Each year, the planet balances its budget. The carbon dioxide absorbed by plants in the spring and summer as they convert solar energy into food is released back to the atmosphere in autumn and winter. Levels of the greenhouse gas fall, only to rise again.

But the budget has gotten bigger. Over the last five decades, the magnitude of this rise and fall has grown nearly 50 percent in the Northern Hemisphere, as the amount of the greenhouse gas taken in and released has increased. Now, new research shows that humans and their crops have a lot to do with it, highlighting the profound impact people have on the Earth's atmosphere.

In a study published Wednesday, Nov. 19, in Nature, scientists at Boston University, the University of New Hampshire, the University of Michigan, the University of Minnesota, the University of Wisconsin-Madison and McGill University show that a steep rise in the productivity of crops grown for food accounts for as much as 25 percent of the increase in this carbon dioxide (CO2) seasonality.

It's not that crops are adding more CO2 to the atmosphere; rather, if crops are like a sponge for CO2, the sponge has simply gotten bigger and can hold and release more of the gas.

With global food productivity expected to double over the next 50 years, the researchers say the findings should be used to improve climate models and better understand the atmospheric CO2 buffering capacity of ecosystems, particularly as climate change may continue to perturb the greenhouse gas budget.

"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 UW-Madison's Chris Kucharik, a co-author of the study and professor in the College of Agricultural and Life Sciences Department of Agronomy and the Nelson Institute for Environmental Studies.

Since the 1960s in the Northern Hemisphere, maize (corn), wheat, rice and soybeans have seen a 240 percent spike in production, particularly concentrated in the midwestern U.S. and in Northern China, the study found.

But until this point, scientists missed the connection between crops and the CO2 seasonality increase.

"Global climate models don't represent the important details of agroecosystems and their management very well," says Kucharik.

It was fall 2013 when the study's lead authors at Boston University approached the UW-Madison scientist and asked him to lend his agricultural land management, carbon cycling and agricultural technology expertise to their examination of the cycle.

Kucharik helped the team determine how the amount of carbon absorbed by the leaves, stems, roots and food-portion of crops may have changed over time. He helped ensure the methodology the team used properly represented agricultural lands and the management practices that drive changes in the carbon balance.

The study found that, while the area of farmed land has not significantly increased, the production efficiency of that land has. Intensive agricultural management over the last 50 years has had a profound impact.

Kucharik attributes this to improvements in plant breeding, post-World War II fertilization innovations, irrigation and other human-powered technologies.

"You get more bang for your buck, more crop per drop," he says.

Cropland makes up just six percent of the vegetated, or green, area of the Northern Hemisphere and yet, it is a dominant contributor to the 50 percent increase in the CO2 seasonality cycle. This, despite the fact that forests and grasslands have also been more productive as the planet has warmed and growing seasons have lengthened.

"That's a very large, significant contribution, and 2/3 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."

Earlier work at UW-Madison enabled the research team to make the necessary calculations to incorporate agriculture into the new modeling approach, Kucharik says.

"The person that led the charge was Navin Ramankutty at SAGE (the Nelson Institute Center for Sustainability and the Global Environment), in Jon Foley's group in the late '90s and early 2000s," says Kucharik. "Those first global maps of agricultural land use over time came out of SAGE and the Nelson Institute."