Teacher Notes for “Food and Climate Change– How can we feed a growing world population without increasing global warming?”1

In the first section of this activity, students analyze information about climate change, global warming and greenhouse gases. Students learn that correlation does not necessarily imply causation, and they analyze the types of evidence that establish causal relationships. In the next two sections, students analyze carbon cycles, how food production results in the release of greenhouse gases, and the reasons why the production of different types of food results in the release of very different amounts of greenhouse gases. In the last section, students propose and research strategies 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” (https://serendipstudio.org/exchange/bioactivities/photocellrespir)

“Food Webs, Energy Flow, Carbon Cycle, and Trophic Pyramids” (http://serendipstudio.org/sci_edu/waldron/#ecolfoodweb)

Internet access is desirable for some parts of this activity. Question 7a in 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 page 6 of these Teacher Notes.

To answer question 17, students will conduct Internet research using the sources listed on pages 8-9 of the Student Handout. Alternatively, you could provide printed copies of these sources.

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

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

Table of Contents Learning Goals – pages 1-3Additional Information and Instructional Suggestions

General instructional suggestions – page 3I. Climate Change, Global Warming and Greenhouse Gases – pages 3-7II. CO2 and the Carbon Cycle – pages 8-9III. Food Production and Greenhouse Gases – pages 9-13IV. How can we feed a growing world population without increasing global warming? –

pages 13-16Sources for Student Handout Figures – page 16

Learning GoalsGiven the nature of the topic, both the learning goals and the general approach are interdisciplinary. In accord with the Next Generation Science Standards2:

1 By Dr. Ingrid Waldron, Department of Biology, Univ. Pennsylvania, © 2019. These Teacher Notes and the Student Handout for this activity will be available at https://serendipstudio.org/exchange/bioactivities/global-warming

2 http://www.nextgenscience.org/sites/default/files/HS%20LS%20topics%20combined%206.13.13.pdf1

Students will increase their understanding of Disciplinary Core Ideas:o 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”

o 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”

o 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.o 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.”

o 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).”

o 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:o 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.”

o 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:o 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.”

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o HS-LS2–7: “Design, evaluate, and refine a solution for reducing the impacts of human activities on the environment and biodiversity.”

o HS-ESS2-4: “Use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.”

o MS-ESS3-5: “Ask questions to clarify evidence of the factors that have caused the rise in global temperatures over the past century.”

o 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 (iwaldron@upenn.edu ). The following paragraphs provide additional information and instructional suggestions.

I. Climate Change, 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.

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, temperature changes of 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 around the same amount. Thus, global temperature trends are plotted, not as absolute temperatures, but as temperature differences, called “temperature anomalies,” relative to some reference temperature.”(http://www.rescuethatfrog.com/history-of-evidence/)

Average global temperature has increased ~1°C or ~2°F, 1920-2017 (https://www.nytimes.com/interactive/2018/01/18/climate/hottest-year-2017.html; https://www.eurekalert.org/pub_releases/2018-01/uoa-rji012418.php). As discussed on the bottom of page 3 of the Student Handout, this seemingly small increase in global temperature has had multiple effects (https://science2017.globalchange.gov/; https://nca2018.globalchange.gov/; https://www.ipcc.ch/sr15/chapter/chapter-3/; “Increased Occurrence of Record-Wet and Record-Dry Months Reflect Changes in Mean Rainfall” Geophysical Research Letters 45, 2018).

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On average, global sea level has risen by about 8 inches since 1880. Rising sea levels have been caused by increased melting of glaciers and the expansion of ocean water as the oceans have warmed. Scientists expect global sea level to rise another 1-4 feet by 2100 (https://science2017.globalchange.gov/chapter/12/). This would probably result in substantial flooding in many cities in the US (e.g. Miami, New York City) and around the world. The amount of flooding would depend on actions taken to reduce the risk of flooding and whether global warming results in the expected increase in hurricanes and typhoons. The potential for much larger future rises in sea level is described in http://www.johnenglander.net/chart-of-420000-year-history-temperature-co2-sea-level/.

Page 2 of the Student Handout introduces students to the concept that correlation does not necessarily imply causation. You can discuss the generality of this concept, using 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 mentions that men’s height has increased during the last century (see figure 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.)

Birth cohort (dates of birth)(http://www.thelancet.com/cms/attachment/2060860246/2062777442/gr1.jpg)

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Adult

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

On pages 2-3 of the Student Handout, students learn about the types of evidence that are used to establish causation and then analyze evidence that increases in CO2 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 the 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 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 actual greenhouses work by a different mechanism. When the sun’s light energy is absorbed by the dirt, plants and other objects inside the greenhouse, it is converted to heat energy which 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 CO2 and 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 systems and 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.

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The animated graph referred to in question 7a provides an engaging way of presenting the contributions of different factors that influence global temperature. (These causal factors are sometimes referred to as forcings.)

This figure provides similar information and can be used in the Student Handout if you do not have Internet access. If you use this figure, you will need to revise the table in question 7a.

In this figure, natural variability refers to the El Niño/Southern oscillation. The anthropogenic component includes a warming component from greenhouse gas concentrations and a cooling component from anthropogenic aerosols. (Source of figure: https://science2017.globalchange.gov/chapter/3/)

After discussing question 7 with your students, I recommend that you review the answers to question 5 and the different types of evidence that support the interpretation that increased greenhouse gases have been the primary cause of increases in global temperature.

Several resources are available if you want to supplement the brief introduction to climate change, global warming and greenhouse gases in this section of the Student Handout. For example, you may want to use one of the recommended curricula listed on page 1 of “Resources for Teaching about Climate Change” (http://serendipstudio.org/exchange/bioactivities/ClimateChange). This annotated 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 to introduce students to proposals to reduce the amount of global warming and reduce the harmful effects of climate change.

Additional helpful sources for understanding greenhouse gases and global warming include: http://carbonconnections.bscs.org/curriculum/unit-01/lesson-03.php#c

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http://nas-sites.org/americasclimatechoices/more-resources-on-climate-change/climate- change-lines-of-evidence-booklet/

https://www.skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse- effect.htm

https://www.livescience.com/58203-how-carbon-dioxide-is-warming-earth.html

For more information on the observed and predicted effects of global warming, see: https://nca2018.globalchange.gov/ , https://health2016.globalchange.gov/ and

https://climate.nasa.gov/effects/ for the US https://www.ipcc.ch/report/ar5/syr/ for worldwide information.

For information on the effects of global warming on agriculture, see: https://nca2014.globalchange.gov/report/sectors/agriculture for the US https://www.imf.org/external/pubs/ft/fandd/2008/03/pdf/cline.pdf for a global

perspective.

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

(http://www.ipcc.ch/report/graphics/images/Assessment%20Reports/AR5%20-%20Synthesis%20Report/SPM/SPM.04_rev1-01.png)

II. CO2 and the Carbon Cycle – Additional Information and Instructional Suggestions 7

Question 8 reviews the role of photosynthesis and cellular respiration in the carbon cycle.

As you discuss question 9, 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 (http://serendipstudio.org/exchange/files/FoodEnergyWtTN.pdf). If your students have completed “Food Webs, Energy Flow, Carbon Cycle, and Trophic Pyramids” (http://serendipstudio.org/sci_edu/waldron/#ecolfoodweb), you will want to refer back to the section on trophic pyramids in that activity as you discuss question 9 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 that is not needed to replace the H2O lost by breathing and sweating is excreted in the urine.

Some food molecules are not absorbed from the digestive system and leave the body in feces. Beverages and some foods (e.g. leaves, many fruits and vegetables) contain a lot of H2O,

much of which is excreted in the urine.

For your discussion of question 11, it may be helpful to know the magnitudes of various fluxes into and out of the atmosphere and the magnitudes of various carbon pools (e.g. fossil fuels and plants; carbon pools can serve as carbon sources and/or carbon sinks).

(http://globecarboncycle.unh.edu/diagram.shtml)

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The above figure illustrates several important points. CO2 is removed from the atmosphere primarily by photosynthesis and by diffusing into the

ocean. The oceans are a major carbon pool, with dissolved CO2 and carbon in marine organisms and dead organic matter.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 11; the rapid rate of burning fossil fuels is not balanced by the very slow processes that make fossil fuels.

As you discuss questions 10 and 11, 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 atmospheric CO2) and what changes would be necessary to produce stability.

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

Estimates of the contribution 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 emissions is 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 CO2 through our high use of fossil fuels for transportation, manufacturing, generation of electricity, and heating (https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions#agriculture).

The figure on the next page shows that the concentrations of methane and nitrous oxide in the atmosphere increased during the twentieth century. Scientists have estimated the global warming potential of one ton of each gas over 100 years, relative to the global warming potential of one ton of CO2; the estimated global warming potential of methane is 28-36 and the estimated global warming potential of nitrous oxide is 265-298 (https://www.epa.gov/ghgemissions/understanding-global-warming-potentials). These estimates, combined with estimates of the total amount of each gas released during food production, have yielded the following conclusions concerning the relative contributions to global warming of different greenhouse gases released during food production. About half of the contribution of food production to global warming is due to agricultural sources of methane and nitrous oxide. CO2 released as a result of conversion of forests and other natural ecosystems to crop land and pasture contributes roughly 40%, and burning fossil fuels in agricultural machinery contributes roughly 10% (calculated from data on page 17 of Food in the Anthropocene, https://www.thelancet.com/commissions/EAT).

3 CO2 dissolved in water increases acidity, because CO2 + H2O – > H2CO3 – > H+ + HCO3-. Increasing acidity of ocean waters is causing problems for some types of marine organisms.

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(http://nas-sites.org/americasclimatechoices/more-resources-on-climate-change/climate-change-lines-of-evidence-booklet/evidence-impacts-and-choices-figure-gallery/)

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.

(http://www.physicalgeography.net/fundamentals/9s.html)

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Lifecycle assessment is sometimes called cradle-to-grave assessment since the goal is to include greenhouse gas emissions associated with the entire production and disposal process. (It should be mentioned that many lifecycle assessments are incomplete because it is difficult to obtain complete data.) A good introduction to lifecycle assessment is available beginning on page 6 of https://static.ewg.org/reports/2011/meateaters/pdf/methodology_ewg_meat_eaters_guide_to_health_and_climate_2011.pdf?_ga=2.107227778.1489121.1517758754-1352643161.1515275837.

The same source provides a more detailed description of the methods used to generate the data in the figure on page 6 of the Student Handout. Your students may need help to understand that in this figure, the height of each bar indicates total emissions, with postproduction farm gate emissions stacked on top of production emissions (which are all emissions before the product leaves the farm). This figure was chosen because it provides comprehensive estimates of greenhouse gas emissions for a variety of foods and these estimates were produced by a consistent methodology and based on US data. A problem with these estimates is that the measure of greenhouse gases per kilogram of consumed food is distorted by differences in water content. This problem particularly affects estimates for milk, which has a very high water content.

A better measure is the amount of greenhouse gases released per 1000 calories of food consumed, which is introduced on the top of page 7 of the Student Handout. (I am adopting the common usage of calories, which should more properly be called kilocalories.) More complete data from this study are given in the table below.

Type of Food Kg CO2e / Edible 1000 calories Beef 9.60Pork 2.03Dairy 1.85Poultry 1.71Rice* 1.50Eggs 1.45Potatoes 0.96Wheat 0.24(Source of data: https://www.pnas.org/content/111/33/11996)

*Methane production by microbes in rice patties accounts for the relatively high release of greenhouse gases for this plant food.

Methane is produced by certain types of archaea in anaerobic conditions. As explained in the Student Handout, microorganisms produce methane during digestion in ruminant animals such as cows. Of the three main types of animals raised for meat in the US (cows, pigs and chickens), only cows are ruminants. Other ruminants include sheep (lambs), goats and deer.4 In the US cows are the biggest agricultural source of methane. The figure below describes ruminant digestion in a cow.

4 Some animals like horses have only one stomach, but they can live on a diet of grass because microorganisms in a large cecum can digest cellulose.

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(http://www.zo.utexas.edu/faculty/sjasper/images/ruminantdigestion.jpg)

If you want your students to compare digestion in humans vs. cows, you could use the following question.

Explain the reasons for both of these differences between cows and humans: Cows can live on a diet of grass and hey, but humans cannot. A cow releases much more methane to the atmosphere than a human.

For the comparison between beef and chicken in question 14, students should realize that the size of the animal is irrelevant for explaining differences in the amount of greenhouse gases released per kilogram of meat or per 1000 food calories. As shown in the graph on page 6 of the Student Handout, the primary difference in greenhouse gas emissions occurs during the production phase, while post-production emissions are relatively similar.5 For some students, you may need to inform them that chickens are not ruminants, so they can deduce that one reason for the greater greenhouse gas emissions during the production of beef is the substantial methane production by cows, but not chickens. You may also want to discuss that cows are less efficient than chickens at converting feed to meat for human consumption, so more feed production is required for each kilogram of beef (see figure below).

5 The percent of greenhouse gas emissions that occurs after food leaves the farm is 10% for beef vs. 48% for chicken. You may want to ask your students why. The primary reason why the percent is lower for beef is the difference in the denominators for calculating these percentages. The same methodological point is relevant for understanding why buying locally grown foods makes a relatively larger contribution to reducing greenhouse gas emissions from production of vegetables (10-30%), but has a relatively smaller impact for meat (1-3%). It should also be mentioned that fresh vegetables are more often transported by air freight which results in particularly high CO2 emissions.

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(https://www.ewg.org/meateatersguide/a-meat-eaters-guide-to-climate-change-health-what-you-eat-matters/climate-and-environmental-impacts/)

IV. How can we feed a growing population without increasing global warming? – Additional Information and Instructional Suggestions

For the population growth figure (on the bottom of page 7 of the Student Handout), the dashed line gives the median estimate of future population growth and the shaded area indicates the 95% confidence intervals for these predictions (https://www.nextbigfuture.com/wp-content/uploads/2017/07/23c953c49ec8e57b9314f83f2694bdae.png?x71037).

Incomes have increased in many developing countries in recent decades, which has resulted in increases in meat and dairy consumption (see table below). Notice that the levels of meat and dairy consumption are still much lower in developing countries than the levels in developed countries.

Per capita Food ConsumptionDeveloping Countries Developed Countries1969-1971 2005-2007 1969-1971 2005-2007

Meat (carcass weight; kilograms/person/year) 11 28 63 80

Milk and dairy (excluding butter; kilograms fresh milk equivalent/person/year)

29 52 189 202

(Source of data: Annual Review of Environment and Resources, 2015, 40:177-202)

In discussing solutions to global warming-related problems, it is useful to distinguish between mitigation and adaptation. Mitigation refers to “reducing the amount and speed of future climate change by reducing emissions of heat-trapping gases or removing carbon dioxide from the atmosphere.” Adaptation “refers to actions to prepare for and adjust to new conditions, thereby reducing harm or taking advantage of new opportunities. Mitigation and adaptation actions are

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linked in multiple ways, including that effective mitigation reduces the need for adaptation in the future. Both are essential parts of a comprehensive climate change response strategy. The threat of irreversible impacts makes the timing of mitigation efforts particularly critical.” (http://nca2014.globalchange.gov/highlights/overview/overview#intro-section-2). This learning activity focuses on one aspect of mitigation and does not discuss adaptation, although adaptation will also be needed. An introduction to adaptation and mitigation in agriculture is available at https://csa.guide/csa/what-is-climate-smart-agriculture.

Scientists have concluded that changes in food production and changes in diet (especially reduced beef consumption) must both be part of any plan to keep global warming below a level that would very likely result in serious damage to our environment (https://www.nature.com/articles/nclimate2353). Useful summaries of ways to reduce greenhouse gas emissions associated with food production are available at: https://www.wri.org/blog/2018/12/how-sustainably-feed-10-billion-people-2050-21-charts on page 20 in “Meat Eater’s Guide to Climate Change + Health – Lifecycle Assessments

Methodology and Results” (https://static.ewg.org/reports/2011/meateaters/pdf/methodology_ewg_meat_eaters_guide_to_health_and_climate_2011.pdf?_ga=2.107227778.1489121.1517758754-1352643161.1515275837)

For a more detailed and comprehensive discussion of healthy diets, environmental effects of the production of different types of food, and a plan for feeding a growing world population while limiting greenhouse gas emissions and other environmental effects, see “Food in the Anthropocene: the EAT-Lancet Commission on healthy diets from sustainable food systems” (https://www.thelancet.com/commissions/EAT; see e.g. Table 1 on page 5 and pages 24-28). The table below summarizes the relative amounts that various proposed strategies could contribute to decreases in global greenhouse gas emissions in 2050. Notice that global adoption of the proposed healthy diet, including reduced consumption of animal products, has the greatest potential to reduce greenhouse gas emissions that will result from food production.

% Decrease in Greenhouse Gas Emissions Compared to Continuing Current Practices, 2050Proposed Strategy % DecreaseAdoption of the proposed healthy diet

~50%

Improved agricultural practices ~10%Reducing waste by half ~7%All three strategies combined ~60%

(calculated from data in Figure 6 on page 27 of https://www.thelancet.com/commissions/EAT)

In the list of Reliable Relevant Sources in the section, “Healthy eating can reduce greenhouse gas emissions”, reference c proposes a tax on meat as one way to reduce meat consumption. To help students understand the rationale for these taxes, you may want to introduce the idea of an externality; an externality is a consequence of a commercial activity that adversely affects other people, but is not included in the price to the consumer. For example, consumers do not pay for the costs of harm to others caused by greenhouse gas emissions and water pollution due to production of food or other goods. One general approach to the problems of externalities is to use taxes to offset the cost to society of externalities and to shift people’s behavior away from behaviors that result in externalities and toward behaviors with the maximum benefit for society.

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Taxes on meat could be viewed as offsetting the cost of greenhouse gas emissions and other environmental problems associated with meat production.

Important criteria for assessing proposals include effectiveness, technical feasibility, cost, and whether the proposed actions would result in harmful and/or beneficial “side effects”. Examples of potential beneficial side effects include:

better health associated with plant-based diets6

reduced eutrophication of aquatic ecosystems if excess fertilizer use is avoided by applying optimum amounts at optimum times.

For some proposed actions, technological advances could change the effectiveness, feasibility, cost and side effects. In addition to technical feasibility, we should consider societal and political feasibility, taking into account that this may change as the effects of global warming become more evident and people become more informed. If the open-ended version of question 16 would be too challenging for your students, you could use this substitute.

16. This table suggests several criteria to evaluate each proposed strategy to reduce the amount that future food production will contribute to greenhouse gas release. Complete the second column of this table.

A good proposed strategy should: Do you agree with this proposed criterion? If you disagree, how would you change the criterion?

be expensive

be feasible

have harmful side effects

significantly reduce the release of greenhouse gases

You should have a class discussion of question 16 before your students begin question 17.

Question 17 leaves the format of the student reports up to you. Depending on your students, you may want to remind them of the following:

Each student should think about which information in the source articles is relevant to evaluating and/or improving their proposal. They should not include information that is not relevant for this purpose!

If you allow a few quotations, these should be given in quotation marks with a specific citation. With the exception of explicit quotations, the reports should be written entirely in the student’s own words! To accomplish this, it is helpful to put the source articles aside while drafting the report and then later use the original sources to check accuracy, as needed.

Students should include at least one reference for each major assertion in the report.

6 If students want additional recommendations for a healthy diet, a useful source is Healthy Eating Plate (https://www.hsph.harvard.edu/nutritionsource/healthy-eating-plate/).

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You may want to have students present their findings as part of a class discussion of the advantages and disadvantages of the various proposals to reduce the contribution of food production to global warming. If you plan to do this and you don’t already have a format for student presentations, you may find the following suggestions helpful.

Be clear about what students should include in their presentation. Set a time limit for each presentation.

Students should prepare a poster or 1-3 slides with a few major points written in large enough letters so everyone will be able to read them. If graphics are included, they should be clear and large enough for everyone to see.

Students should prepare a script and rehearse their presentation. If you can, review some or all of the material that students have prepared before their

class presentation.

The Student Handout provides a list of reliable and accessible sources that will allow students to make an initial analysis, but will not provide the basis for an in-depth evaluation of their proposal on each of the criteria. To accomplish that, you would need quite a bit more time and sources. You may want to consult advice available at

https://www.nextgenscience.org/sites/default/files/evidence_statement/black_white/HS- LS2-7%20Evidence%20Statements%20June%202015%20asterisks.pdf

https://www.nextgenscience.org/sites/default/files/evidence_statement/black_white/HS- ETS1-1%20Evidence%20Statements%20June%202015%20asterisks.pdf

https://www.nextgenscience.org/sites/default/files/evidence_statement/black_white/HS- ETS1-3%20Evidence%20Statements%20June%202015%20asterisks.pdf .

At the end of this activity, students may appreciate a discussion of possible individual actions that they could take to reduce global warming. In this context, you may want to consider the findings of a study that “recommends four widely applicable high-impact (i.e. low emissions) actions with the potential to contribute to systemic change and substantially reduce annual personal emissions:

having one fewer child (average for developed countries of 58.6 tons CO2 equivalent (tCO2e) emission reductions per year)

living car free (2.4 tCO2e saved per year) avoiding airplane travel (1.6 tCO2e saved per round-trip transatlantic flight) eating a plant-based diet (0.8 tCO2e saved per year).”

(http://iopscience.iop.org/article/10.1088/1748-9326/aa7541) This research article presents evidence that adoption of these actions (especially by teenagers and young adults) would make a much greater long-term contribution to reducing greenhouse gas emissions than some frequently recommended personal actions such as recycling, upgrading light bulbs, hang drying laundry, and washing clothes in cold water. Additional effective personal actions include purchasing green energy for the home and buying a more fuel efficient car. (Additional information on this study is available at http://iopscience.iop.org/article/10.1088/1748-9326/aa8467 and https://www.sierraclub.org/sierra/we-re-teaching-kids-wrong-ways-fight-climate-change.)

If your students would like to calculate their carbon footprint for travel, home, shopping and food, they can use the footprint calculator available at https://www.nature.org/greenliving/carboncalculator/index.htm. They will probably need to check with an adult in their household to get some of the information needed to complete the carbon footprint calculation.

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Sources for Student Handout Figures figure on page 1 modified from http://www.rescuethatfrog.com/history-of-evidence/ figure on page 2 modified from

https://www.ucapture.com/wp-content/uploads/2016/07/Global-Warming-Infographic.png

figure on page 4 modified from http://www.bbc.co.uk/schools/gcsebitesize/science/images/bi01002.gif

figure on page 5 modified from http://www.shmoop.com/images/biology/biobook_eco_11.png

figure on page 6 modified from https://www.ewg.org/meateatersguide/ figures on page 7 modified from

http://english.cas.cn/newsroom/research_news/201705/W020170518587070843478.png and https://www.nextbigfuture.com/wp-content/uploads/2017/07/23c953c49ec8e57b9314f83f2694bdae.png?x71037

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