'Green Revolution' changes breathing of the biosphere: Stronger seasonal oscillations in carbon dioxide linked to intensive agriculture

That's the key finding of a new atmospheric model developed by University of Maryland researchers, which estimates that on average, the amplitude of the seasonal oscillation of carbon dioxide in the atmosphere is increasing at a rate of 0.3 percent every year. A study based on the results of the model, called VEGAS, was published Nov. 20, 2014 in the journal Nature.

"What we are seeing is the effect of the Green Revolution on Earth's metabolism," said UMD Atmospheric and Ocean Science Professor Ning Zeng, the lead developer of VEGAS, a terrestrial carbon cycle model that, for the first time, factors in changes in 20th and 21st century farming practices. "Changes in the way we manage the land can literally alter the breathing of the biosphere."

Scientists have known since the 1950s that carbon dioxide levels in the atmosphere hit an annual low during late summer and early fall in the Northern Hemisphere, which has a greater continental landmass than the Southern Hemisphere, and therefore has more plant life. The atmosphere's carbon dioxide level falls in spring and summer as all the hemisphere's plants reach their maximum growth, taking in carbon dioxide and releasing oxygen. In the autumn, when the hemisphere's plants are decomposing and releasing stored carbon, the atmosphere's carbon dioxide levels rapidly increase.

In a set of historic observations taken continuously since 1958 at Hawaii's Mauna Loa Observatory, and later in other places including Barrow, Alaska, researchers have tracked these seasonal peaks and valleys, which clearly show an increase in the atmosphere's overall level of carbon dioxide, Earth's main greenhouse gas. Between 1961 and 2010, the seasonal variation has also become more extreme. Carbon dioxide levels are currently about 6 parts per million higher in the Northern Hemisphere's winter than in summer.

While the forces driving the overall increase in carbon dioxide are well understood, the reasons behind the steepening of the seasonal carbon dioxide cycle are harder to pin down. Because plants breathe in carbon dioxide, higher atmospheric levels of the gas can stimulate plant growth, and this so-called "carbon dioxide fertilization effect" probably plays a role. Climate scientists also point to the warming in the Northern Hemisphere high latitudes that makes plants grow better in cold regions as an important factor. But even taken together, those factors cannot fully account for the trend and spatial patterns toward increasing seasonal change, said Zeng.

Zeng points out that between 1961 and 2010, the amount of land planted with major crops grew by 20 percent, but crop production tripled. The combination of factors known as the Green Revolution--improved irrigation, increased use of manufactured fertilizer, and higher-yield strains of corn, wheat, rice and other crops--must have led not only to increased crop productivity, but also to increases in plants' seasonal growth and decay and the amount of carbon dioxide they release to the atmosphere, he reasoned.

UMD graduate student Fang Zhao and other collaborators worked with Zeng, who developed the first of several versions of the VEGAS model in 2000, to add information on worldwide crop production. The researchers combined country-by-country statistics collected yearly by the United Nations Food and Agricultural Organization (FAO) with climate data and observations of atmospheric carbon dioxide levels from several sites. To ensure that their results did not overstate the Green Revolution's effect, the researchers ran their model using an estimate of worldwide crop production slightly lower than the FAO statistics.

Once the Green Revolution was factored in, VEGAS' results generally tracked the actual carbon dioxide peaks and valleys recorded at Mauna Loa. Between 1975 and 1985, carbon dioxide levels rose faster at Mauna Loa than they did in the model, but this could be due to regional weather patterns, Zeng said.

Other atmospheric models factor in changes in land use, from natural vegetation to cropland, Zeng said, but the VEGAS results described in Nature are the first to track the effect of changes in the intensity of farming methods. There are still many unknowns. For example, the Green Revolution has not affected all parts of the world equally, and there isn't enough detailed information about changing farming practices over the past 50 years to build those detailed variations into the model.

"We dealt with the unknowns by keeping it simple," said Zeng. "My education was mostly in physics, and physicists are brave about making the simplifying assumptions you have to make to reach a general understanding of some important force. Our goal was simply to represent the intensification of agriculture in a model of the carbon cycle, and we have accomplished that."