Levels of carbon dioxide in our atmosphere shows seasonal changes, dropping during the summer as growing plants inhale, then rising as they exhale and begin decomposing the their growing season, and now researchers say they've determined a significant role of agriculture in that phenomenon.

During the past 50 years, the amount of the seasonal swings in CO2 has climbed by as much as half, and a research team has shown agricultural production may be generating as much as a quarter of the increase in the seasonal carbon cycle.

"In the Northern Hemisphere, there is a strong seasonal cycle of vegetation," says Mark Friedl, a Boston University professor and lead author of a paper published in Nature.

"Something is changing about this cycle; the ecosystems are becoming more productive, pulling in more atmospheric carbon during the summer and releasing more during the dormant period," he says.

The findings highlight the significant impact people, with their agricultural activities, can have on the Earth's atmosphere, the researchers say.

Crops have not changed in the amount of CO2 they can release, but the world acreage devoted to agriculture has, the researchers say, pointing out that global food productivity is predicted to at least double in the next 50 years.

In the Northern Hemisphere since the 1960s crops including corn, rice, wheat and soybeans have seen a 240 percent increase in production.

"This is another piece of evidence suggesting that when we (humans) do things at a large scale, we have the ability to greatly influence the composition of the atmosphere," says study co-author Chris Kucharik of the University of Wisconsin-Madison.

Although accounting for just 6 percent of the green, or vegetated, land area in the Northern Hemisphere, cropland has been the major contributor to the increase of 50 percent in the seasonal cycling of CO2, researchers say.

"That's a very large, significant contribution, and two-thirds of that contribution is attributed to corn," says Kucharik. "Corn once again is king, this time demonstrating its strong influence on the seasonal cycle of atmospheric CO2."

It's not that the amount of cropland has increased -- it's been about the same during the 50 years of the increase in CO2 cycles -- it's that crop production has seen extraordinary increases in that time.

"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 such a small land area can actually affect the composition of the atmosphere is an amazing fingerprint of human activity on the planet."

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