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."