Analyze This: Climate change could make food less healthy
As levels of carbon dioxide — CO2 — in the atmosphere have been rising in recent decades, Earth has been warming. That’s because as a greenhouse gas, CO2 traps heat in Earth’s atmosphere. That warming is one symptom of climate change. And it has the potential to affect food in many ways. Rising temperatures and the changes in rainfall that it will bring should impact how much and where crops grow. Data now show that rising levels of CO2 also can affect how nutritious a crop will be. Some of those data were reported last year in Annual Review of Public Health. Indeed, it noted that several studies have come to this conclusion.
Samuel Myers is an environmental health scientist at Harvard University in Cambridge, Mass. He was part of a team that has studied the potential effects of climate change on nutrition. In one 2014 study, his group looked at six major food crops: wheat, rice, field peas, soybeans, maize (corn) and sorghum. They exposed plants to different amounts of CO2. Some got levels of between 363 and 386 parts per million (ppm). Such concentrations were typical at that time. (CO2 levels have since risen.) Other plants were exposed to more of that greenhouse gas as they grew — 546 to 586 ppm. Such levels are expected to develop within the next 50 years or so.
After harvesting the plants, the researchers measured their levels of vitamins, minerals and other nutrients. And plants grown with more CO2 were less nutritious. For instance, wheat and rice had lower protein levels. They also had less zinc and iron, as did the peas and soybeans. Some two billion people worldwide already don’t get enough of these minerals. Most people depend on cereal crops, such as wheat and rice, to meet their dietary needs for both zinc and iron. If crop levels of such nutrients fall, people may face an even greater risk of falling ill.
People who substitute sugars and starches for protein face a greater risk of high blood pressure and heart disease, U.S. studies have shown.
Scientists don’t yet know why CO2 impacts levels of these nutrients. But the new findings suggest scientists may want to try breeding new varieties of crops that are less affected by CO2. That way people will still get the most benefits from their greens and grains.
Analyze the graph and answer the questions below:
- A scatter plot is a set of data points plotted along vertical and horizontal axes. Does this graph count as a scatter plot? Why or why not? What is another type of graph that could be used to display these data? Explain your answers.
- Analyze the variables represented on the x-axis and y-axis in the graph.
- Name the dependent and independent variables.
- What do the negative numbers on the y-axis represent?
- How much higher were zinc levels in wheat exposed to elevated CO2 levels? What is the approximate percent change in zinc content for wheat? What is the difference in protein between plants that were exposed to normal levels of CO2 and those exposed to elevated levels? What is the approximate percent change in protein for soybeans?
- Consider the data represented on the graph: Why are both positive and negative numbers included on the y-axis?
- If you excluded soybean data from the graph, how could you change the range of values and the description of the trends seen on the y-axis?
- The lines above and below each point on the graph are known as error bars. Because no experiment will ever be perfect, an error bar is a line through a point on a graph which shows the degree of uncertainty or variation in findings. Error bars help to represent the overall range of data. The error bars on this graph represent 95 percent confidence intervals. Each confidence interval covers a range of values. The true value — how much nutrition is really in the crop — is likely to be within that range. When a confidence interval is described as “95 percent,” it means that 95 out of 100 times, the data that a scientist collected would fall between the bars. So the error bars on this graph represent what scientists could expect to find 95 times out of every 100 times that they measured the nutrients
- In rice, compare the percent change in zinc, iron and protein between plants grown in normal and in elevated levels of CO2. For which nutrient are the error bars on the graph greatest? For which nutrient are they smallest?
- Imagine you were to run another trial for the iron and protein levels in rice exposed to elevated CO2. Would you expect the average values that you measured to lie between the error bars on the graph for each nutrient? Or would you expect it to lie outside of the error bars? Explain your answer.
- What does each point on the graph represent? Based on the graphed data, what is the approximate percent change in zinc content for wheat? How about for rice and soybeans? Can you say with confidence that the percent difference in zinc for wheat is larger than its percent difference in soybeans? Explain. Can you confidently say that the percent difference for zinc in wheat is larger than its percent decrease in rice? Explain.
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carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cereal grain A plant in the grass family that provides an edible seed, which serves as a food staple (such as wheat, barley, corn, oat and rice).
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
concentration (in chemistry) A measurement of how much of one substance has been dissolved into another.
confidence interval An estimated range of values — derived from a set of data — that are likely to contain the real value. This range is used to understand the amount of uncertainty in a sample of data. Confidence intervals are usually expressed as a percentage. For example, a 95 percent confidence interval means that in 95 tests out of 100, the result a scientist obtained would likely fall within that range.
dependent variable A variable that changes during a scientific experiment based on other factors in the experiment. A researcher does not control it. For example, a test score is a dependent variable. It depends on how much someone studied, how difficult the test is and even how much sleep a student got the night before.
error bar A line (it can be vertical or horizontal) drawn through a point or a bar on a graph. The distance from one end of the line to the other represents how precise a measurement is, or how far the real value of something might fall from the data point reported in the experiment.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
independent variable A variable that changes during a scientific experiment but that is not changed by the other variables that are being measured. It is controlled by the scientist. Note: The independent variable doesn’t always change. For example, age or sex (male or female) may be an independent variable.
iron A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
mean One of several measures of the “average size” of a data set. Most commonly used is the arithmetic mean, obtained by adding the data and dividing by the number of data points.
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.
nutrition (adj. nutritious) The healthful components (nutrients) in the diet — such as proteins, fats, vitamins and minerals — that the body uses to grow and to fuel its processes. A scientist who works in this field is known as a nutritionist.
protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.
scatter plot (or scatter diagram) A graph in which values for two variables are plotted as dots along horizontal and vertical axes.
starch A soft white chemical made by all green plants. It’s a relatively long molecule made from linking together a lot of smaller, identical building blocks — all of them glucose, a simple sugar. Plants and animals use glucose as an energy source. Plants store that glucose, in the form of starch, as a reserve supply of energy. Animals that consume starch can break down the starch into glucose molecules to extract the useful energy.
variable (in math or research) (in mathematics) A letter used in a mathematical expression that may take on different values. (in experiments) A factor that can be changed, especially one allowed to change in a scientific experiment. For instance, when researchers measure how much insecticide it might take to kill a fly, they might change the dose or the age at which the insect is exposed. Both the dose and age would be variables in this experiment.
x axis (in mathematics) The horizontal line at the bottom of a graph, which can be labeled to give information about what the graph represents.
y axis (in mathematics) The vertical line to the left or right of a graph, which can be labeled to give information about what the graph represents.
zinc A metallic element that in its pure form is ductile (easily deformed) and that is an essential micronutrient in plants and animals.
Journal: S.S. Myers et al. Increasing CO2 threatens human nutrition. Nature. Vol. 510, June 5, 2014, p. 139. doi: 10.1038/nature13179.
Website: Climate Change: Vital Signs of the Planet: Carbon Dioxide. NASA.
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