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.
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