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Direct human influence on atmospheric CO2 seasonality from increased cropland productivity, study

20 November, 2014
Boosts in productivity of corn and other crops modify Northern Hemisphere carbon dioxide cycle.  sciencedaily.com. November 19, 2014. Summary: In the Northern Hemisphere, there’s a strong seasonal cycle of vegetation. Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer as plants "inhale," then climb again as they exhale after the growing season. During the last 50 years, the size of this seasonal swing has increased by as much as half, for reasons that aren’t fully understood. Now a team of researchers has shown that agricultural production may generate up to a quarter of the increase in this seasonal carbon cycle, with corn playing a leading role.
Direct human influence on atmospheric CO2 seasonality from increased cropland productivity, study
Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer as plants "inhale," then climb again as they exhale after the growing season.  Credit: © Željko Radojko / Fotolia

Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer as plants "inhale," then climb again as they exhale after the growing season.
During the last 50 years, the size of this seasonal swing has increased by as much as half, for reasons that aren’t fully understood.
Now a team of researchers has shown that agricultural production may generate up to a quarter of the increase in this seasonal carbon cycle, with corn playing a leading role.
"This study shows the power of modeling and data mining in addressing potential sources contributing to seasonal changes in carbon dioxide," says Liz Blood, program director for the National Science Foundation’s MacroSystems Biology Program, which funded the research. "It points to the role of basic research in finding answers to complex problems."
In the Northern Hemisphere, there’s a strong seasonal cycle of vegetation, says scientist Mark Friedl of Boston University (BU), senior author of a paper reporting the results in this week’s issue of the journal Nature.
"Something is changing about this cycle," says Friedl. "Ecosystems are becoming more productive, pulling in more atmospheric carbon during the summer and releasing more during the dormant period."
Most of this annual change is attributed to the effects of higher temperatures driven by climate change–including longer growing seasons, quicker uptake of carbon by vegetation and the "greening" of higher latitudes with more vegetation."But that’s not the whole story," says Josh Gray of BU, lead author of the paper. "We’ve put humans and croplands into the story."
The scientists gathered global production statistics for four leading crops–corn, wheat, rice and soybeans–that together represent about 64 percent of all calories consumed worldwide.
They found that production of these crops in the Northern Hemisphere has more than doubled since 1961 and translates to about a billion metric tons of carbon captured and released each year.
These croplands are "ecosystems on steroids," says Gray, noting that they occupy about 6 percent of the vegetative land area in the Northern Hemisphere, but are responsible for up to a quarter of the total increase in seasonal carbon exchange of atmospheric carbon dioxide.
The growth in seasonal variation doesn’t have a huge impact on global terrestrial carbon uptake and release, he says, since carbon gathered by the crops is released each year.
However, understanding the effects of agricultural production, the researchers maintain, will help improve models of global climate, particularly in discovering how well ecosystems will buffer rising levels of carbon dioxide in the future.
The BU investigators collaborated with a team of scientists, including Eric Kort of the University of Michigan, Steve Frolking of the University of New Hampshire, Christopher Kucharik of the University of Wisconsin, Navin Ramankutty of the University of British Columbia and Deepak Ray of the University of Minnesota.
The work highlights extraordinary increases in crop production in recent decades.
"These indications of increased productivity speak well for agriculture," says Tom Torgersen, program director for the National Science Foundation’s Water Sustainability and Climate Program, which also funded the research. "But such enhanced agricultural productivity makes significant demands on water supplies, which will require further investigation. "Adds Friedl, "It’s a remarkable story of what we’ve done in agriculture in general. And in particular in corn, which is one crop that’s just exploded."
Corn alone accounts for two-thirds of the crop contribution to the increased seasonal exchange in carbon, he says. Almost 90 percent is produced in the midwestern United States and China.
"Over the last 50 years, the area of croplands in the Northern Hemisphere has been relatively stable, but production has intensified enormously," Friedl says.
"The fact that this land area can affect the composition of the atmosphere is an amazing fingerprint of human activity on the planet."
Journal Reference: Josh M. Gray, Steve Frolking, Eric A. Kort, Deepak K. Ray, Christopher J. Kucharik, Navin Ramankutty, Mark A. Friedl. Direct human influence on atmospheric CO2 seasonality from increased cropland productivity. Nature, 2014; 515 (7527): 398 DOI: 10.1038/nature13957
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Read also: ‘Green Revolution’ changes breathing of the biosphere: Stronger seasonal oscillations in carbon dioxide linked to intensive agriculture.sciencedaily.com.November 19, 2014. Summary: The intense farming practices of the ‘Green Revolution’ are powerful enough to alter Earth’s atmosphere at an ever-increasing rate, boosting the seasonal amplitude in atmospheric carbon dioxide to about 15 percent over the past five decades. That’s the key finding of a new atmospheric model, 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.
The trend line of atmospheric carbon dioxide readings in Barrow, Alaska from 2003 to 2012 is superimposed on an artist’s rendering of intensive farming in the Corn Belt of the Midwestern United States.
Credit: Fang Zhao and Ning Zeng/Shutterstock


The  intense farming practices of the "Green Revolution" are powerful enough to alter Earth’s atmosphere at an ever-increasing rate, boosting the seasonal amplitude in atmospheric carbon dioxide to about 15 percent over the past five decades.
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."

Journal Reference: Ning Zeng, Fang Zhao, George J. Collatz, Eugenia Kalnay, Ross J. Salawitch, Tristram O. West, Luis Guanter. Agricultural Green Revolution as a driver of increasing atmospheric CO2 seasonal amplitude. Nature, 2014; 515 (7527): 394 DOI: 10.1038/nature13893

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