A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in Earth’s atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Without greenhouse gases, the average temperature of Earth’s surface would be about 18 °C rather than the present average of 15 °C (59 °F).
Farms emitted 6 billion tones of GHGs in 2011, or about 13 percent of total global emissions. That makes the agricultural sector the world’s second-largest emitter, after the energy sector (which includes emissions from power generation and transport). The 10 countries with the largest agricultural emissions in 2011 were (in descending order): China, Brazil, United States, India, Indonesia, Russian Federation, Democratic Republic of Congo, Argentina, Myanmar, and Pakistan. Together, these countries contributed 51 percent of global agricultural emissions.
Most farm-related emissions come in the form of methane (CH4) and nitrous oxide (N2O). Cattle belching (CH4) and the addition of natural or synthetic fertilizers and wastes to soils (N2O) represent the largest sources, making up 65 percent of agricultural emissions globally. Smaller sources include manure management, rice cultivation, field burning of crop residues, and fuel use on farms.
From 1990 to 2010, global agricultural emissions increased 8 percent. They are projected to increase 15 percent above 2010 levels by 2030, when they will amount to nearly 7 billion tons per year. These increases are mainly driven by population growth and changes in dietary preferences in developing economies. Agricultural emissions growth will be greatest in Asia and sub-Saharan Africa, which will account for two-thirds of the increase in overall food demand over first half of the 21st Century. The production of vegetable oils and animal products products with a high GHG intensity are expected to grow the most amongst agricultural outputs.
Changes in both farming practices and food demand offer big opportunities. On the supply side, crop management practices such as improved fertilizer management and conservation tillageoffer the greatest reduction potential at relatively low costs. Better managing grazing lands such as by rotational grazing and altering forage composition and restoring degraded lands and cultivated organic soils into productivity are also important.
On the supply-side, shifting away from meat and particularly beef consumption offers the most potential for reducing emissions. Also, recent WRI research shows that about 24 percent of all calories currently produced for human consumption are lost or wasted in the food supply chain. Consequently, reducing food losses and wastes can also play an important role.