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5. Climate Change, Agriculture, and Famine

Published onApr 10, 2020
5. Climate Change, Agriculture, and Famine

Climate change poses unprecedented challenges to US agriculture because of the sensitivity of agricultural productivity and costs to changing climate conditions.

USDA Technical Bulletin 1935, 2012

Other chapters described some of the effects of climate change that impact humans directly: the effects of heat, disease, and so on. Although there are genetic differences among us that make us more or less susceptible to various diseases, we are all one species. We have similar, but not identical, sensitivities to the stresses of all types that are expected as the result of climate change. This chapter turns to agriculture, the cultivation of crops, animals, and other forms of life that provide the food, fuel, and other needs of civilization. In agriculture, it is necessary to consider multiple species, not just humans. Not surprisingly, the differences among species are immense—and so are the responses to a changing climate.

The ability to grow adequate amounts of food is a prerequisite to good health. For each of the species that make up the total output of crops and animals that constitute agriculture, there are defined optimal and limiting temperatures for growth, reproduction, and development. Different species have vastly different susceptibilities to numerous diseases, insects, and weeds; different needs for water; and different responses to the increase in atmospheric concentration of CO2. The concentration of this long-lived greenhouse gas is rising and is certain to continue to rise. We will likely require different mitigation and adaptation strategies for each component of agriculture and for agriculture as a whole.

The task of providing adequate nutrition for the growing number of the earth’s inhabitants would be daunting even without climate change. We already fail to feed everyone adequately. The 2014 publication titled Hunger Report: Ending Hunger in America, published by the Bread for the World Institute, reports that between 2010 and 2012, 14.7 percent of Americans had an insecure food supply, and that in 2012, over forty-six million relied on the Supplemental Nutrition Assistance Program (SNAP) to feed themselves and their families.1 Many of those who are hungry are children.

Although there are fewer people who suffer from malnutrition than in the past, the problem is still one of “staggering size,” as reported in the 2015 Global Nutrition Report published by the International Food Policy Institute.2 The institute found that malnutrition affects all countries and one in three people on the planet. Malnutrition, particularly when it takes the form of undernutrition, results in stunting. Stunting is present when a child is two standard deviations below World Health Organization median values for either height or weight for age. This translates into a bit more than the lowest two percent of the weight that separates the top half from the bottom half of children at any given age. Undernutrition and stunting have profound effects on the developing brain. The result is a lifelong impairment of brain function, including intelligence. This decreases the probability that individuals and societies will thrive. The World Bank reports that in some countries in the years between 2009 and 2013, 45 percent of children under five years of age did not receive adequate nutrition. The effects of the expected changes in our climate will increase the difficulties associated with feeding everyone and could make problems associated with undernutrition much worse. The challenge is enormous.

Climate change already affects agriculture in this country. We are one of the leading producers of agricultural commodities. According to the Food and Agriculture Organization of the United Nations, the United States produces about 35 percent of the world’s corn, 32 percent of the soybeans, and smaller but significant amounts of other leading crops such as rice and wheat, about 1 percent and 8 percent, respectively. What happens here has already affected, and will continue to affect, others around the world. For additional details on crop production, see table 5.1.

Table 5.1 Agricultural commodity production, 2013, in millions of metric tons3


US yield

World yield













Effect of Atmospheric CO2 on Plant Growth

“Carbon dioxide is plant food, bring it on!” So goes the cry of climate change deniers. There is some truth to what they say. However, like virtually every other detail about climate change, it is not nearly that simple.

Six hundred million years ago, when the ancestors of the plants we recognize today began to develop, the earth’s atmosphere contained huge amounts of CO2.4 Eventually, as plants proliferated, carbon dioxide levels fell as a result of the combined effects of a reduction in natural emissions of the gas and carbon trapping by plants. When these plants died, many were buried and eventually formed the fossil fuels we extract and burn today. Although there were fluctuations, atmospheric levels of CO2 were still high until just over twenty-four million years ago, when they fell to around 300 ppm. This concentration remained relatively constant until the onset of the industrial revolution (see chapter 2).5 From an evolutionary perspective, plants evolved during periods of high concentrations of CO2. Compared to the conditions that were present when they first appeared, today many plant species are relatively starved for CO2. Many plants would like more of the gas and would grow more rapidly with a higher CO2 concentration.

Virtually all plants of interest fall into one of two groups: those that respond to increases in the concentration of CO2 and those that do not. Which group a plant falls into depends on how its photosynthetic pathways function. About 95 percent of plants produce a three-carbon compound during the initial step of this process that converts CO2 into sugars. These are referred to as C3 plants. Soybeans, rice, and alfalfa are good examples of C3 plants. Plants in this group are responsive to changes in the concentration of CO2. Typically, their growth rates increase as CO2 concentrations rise, at least in short-term studies. The less common C4 plants produce a four-carbon compound during the initial step of photosynthesis. Corn and sorghum are examples. C4 plants do not respond to increases in CO2 concentrations because the enzymes in their metabolic pathways are saturated with the gas, and therefore increases have no effect on growth. Think of people trying to get into a bank via a revolving door. When there are only a few people (molecules of CO2) trying to get in, an increase in the number of those trying to enter results in an increase in the number getting into the bank. This is the C3 situation: adding more CO2 (people) results in an increase in photosynthesis. However, when there are crowds of people outside the bank, the revolving door can’t let more people in; the door is saturated. This is the C4 condition.

Most short-term research shows that plants grow more rapidly and accumulate mass as the ambient CO2 concentration increases.6 To illustrate this, consider a study of grasslands in Texas that contained a mixture of C3 and C4 species from which cattle had been excluded.7 Four years later, this plot was exposed to a gradient in the concentration of CO2 that ranged from 200 ppm to 560 ppm. From chapter 2, you may remember that the ambient CO2 concentration is now about 400 ppm. Researchers found that the aboveground biomass increased between 121 and 161 grams per square meter for a 100 ppm rise in the CO2 concentration. Belowground biomass was not measured. In response to the added CO2, the composition of the grassland shifted from C4 grasses to C3 flowering plants that are not grasses (technically, the shift was to forbs, flowering plants with leaves and stems). Neither the grasses nor the flowering plants were more desirable; they were just different.

This study of Texas grasslands is representative of what might be expected to happen in a mixed plot of C3 weeds and C4 crops as CO2 concentrations increase. Theoretically, when a C3 weed invades a C4 crop, the weed is more likely to win when the CO2 concentration rises. Table 5.2 shows some of the winners and losers in some pairings of crops and weeds of interest. Under laboratory conditions, it has been shown that C3 weeds often muscle out C4 crops, resulting in a decreased yield of the crop.8

Table 5.2 Crops vs. weeds9



High CO2 favors

C4 Crop—C4 Weed


Redroot Amaranth


C4 Crop—C3 Weed


Rough or common cocklebur




C3 Crop—C4 Weed


Johnson grass



Johnson grass



Echinochloa glabrescens



Redroot amaranth


C3 Crop—C3 Weed


Creeping thistle









Plantago (a genus with 200 species—e.g., plantain)



English or narrowleaf plantain

An important and convincing challenge to the “CO2 is fertilizer” position, touted by climate change deniers, arises from a recent study of trees in the Amazon River basin.10 The study was designed to answer the question “Does a rising atmospheric CO2 concentration cause trees to trap more of this greenhouse gas?” In a massive undertaking, teams of scientists made direct measurements of all trees with a diameter of 100 mm or more in 321 different plots of land scattered throughout the Amazon basin. Candidate plots were excluded if there was evidence for recent human activity within their boundaries. In addition, the plots were selected to provide lots of diversity to make them as representative as possible of the many different ecosystems in the entire basin. Periodic measurements began in 1983, when twenty-five plots were marked out, and continued into the middle of 2011, when the full number of plots was under surveillance. Tree size data from serial measurements in the plots were used to compute the amount of carbon that had been trapped, and trapped carbon data for all trees were summed for all of the plots.

During the study, the atmospheric CO2 level rose steadily. The results of the investigation were striking. Individual trees did indeed grow more rapidly in response to the rising CO2 and, accordingly, trapped more CO2. However, this effect was overwhelmed by the fact that trees were not living as long as they had in the past. This resulted in a net decrease in the rate at which CO2 was trapped. The effect was not trivial. The authors of the paper wrote, “The above-ground biomass declined by one-third during the past decade compared to the 1990s.”11 When these data are combined with the fact that massive amounts of tropical forests are cleared each year, there is real cause for concern about the ability of the natural system to mitigate climate change.

In her book The Sixth Extinction: An Unnatural History, Elizabeth Kolbert paints a vivid picture of the diversity to be found in the Amazon basin.12 As she was guided through the plots, similar to those described previously, she was told to look carefully at the leaves on the plants and trees. As Kolbert climbed a hill, her guide told her that after walking a few hundred feet she would not see these same leaves again. The diversity was so enormous and the plant life was so highly adapted to a narrow range of temperatures and humidity that small changes in elevation moved an observer into an entirely new ecosystem. Although the authors of the study documenting the decline of the Amazonian carbon sink did not reference specific species in their report, it seems quite likely that as CO2 levels rise, the diversity in the ecosystem will fall, just as was seen in the Texas prairie study. This will make the Amazon even more susceptible to the effects of climate change. A loss of diversity may be as disastrous as the impact on the carbon sink.

In addition to the atmospheric CO2 concentration, plant growth depends on many other factors. The amount of nitrogen in the soil is critical. This is why farmers add nitrogen to their fields and homeowners apply nitrogen-rich fertilizers to their lawns. As you might expect, the balance between nitrogen and CO2 is important. In a report examining the interaction between these two chemicals, the authors noted that most of the studies that examined the effect of CO2 were of a relatively short duration. Therefore, they embarked on a six-year investigation that examined the CO2–nitrogen relationship and found, as the title of their paper states explicitly, that nitrogen limitation constrains sustainability of ecosystem response to CO2.13 The nutrient content of the soil for plants grown under conditions of an elevated concentration of CO2 is arguably more important than whether the plant grows faster. In other words, the CO2 fertilization effect is subject to considerable modification by the nitrogen concentration in the soil. This is likely to be most important in parts of the world where (1) the soil condition is poor because the same crop is planted year after year, (2) the soil is damaged, and (3) farmers can’t afford chemical fertilizers. This is the case in much of sub-Saharan Africa. (Chapter 10 describes efforts to adapt to and overcome the poor soil condition in parts of Africa.)

In addition to the complex relationship between CO2 levels and plant growth, the nutritional content of some crops is also affected by CO2. Inadequate nutrition is an important determinant of the global burden of disease, as discussed in chapter 1 and shown explicitly in table 1.2. Therefore, more in-depth studies of the numerous effects of climate change on crops are warranted. Limited amounts of iron and zinc are particularly problematic in many parts of the world. In a recent study that combined new data with that which had already been published, the study authors noted that around two billion people are deficient in zinc and iron.14 They reported that increasing the concentration of CO2 in the ambient air led to a reduction in these two critical elements in C3 grains and legumes. These two classes of food supply most of the micronutrients in regions where dietary deficiencies are rampant. The study authors also found reductions of protein in many C3 crops other than legumes. Finally, they found variable effects of CO2 on the concentration of a molecule (phytate) that inhibits the uptake of zinc in the human gastrointestinal system. Phytate levels were reduced in wheat, a C3 grass. This might mitigate the effects of reduced zinc. Plant phytate concentrations are used to model zinc metabolism, so a thorough understanding of the behavior of this molecule is important. Some of the study results are shown in figure 5.1.

Figure 5.1 Percentage change in nutrients at elevated CO2 concentrations relative to the ambient CO2 concentration. Numbers in parentheses refer to the number of comparisons in which replicates of a particular cultivar grown under one set of growing conditions in one year at elevated [CO2] have been pooled and for which mean nutrient values for these replicates are compared with mean values for identical cultivars under identical growing conditions, except grown at ambient [CO2]. In most instances, data from four replicates were pooled for each value. Error bars represent 95 percent confidence intervals of the estimates, and [CO2] represents the atmospheric concentration of CO2. Reproduced with permission from S. S. Myers, A. Zanobetti, I. Kloog, et al., “Increasing CO2 Threatens Human Nutrition,” Nature 510, no. 7503 (2014): 139–142.

To summarize, although it is clear that some plants (notably the C3 species) grow more rapidly in response to an increase in the CO2 concentration, this increase comes with a price. Protein yields in these plants may be low, other nutrients may be reduced, and the CO2 fertilizer effect may not be permanent. Under conditions of an elevated CO2 concentration, plant diversity may fall and weeds and invasive species may flourish. In other words, when something looks like it is too good to be true, it probably is.


Ragweed (Ambrosia artemisiifolia) is common throughout North America and can be found in Europe. Its Latin name, suggesting a relationship to the nectar of the gods and to asters, gives this plant an undeserved positive spin. It produces pollen that is highly allergenic and acts as a potent trigger for attacks of asthma. It is also responsible for much of the suffering among those afflicted by hay fever. The lengthening of the growing season and increases in the atmospheric CO2 concentration that characterize climate change favor the growth of this pest.

Plant scientists interested in ragweed have grown it under strict laboratory conditions.15 In one of the most comprehensive studies, a group of scientists tested the hypothesis that a longer growing period and elevated CO2 levels would lead to the production of more of the offensive pollen. They released dormant ragweed seeds during three successive fifteen-day intervals. At each of the three releases, the CO2 concentration was identical to the level in the atmosphere or increased to 700 ppm. The heights and weights of the plants were measured periodically along with the number, length, and weight of the clusters of flowers and the time at which the flowers opened. The scientists placed bags over the flowers to quantify pollen production. The seeds planted in the first group outgrew those planted later by almost every measure. They were bigger and produced more clusters of flowers that were heavier and yielded almost 55 percent more pollen than seeds planted in the third cohort. These results were further augmented when plants were grown in the CO2-enriched atmosphere. This interaction between the longer growing season and enhanced pollen production at high CO2 concentrations predicts more suffering for asthmatics and hay fever patients as the climate warms.

Effects of Temperature on Plant Growth

The relationships between the ambient temperature and plant growth are complex. Each species has a temperature below which growth does not take place and a temperature above which growth fails. In between, there is an optimum temperature. In addition to growth, temperature affects pollination and other aspects of plant reproduction. Thus disruptions, particularly warming, may have substantial impacts on plants—including corn and soybeans, the most prevalent and valuable crops grown in the United States.

Corn is an important source of food for Americans, particularly when one considers the prevalence of corn-derived products such as high fructose corn syrup in contemporary diets and so-called nutraceuticals (nutrients and dietary supplements, as well as a variety of foods that may or may not have direct links to corn). Corn is also the primary source of carbohydrates that are used in fermentation reactions to produce the ethanol that makes up 10 percent of the gasoline mixture at the pump. In addition, farmers who raise hogs, poultry, and cattle, both for meat and dairy production, rely heavily on corn and soybeans for feed.

Several temperatures are important when discussing the relationship between temperature and yield. The first of these is the optimum temperature for grain yield, which the US Department of Agriculture lists for a number of crops. For corn, it is between 18°C and 22°C; for rice, it is somewhat higher, between 23°C and 26°C; and for wheat, it is lower at 15°C.16

An extensive analysis of the impact of temperature on corn and soybean yields was published in a 2009 report.17 For both crops, the yield increases slightly as degree-days increase up to a critical level (where a degree-day is a function of the average temperature on a given day). Beyond a critical temperature, yields fall—and fall rapidly. For corn, the critical temperature is 29°C, and it is 30°C for soybeans. There is likely to be a major impact on agricultural production as daily temperatures increase above critical levels in parts of the country where these are the dominant crops. This is particularly true for the Midwestern part of the United States and for Africa.

As temperatures continue to rise, farmers may encounter the failure temperature. This is exactly what it sounds like—the temperature that results in failure of the crop, not just a reduction in yield. Frequently, this failure occurs during pollination, the most temperature-sensitive portion of crop production. The US Department of Agriculture also lists failure temperatures for various crops. For beans, it is 32°C; for wheat, it is 34°C; for rice, sorghum, and corn, it is 35°C; and for soybeans, the failure temperature is 39°C.

The 2009 report on temperatures and crop production made yield projections based on climate models commonly in use at the time.18 The most optimistic of these, the B1 Hadley III warming scenario, predicts a temperature increase of about 1°C above current temperatures at the end of the century. This is expected to be associated with a yield reduction of approximately 45 percent for corn and 35 percent for soybeans. The least optimistic scenario, the A1FI Hadley warming scenario (where FI indicates fossil fuel intense—i.e., burning lots of fossil fuels) predicts an increase of about 3.5°C above current temperatures by the end of the century. At this level of warming, scientists predict that there will be an 80 percent reduction of corn yields and slightly lower reductions for soybean yields.

These data are critical predictors of the future, but evidence of what has happened already is perhaps even more important. Databases that are available to the public are extremely useful for this purpose. One such study was published in 2011 and has been widely cited.19 For all nations, the authors retrieved information on crop locations, monthly temperatures, and precipitation for four key crops: maize or corn, wheat, rice, and soybeans. These crops were chosen in part because they account for three-fourths of global consumption of calories. Temperatures were relatively stable between 1960 and 1980. However, for the next two decades temperatures warmed and varied substantially, as discussed in chapter 2. During those decades, 75 percent of all countries had a one-standard-deviation increase in temperature trends in growing regions for wheat, 65 percent had an increase of a similar magnitude in regions growing maize and rice, and 53 percent had an increase in regions growing soybeans.

The authors compared actual yields to those that were predicted by modeling in the absence of a temperature change. The observed global yields for maize and wheat production fell by 3.8 percent and 5.5 percent, respectively. Among variations in rice and soybean production, temperature changes had little effect on worldwide yields. However, as might be expected, there were larger effects observed in some countries. For example, maize production in Brazil and China fell by about 7.5 percent, wheat production in Russia was depressed by almost 15 percent, and soybean production was down by about 4 percent in Brazil and Paraguay. The authors pointed out that any expected increases in yields due to rising CO2 and improvements in technology were obliterated by the effects of climate change.

Other Factors Affecting Agriculture

Thus far, I have focused on the effects of an increasing atmospheric CO2 concentration and temperature in this chapter. These variables have major effects on agriculture and occur with some degree of evenness and predictability worldwide. However, climate change will also have nonuniform effects that are certain to affect crop production. These include the amount and intensity of precipitation, the prevalence of severe storms, changes in the concentration of ozone, proliferation of insect pests and impacts on insect pollinators, the effects on plant pathogens (yes, plants get sick too!), soil degradation, the effects of wind, and others. Most of these effects are worthy of books themselves, and details are beyond the scope of this chapter. Interested readers should consult the US Department of Agriculture 2013 publication Climate Change and Agriculture in the United States: Effects and Adaptation, an excellent and objective source.20 However, it is worthwhile to consider the impacts of drought on agriculture in this chapter—something that seems self-evident.

Droughts have been the nemesis of the farmer throughout recorded history. This continues to the present day. In his book Collapse: How Societies Choose to Fail or Succeed, Jared Diamond illustrates the importance of drought in the failure of the Indian tribes of the Southwest, Mayan civilizations, and others.21

Major droughts had important impacts on agricultural productivity and water use in 2012 and 2014. The 2012 drought was the worst to occur in the midsection of the United States since the 1930s. Real and anticipated crop loss in the United States led to rapid and substantial worldwide increases in the price of food. On August 30, 2012, the World Bank reported that world food prices had increased by 10 percent in July due to the Midwestern drought. An analysis published by Bloomberg in January 2013 said that the drought had severely depleted soil moisture levels, as indicated by the Palmer Index, a widely used measure of drought severity.22 Experts quoted in the report predicted that it might take between eighteen and fifty-one months to make a complete recovery, even if normal amounts of rain fell. At the time of this writing, an ongoing drought has affected major portions of California’s Central Valley and adversely impacted food production for much of the nation. In response, Governor Brown of California announced rules to curtail water use substantially in nonagricultural regions of the state. Water shortages throughout much of the Southwestern part of the country, including the Rio Grande River system, have forced curtailment of water-intensive activities.

Sub-Saharan Africa has also been hard hit by drought. This part of the world was already economically disadvantaged and, in some areas, ravaged by violence. South Sudan was particularly devastated. On April 3, 2014, a UN official told the New York Times that 3.7 million South Sudanese people, or one-third of the population, was on the verge of starvation.23 He predicted that unless $230 million in food aid was forthcoming within a two-month window, deaths due to starvation could rival those from the 1980s drought, when hundreds of thousands died in Ethiopia.


Famine frequently follows in the footsteps of drought. In the summary findings of a 1996 policy research working paper, the author defined famine as “widespread, usually life-threatening hunger or starvation.”24 The author listed the following risk factors that increase the vulnerability of a region to famine: poverty, a weak social and physical infrastructure, a weak and unprepared government, and a relatively closed political regime.

In the Key Risks section of the Summary for Policymakers, the IPCC Fifth Assessment Report stated with high confidence that there is a high “risk of food insecurity and breakdown of food systems linked to warming, drought, flooding, and precipitation variability and extremes, particularly for poorer populations in urban and rural settings.”25 The current state of nutrition leaves much to be desired; figure 5.2 shows the percentage of undernourished people in various regions of the world. The 2014 UN Food and Agriculture Organization report estimates that between 2012 and 2014 there were 805 million people who were undernourished.26 This number, while still too large, represents a reduction of more than one hundred million compared to the number affected in the 1990 to 1992 interval. The report goes on to say that although sixty-three nations reached the Millennium Development Goal for hunger abatement large portions of the world—including sub-Saharan Africa, the Caribbean, Southern Asia, and Oceania—have not. Oceania is the region with the smallest population, but the absolute number of hungry individuals has increased, and rising rates of obesity, considered by some to be a form of malnutrition, are problematic.

Figure 5.2 Undernourishment by region. The percentage of people in each region who are undernourished, adapted from FAO, IFAD, and WFP, The State of Food Insecurity in the World 2014: Strengthening the Enabling Environment for Food Security and Nutrition (Rome: FAO, 2014). Millennium Development Goals remain unmet in sub-Saharan Africa, the Caribbean, Southern Asia, and Oceania.

Famine and Violence

On December 17, 2010, Tarek al-Tayeb Mohamed Bouazizi, a young Tunisian street vendor, set himself on fire to protest alleged harassment and confiscation of his goods by local authorities. This act, which garnered worldwide attention, came at a time of rising food prices and led to demonstrations and to what is now referred to as the Arab Spring, which reached its culmination with the downfall of the Tunisian and later the Egyptian governments.

The link between food and violence is not new. An analysis of food and violence in North Africa and the Middle East resulted in a model that may predict unrest from food.27 Peaks in the UN Food and Agriculture Organization’s Food Price Index that coincide with food shortages demonstrate a remarkable concordance with the timing of food riots that occurred in that region, which suggests that adequate amounts of food and national security and tranquility may be intimately linked. For additional details concerning food riots, refer to chapter 8.

The Trajectory toward the Future

This chapter only begins to scratch the surface of the extraordinarily complex relationship between climate change, agriculture, and the ability of the world to feed itself. In his pessimistic 1798 publication An Essay on the Principle of Population, Thomas Malthus wrote that if disease and pestilence did not kill us off, “gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population with the food of the world.”28 Thus far, with notable exceptions, we have avoided this Malthusian catastrophe. There have been exceptions, such as the Anasazi, ancestors of the Pueblos, who are often thought to have been driven from their land by periods of climate instability and drought. It remains to be seen whether our increasingly globalized society will be able to feed itself in the face of climate change.

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