Agriculture: Vulnerability to Climate Change

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Agriculture: Vulnerability to Climate Change


Agriculture both contributes to and is vulnerable to climate change. It contributes to climate change because it is a source of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). It is vulnerable to climate change because all agriculture depends on acceptable temperature ranges and patterns of rainfall for raising crops and livestock.

Climate change may create opportunities or benefits for some forms of agriculture in some parts of the world. For example, wine-grape growing (viticulture) has been enhanced throughout most of the wine regions of Europe and North America by climate changes since 1970. Also, by 2100, atmospheric CO2 levels are likely to be about twice what they are in the first decade of the 2000s, a concentration often used by commercial greenhouse growers to speed plant growth. The benefits for open-air agriculture of this CO2 fertilization effect, as it is called, have been questioned by recent research. Possible benefits to some agriculture must be weighed with shifts in precipitation patterns, more frequent droughts and floods, more frequent extreme weather, and rising temperatures in assessing the vulnerability of world agriculture to climate change.

Historical Background and Scientific Foundations

Agriculture is the most fundamentally life-sustaining of all human activities: sharp cutbacks in all other forms of well-being would be survivable for most people if only enough food could still be grown and distributed. Yet hundreds of millions of people—as of 2008, about 820 million—are chronically undernourished and at severe risk if their access to food diminishes any further. It is not surprising that the possible impacts of climate change on agriculture and food security has been of concern ever since the scientific study of global warming began in earnest in the 1960s.

In the 1970s and 1980s, studies suggested that increasing CO2 levels would stimulate crop yields and increase the drought tolerance of crop plants. If true, this offered hope that the primary cause of global warming—high atmospheric CO2—might offset some of its own ill-effects, at least in regard to the human food supply. It also became clear that the effects of global climate change on agriculture are bound to be a mixed bag regardless of CO2 fertilization. Climate change will bring increased rainfall and longer growing seasons to some areas, helping agriculture in those areas even as it injures it in others. The question is, which parts of the world or which types of agriculture may benefit or be harmed by climate change, and whether the global balance will be positive.

Beginning in the early 1990s, scientific consultative groups commissioned by the United Nations, including the Intergovernmental Panel on Climate Change (IPCC), began to study the causes, likely impacts, and possible mitigations of climate change in a systematic way, assembling and sifting world scientific opinion to arrive at an informed consensus. These studies have been made available in a series of four Assessment Reports that outline in detail the state of scientific knowledge on climate change, including the vulnerabilities and possible opportunities of agriculture.

The increasing productivity of most agriculture worldwide—due to improvements in fertilization, breeding, pest and disease control, and other technical aspects—have long made it difficult to discern whether climate changes (or CO2 levels) are yet affecting agriculture. Since 2001, however, evidence has appeared of climate changes that directly affect agriculture. These include changes in the length of the growing season (time between last spring frost and first autumn frost) and the number of growing-degree days per season.

Such changes have been observed in North America and in temperate Eurasia and have been, as far as they go, beneficial to many crops. However, increased droughts and heat are also damaging crops in other areas. In India, for example, the exceptionally severe 2007 monsoon season flooded nearly 8,000 square miles (20,700 square kilometers) of agricultural land, destroyed more than 130,000 homes, and killed at least 1,428 people.

Climate change does not affect agriculture by itself. Rather, it is only one of several fundamental problems besetting the current world agricultural system, under which inhabitants of developed countries are increasingly obese, while 820 million people elsewhere are chronically malnourished, including more than 150 million children under the age of five. These fundamental problems or vulnerabilities can be outlined as follows:

  • Social vulnerability. Social vulnerability includes the effects on food supply of population growth, poverty, hunger itself (which reduces productivity and educational performance), gender inequality, and lack of services and resources, including technology. Social vulnerability affects much of Europe, Asia, and South America, but is most severe in sub-Saharan Africa (Africa south of the Sahara desert). Most of today's 820 million undernourished people live in 84 developing countries holding 4 billion people as of 2002, a number projected to grow to 7 billion by 2050.
  • Economic vulnerability. Poorer countries find it difficult to compete in markets for oil, fertilizer, and food with richer countries. In 2002, 85% of the world's income went to the richest 20% of its population while only 1% of income went to the poorest 20%. Since 1990, under economic globalization, farmers in developing countries have experienced a decline in real income (buying power).
  • Environmental vulnerability. Environmental threats to agriculture include salinization (increasing salt content of soil due to irrigation, which can render land useless for agriculture); loss of soils to erosion (an intractable problem with almost all forms of till agriculture, i.e., farming in which plows are used to turn the soil); water shortages; urban sprawl; and decreased biodiversity due partly to the replacement of locally adapted crop varieties with commercially produced green-revolution hybrids that have increased productivity while increasing dependence and insecurity. Many aspects of climate change will add to environmental vulnerability, especially in the tropical and subtropical parts of the world where most of the world's malnourished persons already live and where agriculture is already suffering most from social, economic, and environmental vulnerability.


CO2 FERTILIZATION EFFECT: Acceleration of plant growth by heightened atmospheric CO2 concentrations. The effect has been suggested as a benefit to agriculture from artificially increased atmospheric CO2 and as a source of negative climate-change feedback through the accelerated growth of forests. Studies have found that crops benefit only slightly from the effect and that its effect on forests is mostly limited to younger trees.

EROSION: Processes (mechanical and chemical) responsible for the wearing away, loosening, and dissolving of materials of Earth's crust.

FOOD SECURITY: Reliable access by a person or group to adequate food. Persons who do not have adequate food, or have adequate food at the moment but do not know how they will continue to get food, have low food security.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

MONSOON: An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.

SALINIZATION: Increase in salt content. The term is often applied to increased salt content of soils due to irrigation: salts in irrigation water tend to concentrate in surface soils as the water quickly evaporates rather than sinking down into the ground.

A fundamental challenge to agriculture in much of the world could be changed precipitation patterns. Two-thirds of the world's people live in regions that receive, all told, only a fourth of the world's rain; worldwide,

70% of freshwater supplies already go to agriculture (for irrigation), and this figure can be as high as 90% in drier countries. As of 2002, about 30 countries were facing chronic water shortages, a number expected to exceed 50 by 2050. Water shortages will impact both health and agriculture directly.

In temperate northerly regions, moderate warming—1.8–5.4°F (1–3°C)—will probably benefit agriculture by increasing yields. However, even higher levels of warming, which are possible, will have negative impacts on agriculture even in temperate and northerly regions. Experts warn that even mild warming will cause decreased crop yields in south-erly regions. In 2007, Jacques Diouf, the director general of the United Nations Food and Agricultural Organization, warned that even slight global warming could reduce crop yields in southern regions, particularly due to increased droughts and floods. “Rain-fed agriculture in marginal areas in semiarid and subhumid regions is mostly at risk,” Diouf was quoted in the New York Times as saying; “India …could lose 125 million tons of its rain-fed cereal [grains] production, equivalent to 18% of its total production.”

Despite all the vulnerabilities mentioned earlier, United Nations scenarios for possible world development predict that the number of malnourished people may more than halve by 2080, from 820 million to 100–380 million. However, even if these optimistic forecasts are correct, the IPCC notes that many fewer people would be malnourished were it not for the effects of climate change. Also, such forecasts do not take into account the possibility of abrupt climate change from unexpected feedbacks, such as many scientists warn have occurred numerous times in Earth's past.

Impacts and Issues

As mentioned earlier, it was long hoped that increased CO2 in the atmosphere would offset some of the harms caused by global warming by accelerating plant growth and making plants more drought-resistant. Greenhouse (indoor) studies do indicate greatly improved yields with higher CO2. Starting in the 1980s, a system of FACE (free air carbon dioxide enrichment) facilities has been set up around the world. At FACE installations, plants grow in open-air, real-world conditions surrounded by a ring of vertical pipes that can emit carbon dioxide. A computer uses readings of wind speed and direction to control which pipes emit how much carbon dioxide, assuring that the plants inside the ring experience fairly steady elevated CO2.

Mixed Results

The results of FACE experiments have dimmed hopes for a neat offsetting by heightened CO2 of global warming's harms to agriculture. Plants at FACE installations do produce larger yields. However, they also make less protein, and make proteins of different kinds (wheat, for example, makes about 20% less gluten proteins under FACE conditions), and contain fewer minerals such as calcium and zinc. Crops may therefore become less nourishing as CO2 levels rise, and the grassy plants that feed grazing livestock may be less nourishing for them as well. There may also be effects on non-agricultural ecosystems. Experts are more or less evenly divided over the question of whether protein levels could be kept up in some crops by adding more nitrogen fertilizer to soils.


A majority of climate change researchers assert that Africa's poorest regions are most vulnerable to increased threats of drought, flooding, severe weather, and disease associated with climate change. The World Bank estimates that crop yields in Africa could drop by 25% in the coming decade because of climate change. Such a significant crop failure could result in growing poverty and widespread famine in several regions of Africa. In October 2007, the World Bank called on developed nations and global businesses to invest in helping Africa's farms adapt to climate change. Acknowledging that much of Africa's agricultural infrastructure is fragile, the World Bank stressed the need for investment in farming technology to increase crop yield and prevent food scarcity. However, the World Bank also noted that investment in Africa's farms must coincide with increased access to less-expensive alternative energy sources in under-developed regions.

However, yield only increases about half as much under open-air conditions as in greenhouses, and for some plants, it does not increase at all. The FACE results are significant because computer models of the impact of climate change on global food supply, such as those on which the IPCC's predictions have been based, have assumed a 20–30% increase in yields with heightened CO2. Observed yield increases are about half this large. Therefore, standard predictions of global agricultural output may be based on over-optimistic assumptions about CO2 fertilization.

Primary Source Connection

Global climate change could have a major impact on agriculture by affecting crop production. Areas that were once suitable for growing a particular crop may become unsuitable as temperatures increase and droughts or floods become more common. This article examines the possible impact of climate change on agriculture in the United States in the twenty-first century.

The National Assessment Synthesis Team is an advisory committee of the United States Global Change Research Program (USGCRP). USGCRP is responsible for advising Congress and the president on issues related to global climate change.


It is likely that climate changes and atmospheric CO2 levels, as defined by the scenarios examined in this Assessment, will not imperil crop production in the US during the 21st century. The Assessment found that, at the national level, productivity of many major crops increased. Crops showing generally positive results include cotton, corn for grain and silage, soybeans, sorghum, barley, sugar beets, and citrus fruits. Pastures also showed increased productivity. For other crops including wheat, rice, oats, hay, sugar cane, potatoes, and tomatoes, yields are projected to increase under some conditions and decline under others.

Not all agricultural regions of the United States were affected to the same degree or in the same direction by the climates simulated in the scenarios. In general the findings were that climate change favored northern areas. The Midwest (especially the northern half), West, and Pacific Northwest exhibited large gains in yields for most crops in the 2030 and 2090 timeframes for both of the two major climate scenarios used in this Assessment, Hadley and Canadian. Crop production changes in other regions varied, some positive and some negative, depending on the climate scenario and time period. Yields reductions were quite large for some sites, particularly in the South and Plains States, for climate scenarios with declines in precipitation and substantial warming in these regions.

Crop models such as those used in this Assessment have been used at local, regional, and global scales to systematically assess impacts on yields and adaptation strategies in agricultural systems, as climate and/or other factors change. The simulation results depend on the general assumptions that soil nutrients are not limiting, and that pests, insects, diseases, and weeds, pose no threat to crop growth and yield. One important consequence of these assumptions is that positive crop responses to elevated CO2, which account for one-third to one-half of the yield increases simulated in the Assessment studies, should be regarded as upper limits to actual responses in the field. One additional limitation that applies to this study is the models' inability to predict the negative effects of excess water conditions on crop yields. Given the “wet” nature of the scenarios employed, the positive responses projected in this study for rainfed crops, under both the Hadley and Canadian scenarios, may be overestimated.

Under climate change simulated in the two climate scenarios, consumers benefited from lower prices while producers' profits declined. For the Canadian scenario, these opposite effects were nearly balanced, resulting in a small net effect on the national economy. The estimated $4-5 billion (in year 2000 dollars unless indicated) reduction in producers' profits represents a 13–17% loss of income, while the savings of $3-6 billion to consumers represent less than a 1% reduction in the consumers food and fiber expenditures. Under the Hadley scenario, producers' profits are reduced by up to $3 billion (10%) while consumers save $9–12 billion (in the range of 1%). The major difference between the model outputs is that under the Hadley scenario, productivity increases were substantially greater than under the Canadian, resulting in lower food prices to the consumers' benefit and the producers' detriment.

At the national level, the models used in this Assessment found that irrigated agriculture's need for water declined approximately 5–10% for 2030 and 30-40% for 2090 in the context of the two primary climate scenarios, without adaptation due to increased precipitation and shortened crop-growing periods.

A case study of agriculture in the drainage basin of the Chesapeake Bay was undertaken to analyze the effects of climate change on surface-water quality. In simulations for this Assessment, under the two climate scenarios for 2030, loading of excess nitrogen into the Bay due to corn production increased by 17–31% compared with the current situation.

Pests are currently a major problem in US agriculture. The Assessment investigated the relationship between pesticide use and climate for crops that require relatively large amounts of pesticides. Pesticide use is projected to increase for most crops studied and in most states under the climate scenarios considered. Increased need for pesticide application varied by crop—increases for corn were generally in the range of 10–20%; for potatoes, 5–15%; and for soybeans and cotton, 2–5%. The results for wheat varied widely by state and climate scenario showing changes ranging from approximately 15 to þ15%. The increase in pesticide use results in slightly poorer overall economic performance, but this effect is quite small because pesticide expenditures are in many cases a relatively small share of production costs.

The Assessment did not consider increased crop losses due to pests, implicitly assuming that all additional losses were eliminated through increased pest control measures. This could possibly result in underestimates of losses due to pests associated with climate change. In addition, this Assessment did not consider the environmental consequences of increased pesticide use.

Ultimately, the consequences of climate change for US agriculture hinge on changes in climate variability and extreme events. Changes in the frequency and intensity of droughts, flooding, and storm damage are likely to have significant consequences. Such events cause erosion, water-logging, and leaching of animal wastes, pesticides, fertilizers, and other chemicals into surface and groundwater.

One major source of weather variability is the El Niño/ Southern Oscillation (ENSO). ENSO effects vary widely across the country. Better prediction of these events would allow farmers to plan ahead, altering their choices of which crops to plant and when to plant them. The value of improved forecasts of ENSO events has been estimated at approximately $500 million per year. As climate warms, ENSO is likely to be affected. Some models project that El Niño events and their impacts on US weather are likely to be more intense. There is also a chance that La Niña events and their impacts will be stronger. The potential impacts of a change in frequency and strength of ENSO conditions on agriculture were modeled. An increase in these ENSO conditions was found to cost US farmers on average about $320 million per year if forecasts of these events were available and farmers used them to plan for the growing season. The increase in cost was estimated to be greater if accurate forecasts were not available or not used.

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See Also Agriculture: Contribution to Climate Change; Fisheries; Methane.



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Web Sites

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Larry Gilman

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