Food, the Carbon Cycle, and Global Warming How can we feed a growing world population without increasing global warming? 1

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1 Food, the Carbon Cycle, and Global Warming How can we feed a growing world population without increasing global warming? 1 I. Global Warming and Greenhouse Gases This graph shows changes in global temperature and CO2 concentration in the earth s atmosphere. Global temperature varies from year-to-year, due to multiple factors such as volcanic eruptions and El Niño ocean currents. We will focus on the overall temperature trend. 1a. The overall trends in this graph show in global temperature and (a decrease/an increase) in CO2 concentration. (a decrease/an increase) 1b. The global warming trend a. was most rapid from 1850 to b. has occurred steadily in every decade since c. has been more rapid in recent decades. 2. Last winter, two weeks were unusually cold. Your friend complained that she was freezing and said that this was proof that there really isn t any global warming. Do you agree or disagree? Explain why. 1 By Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania Teachers are encouraged to copy this Student Handout for classroom use. A Word file and Teacher Notes with instructional suggestions and background information will be available at 1

2 3. Since 1900, there have been increases in: global temperature CO 2 concentration in the earth s atmosphere men s average height. Do these correlated trends mean that the increase in men s height caused the increase in global temperature? yes no Explain why or why not. 4. Scientists need additional types of evidence to test the hypothesis that increased CO 2 in the earth s atmosphere has caused the increase in global temperature. To test this hypothesis, scientists have: A. investigated whether there is a mechanism that can explain how CO 2 in the atmosphere could cause global warming B. tested the ability of their hypothesis to predict future trends C. evaluated whether other hypotheses could account for the observed trends. As you read the evidence on this page and the next page, use the appropriate letter to label each type of evidence. Sunlight warms the earth, and the earth radiates heat energy back out to space. Laboratory experiments have shown that CO 2 molecules absorb heat energy and then radiate heat energy in all directions. CO 2 molecules in the air absorb some of the heat energy radiated by the earth and then radiate some of the heat energy back to earth. When there is more CO 2 in the atmosphere, more heat energy is radiated back to earth and the earth warms up. This warming effect explains why CO 2 is called a greenhouse gas. Other greenhouse gases include methane (CH 4) and nitrous oxide (N 2O). 5. Fill in label C in this figure to explain how greenhouse gases warm the earth. 2

3 Nineteenth century scientists were able to predict future increases in global temperature, based on their understanding of the warming effects of CO2 in the atmosphere. As scientists have learned more about all the factors that influence global temperature, their predictions of future temperature trends have become increasingly accurate. 6a. Scientists have used scientific data and computer models to estimate the contributions of multiple factors to trends in global temperature. View the animated graph, How Global Warming Stacks Up ( For each factor in this table, summarize how this factor has affected global temperature change since Possible Cause of Change in Global Temperature Changes in the earth s orbit Changes in the temperature of the sun Volcanic eruptions Effect on Global Temperature Trends Forest replaced with types of land-use that reflect more light Changes in ozone in the upper and lower atmosphere Aerosol pollution Greenhouse gases 6b. What do you conclude about the main cause of the increase in global temperature during the last century? You may be wondering, What s all the fuss about a small increase in temperature (~1 C or ~2 F)? One effect has been an increase in sea level, which has contributed to coastal flooding in some areas. Sea level has risen because melting glaciers have added more water to the oceans and because warming of the water in the oceans has increased the volume for a given amount of water. These effects will increase in the future as the effects of higher levels of greenhouse gases continue to warm the earth and the oceans continue to absorb heat. Longterm data and analyses indicate that sea level will rise enough to cause substantial flooding in many cities in the US (e.g. Miami, New York City) and around the world. Other effects of global warming have already been observed and are expected to increase in the future. 3

4 II. CO2 and the Carbon Cycle In this section you will learn about processes that influence the concentration of CO 2 in the atmosphere. This figure shows a carbon cycle in nature. Carbon atoms cycle between CO 2 in the air and organic molecules in living organisms and dead organic matter. 7a. After CO 2 enters a plant (B arrow), how do the carbon atoms in CO 2 become carbon atoms in organic molecules in the plant? 7b. Before CO 2 is released into the air (A arrows), how do the carbon atoms in organic molecules become carbon atoms in CO 2? How is this process useful for animals and plants? 7c. The giraffe weighs much less than the weight of all the food that the giraffe has ever eaten. What happened to all the weight of the food? Where did the atoms in the food molecules go? 8a. Carbon cycles in nature can be balanced, so the concentration of CO 2 in the atmosphere stays relatively constant. However, if the total amount of trees and other plants increased worldwide, CO 2 levels in the atmosphere would decrease. Explain why. 8b. Explain why the concentration of CO 2 in the atmosphere increases when humans burn forests to clear land for agriculture. 4

5 9. This figure shows a more complete carbon cycle. Use the information in this figure to explain how human activities have caused an increase in CO2 concentration in the atmosphere. III. Food Production and Greenhouse Gases Cellular respiration in farm animals and plants produces CO2, but an equal amount of CO2 is taken up by farm plants and used for photosynthesis. This is similar to a balanced carbon cycle in nature. However, other parts of modern food production contribute to increased CO2 in the atmosphere. 10. Describe two ways that food production contributes to increased CO2 in the atmosphere. (Hint: Review your answers to questions 8b and 9.) A. B. 5

6 Food production also contributes to increases in two other greenhouse gases nitrous oxide and methane. Nitrous oxide is released by bacteria that metabolize nitrogen-containing molecules in fertilizer applied to crop land and manure produced by farm animals. A molecule of nitrous oxide has about 300 times as much global warming effect as a molecule of CO Explain how farmers increasing use of fertilizers has contributed to global warming. Cows are a major source of increasing methane in the atmosphere. To understand why, we need to begin with cellulose, the main component of plant cell walls and the most abundant molecule in grass and hay. Animals cannot make enzymes that digest cellulose, but cows have extra stomachs which contain microorganisms that can digest cellulose. Some of these microorganisms produce methane, which is released to the atmosphere when a cow burps or passes gas. A molecule of methane has about 25 times as much global warming effect as a molecule of CO Explain the reasons for both of these differences between cows and humans: Cows can live on a diet of grass and hay, but humans cannot. A cow releases much more methane to the atmosphere than a human. 13. The greenhouse gases released during production of a kilogram of beef cause roughly 10 times more global warming than the greenhouse gases released during production of the same amount of corn, beans, nuts or other plant foods. Explain why more CO2, nitrous oxide, and methane are released during the production of a kilogram of beef vs. a kilogram of a plant food. (Hint: Review the information on this page and your answers to questions 7c-10.) 6

7 A lifecycle assessment estimates the total amount of greenhouse gases released during the entire process of producing a food for human consumption, including everything from fertilizer production to cooking the food. This figure shows the results of lifecycle assessments for twenty different types of food. The amount of greenhouse gases is measured in CO 2 equivalents (CO 2e) which take account of the stronger greenhouse gas effects of methane (CH 4) and nitrous oxide (N 2O). 14a. The amount of greenhouse gases (CO 2e) released per kilogram of meat consumed is for beef and for chicken. 14b. What could be one reason for this difference in the amount of greenhouse gases (CO 2e) released during production of beef vs. chicken? 14c. Complete this table, using information from the Meat Eater s Guide to Climate Change + Health ( What causes the difference in the amount of greenhouse gases released during production of beef vs. chicken? Summarize any additional interesting information you notice in this source. 7

8 IV. How can we feed a growing world population without increasing global warming? This figure shows that the world s population more than doubled from 1950 to The dashed line for future decades shows predicted increases in world population, and the shaded area indicates the uncertainty in the prediction of future trends. 15. If our food production system and food choices remain unchanged, then increased food production for the growing world population will result in increased release of greenhouse gases. Scientists expect even greater increases in greenhouse gases because some people in developing countries are using their increasing incomes to eat more meat (similar to the meat consumption of people in developed countries). Propose several ways to reduce the contribution of future food production to global warming. (To come up with proposals, you may want to review the information on pages 5-7 and skim the list of Reliable Relevant Sources on pages 9-10.) 16. What criteria should be used to evaluate the advantages and disadvantages of each proposal to reduce the amount that food production contributes to global warming? 17. Prepare a brief report that evaluates a specific proposal to reduce the amount that future food production will contribute to global warming. Use the information from relevant sources in the list on pages 9-10 to evaluate the advantages and disadvantages of your proposal. Look for information in your sources to evaluate your proposal according to the criteria developed in question 16. You may also want to use the information in your sources to improve your proposal and/or make it more specific. 8

9 Reliable Relevant Sources How Intensive Farming Can Reduce the Release of Greenhouse Gases a. Intensive farming may ease climate change ( b. Reducing the environmental impact of global diets ( or main.pdf?_tid=02ae35bd c-90ce- 28cf2f92d7f6&acdnat= _94222e87942b8d74b452197c1ab412b7) c. Most new farmland comes from cutting tropical forest, says Stanford researcher ( How Farmers Can Increase Carbon Stored in the Soil and Reduce CO 2 in the Atmosphere a. Organic Farms Could Help Fight Climate Change ( b. Soil Power! The Dirty Way to a Green Planet ( c. Soils cannot halt climate change ( More Efficient Use of Fertilizers to Reduce Nitrous Oxide Emissions a. Fertilizer produces far more greenhouse gas than expected ( b. Management of Nitrogen Fertilizer to Reduce Nitrous Oxide Emissions from Field Crops ( xide_emissions_from_fi) Reducing Waste to Reduce Greenhouse Gases a. What s Food Loss and Waste Got to Do with Climate Change? A Lot, Actually. ( b. UN launches campaign to cut 1.3 billion tons of global food waste ( c. Release: New research finds companies saved $14 for every $1 invested in reducing food waste ( d. Improved Crop Storage Bags Can Improve Harvests and Incomes ( Healthy eating can reduce greenhouse gas emissions. a. Healthy Diet May Reduce Gas; Greenhouse Gas, That Is ( b. Meat and Your Health ( 9

10 c. Tax high carbon food, sugar for climate and health gains study ( d. How the Government Supports Your Junk Food Habit ( Shall we eat insects? a. Five reasons we should all be eating insects ( b. Scientists still hoping to serve up insects as food source ( scientists-insects-food-source.html) c. Exploiting a pest insect species Sphenarium purpurascens for food consumption: ecological, social, and economic repercussions (Abstract available at d. Optional more detailed and comprehensive source: Opportunities and hurdles of edible insects for food and feed ( 10