Agricultural Research: The Key to Coping With Global Change

ARS research shows yields in crops like rice may increase if carbon dioxide levels rise but temperatures stay moderate.

With every exhalation, we release carbon dioxide (CO₂) into the atmosphere. In the wondrous biological process of photosynthesis, plants use the sun's energy to convert this gas to the food we eat and the oxygen we breathe.

Yet this crucial gas may have a dark side. CO₂ and some other gases may be changing our climate.

Atmospheric CO₂ concentrations have risen from 280 to more than 350 parts per million during the last 200 years. At current CO₂ emission rates, that concentration will double again over the next century.

"The so-called greenhouse effect is a natural process that helps keep the planet surface at a comfortable temperature," says Herman S. Mayeux. "The concern is that concentrations of greenhouse gases are increasing in the atmosphere. As a result, the surface temperature of the planet may be rising." Mayeux is the Agricultural Research Service's national program leader for rangelands and global change at Beltsville, Maryland.

Potential temperature increases and the changes in precipitation patterns that could occur because of the rise of greenhouse gas concentrations are known collectively as global climate change.

Scientists can measure an increase in atmospheric gas concentrations, but determining the effect of that rise is difficult because of natural variability in temperature and precipitation. Computer models that simulate atmospheric behavior indicate that global temperatures generally increase as greenhouse gas concentrations rise.

"Because climate and CO₂ play such important roles in agriculture, any long-term changes are of great concern," Mayeux says.

For that reason, ARS scientists nationwide are evaluating U.S. agriculture's contribution to the increase in greenhouse gases, the potential impact of climate change on how we produce food, and the industry's unique opportunities to help mitigate atmospheric change.

The Greenhouse Effect

Solar radiation passes though the atmosphere and warms the Earth's surface. Some is reflected back into the atmosphere and dissipates into space. The greenhouse effect refers to an accumulation of specific gases that absorb the reflected radiation, effectively trapping heat in the lower atmosphere. The most important of these gases are water vapor and CO₂. Smaller amounts of methane, nitrous oxide, chlorofluorocarbons, and ozone also contribute, intensifying the greenhouse effect. But global warming doesn't mean every place on Earth will be warmer. Rather, it indicates a general rise in the planet's average surface temperature. More important than either the rise in gases or temperature would be the potential impacts of these increases--changes in the amount and pattern of rain and snowfall, length of growing seasons, sea level, and storm patterns.

The chart below shows how concentrations of three greenhouse gases changed between 1800 and 1990.

*ppmv - parts per million by volume
**ppbv - parts per billion by volume
Source: United Nations International Panel on Climate Change, 1992

Agriculture's Contribution to Global Change

Scientists use the term "climate forcing" to compare the contribution of different activities to climate change. Climate forcing is a measure combining estimates of greenhouse gas emissions with the absorption of long-wave radiation from the Earth and the estimated lifetime of each gas in the atmosphere.

U.S. agriculture is responsible for less than 1 percent of this forcing, according to the Council for Agricultural Science and Technology, a nonprofit agricultural sciences organization based in Ames, Iowa. Agriculture and industry contribute in various ways to atmospheric concentrations of three greenhouse gases.

Carbon dioxide--Microbes produce CO₂ in soil as they free up carbon molecules while feeding on organic matter. Tillage not only frees CO₂ in bursts of gas, but also lets in oxygen that speeds up microbial action. Crops and other plants reduce atmospheric CO₂ levels as they take it from the air during photosynthesis. Burning forests and grasslands are other sources. But burning fossil fuels like oil, coal, and gas accounts for most of the world's CO₂ emissions.

Methane--This gas is released from many sources, including gas drilling areas, coal mines, landfills, natural water bodies like oceans and lakes, holding ponds for animal waste, and rice paddies. Methane is also produced by the digestive processes of ruminant animals and termites. Some bacteria in soils produce methane, while others transform it to other compounds, effectively removing it from the atmosphere.

Nitrous oxide--The synthetic form of N₂O is the "laughing gas" used by dentists as an anesthetic. Agricultural and natural processes within soils, burning of vegetation and fossil fuels, and the oceans all appear to release N₂O. On farmland, microbes emit it as they feed on nitrogen fertilizers and manure. Fertilization with nitrogen increases emissions of
N₂O from cropland and pasture soils.

The Ozone Hole--A Different Issue

Though sometimes confused, the greenhouse effect and the ozone "hole" are separate phenomena. A form of oxygen, ozone plays two roles in the atmosphere. Near the ground, ozone is an air pollutant and a minor greenhouse gas. In the upper atmosphere, it forms a layer that helps protect us from sunburn and skin cancer by absorbing some of the ultraviolet radiation from the sun.

Click here to learn about ARS research on methyl bromide alternatives.

The "hole" refers to a thinning of this layer because of chemical reactions in the upper atmosphere, especially at the Earth's poles. To combat ozone depletion, several chemicals are targeted for reduction or elimination--including the important agricultural fumigant methyl bromide.

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