Farmers produce much of the human food supply; raw inputs such as cotton, leather, and wool for textiles; and an array of fibers, chemicals, and pharmaceuticals that serve myriad manufacturing processes. Agriculture is a complex system of human activity that is intertwined in the economic, environmental, and social processes on the Earth.
The importance of agriculture to human well-being explains why influential national and international government and nonprofit agencies have focused so much attention on the impact of climate change on agriculture. There is a clear recognition on the part of agencies such as the United States Department of Agriculture (USDA), the International Panel on Climate Change (IPCC), the United Nations Food and Agricultural Organization (FAO), and the World Bank clearly recognize that climate change could have profound effects on the future. Global climate change may have some positive impacts on agriculture. On the other hand, there may be deeply negative impacts if climate change triggers droughts or other catastrophic events that hurt the global food supply. Resource scarcity could, in turn, cause famines and geopolitical conflicts with grave humanitarian consequences.
AGRICULTURAL IMPACTS OF CLIMATE CHANGE
Given that crops and livestock thrive in a relatively narrow set of environmental parameters, it makes sense to explore how climate change will affect agricultural productivity. Factors considered include the impacts of rising temperatures, increased production of carbon dioxide and other greenhouse gases, water supply fluctuations, soil quality variations, sea-level increases, and the introduction of new pests, diseases, and weeds, which could hurt agricultural output. These changes can have different impacts depending on the geographic scale of analysis. Climatic change will have different manifestations at local, regional, and global scales. Impacts will also vary according to the agricultural products under consideration. Some plant or animal species may be very resilient to environmental changes. Others may not adapt so well to change.
One of the ways agriculture contributes to greenhouse gases is by shipping produce hundreds of miles to markets.
Temperature increases will affect crop and livestock production in various ways. A warming climate will extend the frost-free growing season at higher latitudes. Regions that are too cold to support commercial agriculture in northern Canada, Alaska, Scandinavia, and Russia, may become viable agricultural areas if temperatures increase. On the other hand, temperature-sensitive crops may no longer be commercially viable in regions that become too hot or dry. Also, rising temperatures could increase the heat stress on livestock.
Climatic change models predict that regional temperature variations may alter precipitation patterns and the supply of water for agriculture. Areas that are currently too dry may receive more moisture in the future. Areas that are productive now without irrigation may suffer as temperatures increase, because of increased plant evapo-transpiration. Farmers will have to find ways to offset the rising temperatures and corresponding moisture loss if they are to survive. Furthermore, many meteorologists suggest that weather events such as thunderstorms, tornados, and hurricanes may become more intense and occur with greater frequency. This may bring more rain to some regions. On the other hand, severe storms cause strong winds and flooding, which could cause large-scale crop damage.
Many regions of the world, such as the Indian Subcontinent, the Andes region of South America, Kazakhstan, California, and the American High Plains, rely on melt water from glaciers and heavy winter snows to feed streams and rivers that provide water for irrigation. These high altitude water sources have traditionally been viewed as renewable resources that can be depended upon to provide moisture during the growing season and are then replenished by snow falls during the frigid winters. However, rising temperatures have caused glaciers to shrink or disappear and have been linked to reduced snow pack at high altitudes.
A related problem is that many of the most productive rivers to fuel hydroelectricity are fed from the melt water of high altitude glaciers and snow pack. The Yangtze River in China is important for agriculture, and with the Three Gorges hydroelectric power plant, it is also an important energy producer. However, the Yangtze River, like the Colorado River in the United States and the Ganges River in India, is replenished by melt water from glaciers and snowmelt. This shows the complex impacts of rising temperatures that will reduce water for agriculture, but also produce a renewable form of energy to offset carbon dioxide production from fossil fuels. Melting glaciers may also increase sea levels, which could jeopardize agriculture by flooding and accelerated soil erosion in many low-lying areas around the world.
Global warming is caused by increased concentrations of carbon dioxide, nitrous oxides, methane, and other gases produced by the combustion of fossil fuels. The impact of having more carbon dioxide in the atmosphere is difficult to gauge with certainty. Plants consume carbon dioxide in the production of oxygen through photosynthesis. Theoretically, increased levels of carbon dioxide could spur plant growth because increased atmospheric concentrations of carbon dioxide mean that there is more available for plants to use during photosynthesis. In addition, there is a synergistic relationship between carbon dioxide and water uptake. Plants are more efficient users of water as ambient concentrations of carbon dioxide increase.
Unfortunately, there are many factors that could offset the potential productivity gains to agriculture from increased carbon dioxide concentrations. The conditions that increase the growth of commercial crops also increase the growth of traditional weeds and could accelerate the growth of new invasive plant species. Also, increased temperatures will prompt the growth of plant diseases and insects. In order to reduce the impact of pests and pathogens, farmers will have to apply more pesticides, herbicides, and other chemicals, many of which are manufactured from petrochemicals.
Soil dynamics will also be affected by changing temperature regimes. Rising temperatures will increase the rate at which organic material decomposes and possibly decrease the level of moisture in soils. This will lower soil productivity, thus prompting the increased use of fertilizers. This might be mitigated, though, by the growing presence of nitrogen oxides that are also increasing as a result of fossil fuel combustion. Increased temperatures might also accelerate soil erosion in agricultural areas, rising temperatures increase the severity of thunderstorms. Without appropriate adaptation by producers, soil erosion could accelerate as the increased flow and force of water droplets dislodge soil. This is a problem because soil erosion is itself a cause of carbon dioxide release into the atmosphere.
CONTRIBUTIONS TO CLIMATE CHANGE
While agriculture is affected by climate change, agricultural processes also contribute directly and indirectly to global warming. This occurs for many reasons. A direct contribution is agriculture’s reliance on the combustion of fossil fuels such as gasoline, diesel, and propane to power farm equipment, including tractors, combines, grain elevators, grain dryers, and transport trucks for shipping feed and livestock. Agriculture also relies on petrochemicals in the form of herbicides and pesticides. Estimates suggest that agriculture uses 8 percent of all energy consumed in the United States.
In order for farming to occur, land must be cleared of trees and other vegetation. The problem is that forests represent a "sink" or reservoir of carbon that would otherwise be part of the earth’s atmosphere. The process of deforestation releases the sequestered carbon back into the atmosphere as the fallen trees decompose. This process is often accelerated as farmers burn the wood. In rainforest areas, traditional cultures use a farming process called "slash and burn" or swidden agriculture, whereby farmland is carved from the rainforest by cutting down trees. The trees are set on fire and the resulting ash nourishes the soil. After a couple of years, the nutrients are leached out of the soils because of the heavy precipitation in the rainforest. Farmers then move to a new site and repeat the process.
However, the impact of swidden agriculture is small compared to the destruction of tropical and temperate rainforests for the purposes of agriculture and timber production. Brazil is effectively competing with the United States in soybean production by turning its forests into fields. By turning its rainforests into cropland, Brazil is increasing greenhouse gases through deforestation. It also contributes to greenhouse gases because it has heavily invested in the American model of industrial agriculture, which relies on the consumption of fossil fuels to power farm equipment and to manufacture fertilizers, pesticides, and herbicides. While it is easy to blame tropical countries for cutting down their forests to make way for farming, temperate countries in North America and Europe have also plowed under biodiverse prairies and cut down broadleaf forests to make way for agriculture. The United States has the most productive agricultural system in the world. This productivity comes at a cost to the environment.
Agriculture accounts for about 7.4 percent of all carbon dioxide emissions in the United States. In addition to carbon dioxide, agricultural operations contribute more methane and nitrogen oxides to the atmosphere than any other economic sector. Enteric fermentation generates most of the methane released. In simpler terms, this refers to the flatulence released from ruminant livestock, such as cattle, as they digest feed grains. The production of rice also creates enormous amounts of methane. When the rice paddies are flooded, the organic material in the water-covered soils decomposes, anaerobically releasing methane in the process. The fact that rice is a staple crop for hundreds of millions of people around the world explains why it contributes so much methane to the atmosphere.
Nitrous oxides are an important input to industrial agriculture. Crop production depletes the nutrients in the field. Prior to the industrial revolution, farmers managed soil nutrients by rotating their crops. Different crops use different soil nutrients at different rates. Hence, crop rotation from year to year reduced the overall rate at which nutrients were depleted. Farmers also periodically left fields fallow. The nutrients in these fields were replenished as organic material on the soil surface decomposed.
In modern industrial agriculture, crop rotations play a moderate or minor role in the management of soil fertility. Farmers survive on low profit-margins. Hence, they tend to specialize in only one or two crops to achieve economies of scale in production. In the American Midwest, the crops tend to be corn or soybeans. Farmers are not likely to rotate beyond these two crops, nor are they likely to leave fields fallow. This means that farmers must maintain soil productivity through the application of nitrogen to the soils. Nitrogen oxides form when the nitrogen designed to work below the soil comes in contact and binds with oxygen molecules. Farmers also use animal waste as a way to increase nutrients in the soils. Hence, animal waste management practices also contribute to nitrogen oxides in the atmosphere.
Finally, agriculture contributes greenhouse gases because of the national agricultural system and a global agricultural market that depends on shipping commodities hundreds and even thousands of miles to markets. This requires the combustion of gasoline and diesel to operate trucks and refrigerated storage facilities.
AGRICULTURAL SOLUTIONS TO CLIMATE CHANGE
There are many strategies that farmers, businesses, and consumers can adopt to reduce greenhouse gases related to agriculture. First, farmers can replace fossil fuels such as gasoline and diesel with biofuels such as ethanol or biodiesel. Ethanol is a fuel alcohol that is produced by a fermentation process that uses yeast to convert the sugars found in plants into a combustible alcohol fuel. Ethanol can offset varying amounts of fossil fuel-generated carbon dioxide depending on the material used to produce the etha-nol. For example, Brazil, located in a tropical climate, can efficiently grow sugarcane. Sugarcane is an excellent source material for ethanol because the sugars in sugarcane can be easily converted into alcohol. In the United States, corn is the primary feedstock for ethanol. It is more costly to convert corn into sugar because the sugars are bound up in long starch molecules. These carbohydrates must be broken down in order to free up the sugars to be converted into alcohol. Therefore, researchers in the United States are working hard to discover ways to lower the costs of producing corn-based ethanol.
Researchers are also studying how to use other plant materials to produce fuel. Cellulosic ethanol is not yet a commercially-viable strategy, but many predict that it will be in the near future. Cellulose is the fibrous or "woody" part of many plants. For example, high concentrations of cellulose are found in the stock and leaves of corn. It would be beneficial to use this part of the corn for ethanol production because it is usually considered a waste product. Cellulosic ethanol would allow the corn kernel to be used for food rather than as a source material for ethanol. The challenge is that the sugars in the cellulose are tightly bound to starch molecules. Consequently a more expensive, enzyme-driven process must be used to convert the sugars into alcohol. Therefore, cellulosic ethanol is not commercially viable now, but many countries, including the United States, are spending hundreds of millions of dollars to discover how it could become a commercially viable fuel.
In addition to ethanol, research is being conducted on other plant-based alcohol fuels such as methanol and butanol. These fuels can also be produced from organic materials including grains and wood fibers. They are currently not commercially viable, but some researchers claim that they may be even better than ethanol as an alternative fuel. Diesel produced from plant material can also reduce greenhouse gases. Crops known as oilseeds, such as cottonseed, sunflower, soybeans, and canola, can serve as the source material to produce a diesel product that has performance characteristics similar to petroleum-derived diesel, without emitting the same volume of greenhouse gases.
Farmers can also modify their management practices so that farmland can serve as a sink to sequester carbon dioxide. For example, farmers can create buffers comprised of trees, shrubs, and natural grasses along rivers to prevent soil erosion and the loss of nutrients due to runoff. The Conservation Reserve Program in the United States pays farmers to take marginal cropland out of production as a way to reduce soil erosion.
Farmers can also create windbreaks near farmhouses and outbuildings. Windbreaks can create a microclimate that can moderate temperature extremes by blocking cold winds or providing shade on hot summer days. This can lower energy use and costs on farms. Windbreaks can also serve as carbon sinks, whereby trees and other plants absorb carbon dioxide. Farmers can also adopt conservation tillage strategies that leave part of the organic residue on the field after the harvest. This material slows runoff, thereby reducing soil erosion and nutrient loss. The decomposing material also replenishes the nutrients in the soil. All of these practices can reduce the energy used, and greenhouse gases produced, on the farm.
Livestock producers can reduce the impact of their operations by changing how they manage animal waste such as methane and manure. Some farmers are experimenting with anaerobic digesters that convert manure into more manageable waste solids that can be used as an organic fertilizer. The digester also creates methane as a byproduct, which can be captured and used as a renewable fuel.
Finally, some writers argue for a move away from large-scale industrial agriculture to place more emphasis on so-called civic agriculture as a way to slow global warming. Civic agriculture includes local food systems and organic foods. Local food systems reduce greenhouse gases by reducing transportation costs. Traditional food supply chains can stretch thousands of miles from the point of production to the place of consumption. Local food advocates suggest that food could be grown and consumed locally. Organic agriculture reduces the use of greenhouse gases because organic farmers cannot use petroleum-based herbicides or pesticides.
