Researchers looked at the significant impact agricultural crops have on seasonal swings of carbon dioxide in the atmosphere.
The study found agriculture amplifies annual carbon dioxide (CO2) fluctuations when plants suck up the CO2 in the spring and summer and then release when they start to compose in the autumn and winter, the University of Michigan reported.
The researchers noted the seasonal changes are not related to the upward trend of CO2 levels that have been linked to global climate change.
"A simple picture is that plants breathe. You can see the seasonal impact of this in the Keeling curve, the famous graph that shows atmospheric CO2 levels measured from a mountaintop in Hawaii since the late 1950s," said Eric Kort, assistant professor in the U-M Department of Atmospheric, Oceanic and Space Sciences and a co-author the study. "While it's been continually increasing, it wiggles up and down a bit each year, and that's this seasonal breathing of the biosphere."
Over the past five decades the "wiggles" have been growing and moving towards higher northern latitudes. Researchers are unsure of exactly what is causing this phenomenon, in the past it has often been blamed on the "natural system reacting to the altered climate" but these new findings suggest other contributing factors.
"It turns out we can explain about 25 percent of the increase in seasonal swings with croplands, which are not a natural system," Kort said. "It's a different direct human fingerprint."
To make their findings the researchers looked at global production statistics corn, wheat, rice and soybeans, which make up about 64 percent of calories consumed across the globe. This research allowed them to estimate how much carbon was taken up and released by the crops over time; they then compared these results with data on the increased carbon exchanged with Northern Hemisphere.
The findings show production of these crops in the Northern Hemisphere increased by over 240 percent between 1961 and 2008, increasing seasonal carbon dioxide swings by one third of a petagram over the same time period.
"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," said Mark Friedl, a professor in Boston University's Department of Earth and Environment.
The findings predict total global food production will need to double over the next half of the century, leading to increases in cropland and productivity.
The findings were published in a recent edition of the journal Nature.
