex real-world problem, based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and trade-off considerations.”
• Engaging in Argument from Evidence:“Evaluate competing design solutions to a real-world problem based on scientific ideas and principles, empirical evidence, and/or logical arguments regarding relevant factors (e.g. economic, societal, environmental, ethical considerations).”
• Obtaining, Evaluating and Communicating Information: “Critically read scientific literature adapted for classroom use to determine the central ideas or conclusions and/or to obtain scientific and/or technical information to summarize complex evidence, concepts, processes, or information presented in a text by paraphrasing them in simpler but still accurate terms.”
• This activity provides the opportunity to discuss the Crosscutting Concepts:

• Cause and effect: Mechanism and explanation. “Students understand that empirical evidence is required to differentiate between cause and correlation and to make claims about specific causes and effects.… They recognize changes in systems may have various causes that may not have equal effects.”
• Stability and Change: “Students understand much of science deals with constructing explanations of how things change and how they remain stable.… They recognize systems can be designed for greater or lesser stability.”
• This activity helps to prepare students for Performance Expectations:

• HS-LS2-5. “Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.”
• HS-LS2–7: “Design, evaluate, and refine a solution for reducing the impacts of human activities on the environment and biodiversity.”
• HS-ESS2-4: “Use a model to describe how variations in the flow of energy into an out of Earth’s systems result in changes in climate.”
• MS-ESS3-5: “Ask questions to clarify evidence of the factors that have caused the rise in global temperatures over the past century.”
• MS-LS2-3: “Develop a model to describe the cycling of matter… among living and nonliving parts of an ecosystem.”

General Instructional Suggestions

To maximize student learning, I recommend that you have your students complete groups of related analysis and discussion questions individually, in pairs or in small groups and then have a class discussion for each group of related questions.

A key for this activity is available upon request to Ingrid Waldron ( ). The following paragraphs provide additional information and instructional suggestions.

I. Global Warming and Greenhouse Gases – Additional Information and Instructional Suggestions

As you discuss the information and questions on page 1 of the Student Handout, you may want to introduce the distinction between climate and weather. Climate is more long-term, whereas weather is more short-term and subject to considerable variation from day to day. Temperature is just one aspect of climate or weather. Global warming has influenced other aspects of climate and weather (

The figure on page 1 of the Student Handout shows that independent estimates of global temperature trends have been quite similar. Changes in global temperature are estimated relative to the 1891-2010 average as explained in this quote:

“In the late 1970’s, Hansen turned his attention to similar calculations of the effects of Earth’s atmosphere on its surface temperature. As part of this work, he tackled the problem of creating a standardized method for calculating global average temperature trends. The method begins with the recognition that, while absolute temperatures are widely variable from place to place on the Earth, even for locations relatively close to one another, temperaturechangesof nearby locations tend to be very similar. For example, while the absolute temperatures in New York and Pittsburgh might be quite different on a particular day, if one is having a hotter than average month, the other is likely having a month hotter than average by aroundthe same amount. Thus, global temperature trends are plotted, not as absolute temperatures, but as temperature differences, called “temperature anomalies,” relative tosome reference temperature.”

(

Page 2 of the Student Handout introduces students to the concept that correlation does not necessarily imply causation. This illustrates the Crosscutting Concept, Cause and effect: “Students understand that empirical evidence is required to differentiate between cause and correlation and to make claims about specific causes and effects.… They recognize changes in systems may have various causes that may not have equal effects.”

Question 3 describes increases since 1900 in three variables. More specific data on the trends in greenhouse gases and men’s height are shown below. (The height data are for men only because early data from military recruits allows a much longer time series for men than for women.)

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I assume that students will recognize that there is no plausible way that increases in men’s height could cause global warming. This flowchart shows one way that variables can havecorrelated trends even when neither variable is causing the other.

In questions 4-6, students learn about the types of evidence that are used to establish causation and then analyze evidencethat increases in greenhouse gases have been the major cause of the increase in global temperature.You may want to point out to your students that each type of evidence could provide the opportunity to falsify the hypothesis (e.g.if no mechanism could be found to explain how greenhouse gases warm the earth or if the greenhouse gas hypothesis failed to predict future trends). Also, if tests of other hypothesis yield evidence for other causes of the increase in global temperature, then scientists would need to develop a more inclusive revised hypothesis about the causes of global warming. For example, current evidence supports the more inclusive hypothesis that global warming has been caused by increases in multiple greenhouse gases, including CO2, nitrous oxide and methane.

The Student Handout describes the basic mechanism by which greenhouse gases warm the earth. It should be noted that greenhouses work by a different mechanism. The sun’s light energy is absorbed by the dirt, plants and other objects inside the greenhouse and converted to heat energy; this heat energy warms the air, and the warm air is trapped by the walls and ceiling of the greenhouse.

In addition to the evidence included in the Student Handout, more detailed scientific evidence shows a good match between(1) the wavelengths of heat energy absorbed and emitted by CO2and other greenhouse gases in laboratory experiments and (2)the wavelengths of the heat energy that is radiated back to earth and prevented from escaping to space. Scientists’ understanding of weather systemsand the effects of greenhouse gases has become much more sophisticated since the original nineteenth century predictions. As a result, predictions of the effects of greenhouse gases are increasingly accurate, although there is still uncertainty about future trends, due in part to the complex feedback effects in climate models.

After discussing question 6 with your students, I recommend that youreview the answers to question 4 and the different types of evidence that support the interpretation that greenhouse gases have been the primary cause of increases in global temperature.

On the bottom of page 3 of the Student Handout the estimated amount of global warming is somewhat larger than is often quoted because the Student Handout estimate is based on more recent data ( sea level has risen by about 8 inches since 1880, and scientists expect global sea level to rise another 1-4 feet by 2100 ( The amount of flooding that would result from this rise in sea level would depend on actions taken to reduce the risk of flooding and also whether global warming resulted in the expected increase in hurricanes and typhoons. The potential for much larger future rises in sea level is described in

The figure below summarizes current evidence concerning the effects of global warming and climate change.

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This sectionof the Student Handout provides a brief introduction to global warming and greenhouse gases. If you want your students to have a more thorough introduction to global warming and climate change, you may want to use one of the recommended curricula listed on page 1 of “Resources for Teaching about Climate Change” ( list includes resources that can help your students develop more in-depth understanding of the causes and consequences of global warming and climate change. Also included are resources tointroduce students to proposals to reduce the amount of global warming and reduce the harmful effects of climate change.

Additional helpful sources for understandinggreenhouse gases and global warming include:

For more information on the observed and predicted effects of global warming, see:

• and for the US
• for effects in the US by state, region and sector
• for worldwide information.

For information on the effects of global warming on agriculture, see:

• for the US
• for a global perspective.

II. CO2 and the Carbon Cycle – Additional Information and Instructional Suggestions

Questions 7a and 7b review the role of photosynthesis and cellular respiration in the carbon cycle.

As you discuss question 7c, you may want to mention that the average American consumes almost 2000 pounds of food each year and clearly we do not gain nearly that much weight each year ( If your students have completed “Food Webs, Energy Flow, Carbon Cycle, and Trophic Pyramids” ( you will want to refer back to the section on trophic pyramids in that activity as you discuss question 7c in this activity. During the discussion of this question, you will probably want to include the following points.

• Cellular respiration converts many of the molecules in food to CO2 and H2O. CO2 leaves the body via breathing.Any H2O which is not needed to replace the H2O lost by breathing and sweating is excreted in the urine.
• Beverages and some foods (e.g. many fruits and vegetables) contain a lot of H2O, much of which is excreted in the urine.
• Some food molecules are not absorbed from the digestive system and leave the body in feces.

In discussing questions 7-9, you may want to introduce the concepts of:

• carbon fluxes (which are indicated by the arrows in the figures)
• carbon pools or stocks (e.g. fossil fuels and forests); carbon pools or stocks can serve as carbon sources and/or carbon sinks.

This figure shows the magnitude of significant carbon fluxes and carbon stocks.

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This figure illustrates several important points.

• CO2 is removed from the atmosphere primarily by photosynthesis and by diffusing into the ocean.[3]
• The burning of fossil fuels is the major source of imbalance that is causing increases in CO2 concentrations in the atmosphere. You may want to ask your students to identify the part of the carbon cycle that is not in balance in the figure in question 9; the rapid rate of burning fossil fuels is not balanced by the very slow processes that make fossil fuels.
• Major carbon pools include:
• oceans, with dissolved CO2and carbon in marine organisms and dead organic matter
• soils, with soil organisms and dead organic matter.

As you discuss questions 8 and 9, you may want to discuss the Crosscutting Concept, Stability and Change: “Students understand much of science deals with constructing explanations of how things change and how they remain stable.…” You could discuss the reasons for instability (e.g.the increases in atmosphericCO2) and what changes would be necessary to produce stability.

III. Food Production and Greenhouse Gases – Additional Information and Instructional Suggestions

Estimates of thecontribution of food production to the increase in greenhouse gases vary, due to differences in methods such as differences in which activities or geographic regions were included in the estimates. For example, the contribution of agriculture to greenhouse gas emissionsis estimated to be 9% for the US vs.~25-30% worldwide. You may want to ask your students to suggest a likely cause of this difference. Hopefully, your students will recognize the importance of thinking about the denominator as well as the numerator used to calculate these percents. The contribution of agriculture is a smaller part of a bigger total in the US, because we produce so much carbon dioxide throughour high use of fossil fuels for transportation, manufacturing, generation of electricity, and heating and air-conditioning (

The nitrogen cycle is more complex than the carbon cycle and is not included in the Student Handout. A major difference between the nitrogen and carbon cycles is that plants can take up carbon from the CO2 in the air, but they cannot take up nitrogen from the N2 in the air. Instead plants absorb nitrates from the soil. Nitrogen is often a limiting resource for plant growth, which explains why farmers frequently use nitrogen-containing fertilizers.