Last week, a study published in Nature revealed that modern agriculture has played a significant role in increasing seasonal swings of the carbon cycle.  

image expired

Each year, the amount of CO2 in the atmosphere decreases in the spring as plants 'inhale', then increases when CO2 is released back into the atmosphere as plants are harvested or die back in the autumn. Levels of the greenhouse gas fall, only to rise again. However, though scientists have known about this cycle since the 1950s, in recent years, the seasonal swing has been observed to increase, with higher levels of CO2 in the atmosphere in peak season.

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. The Nature research from the States showed that humans' agro-ecosystems have a large impact on this swing.

A steep rise in the productivity of crops grown for food is thought to account for as much as 25 percent of the increase in this carbon dioxide (CO2) seasonality, and the type of crop grown is also thought to play a part.

According to researchers at University of Wisconsin-Madison, it's not necessarily the case 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.

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 is despite the fact that forests and grasslands have also been more 'productive' as the planet has warmed and growing seasons have lengthened.

UW-Madison's professor Chris Kucharik, who helped the research team determine how the amount of carbon absorbed by the leaves, stems, roots and food-portion of crops may have changed over time, said "Global climate models don't represent the important details of agroecosystems and their management very well." Commenting on the proportion of the observed change that can be attributed to agriculture, the UW-Madison professor added, "[It's] a very large, significant contribution, and two thirds of that contribution is attributed to corn."

What does this mean for agriculture?

Speaking to Farming Online on Friday (21 November), the Study's lead author Prof Joshua Gray from Boston University said that, though the researchers did not specifically look at agriculture-related emissions of CO2 or other greenhouse gasses, "These have been estimated elsewhere to be as high as 25 percent of the global total emissions."

Prof Gray explained that the most significant aspect of his team's discovery is that it should lead to an improved understanding of how both managed and natural ecosystems are altering the carbon cycle and responding to the changing climate. The BU Professor added that the Nature study deals only with a seasonal pattern of carbon exchange, as "For the most part, [the process] does not sequester carbon."

He said this is important, as it will make future climate projections more precise, due to a better understanding of the carbon cycle and a more accurate interpretation of agriculture's place within it. Prof Gray continued, "Croplands are not particularly well-represented in the current generation of Earth system models, which may partially explain why it's taken so long to discover this phenomenon. So, our study also indicates the overall importance of these ecosystems in the global carbon cycle; they may be small areas, but they pack a very big punch."

Though he was keen to state that the Nature study did not examine carbon sequestration, but instead looked at a "Different aspect of the carbon cycle – the difference between seasonal patterns of temporary storage, and release, of carbon," Prof Gray tentatively offered some insight into areas where the findings dovetail with research on agricultural practices. He continued, "The really, really big asterisk on all of this is that the intensive farming practices that have enabled the cropland productivity increases are themselves significant sources of CO2 and Greenhouse gasses, it's just that those emissions are more or less constant throughout the year and so don't strongly influence seasonality."

"When we talk about mitigating carbon emissions, what we really mean is finding ways to sequester more carbon long term," Prof Gray said. "Regarding getting croplands to sequester more carbon… it's not really our field, but there is a robust literature around soil carbon sequestration under different tillage [or] management schemes.

Prof Gray said the results of other studies broadly suggest that some conservation practices – potentially including no-till farming – could help sequester carbon if they are adopted at larger scales.

Even so, he said, "Most of this signal [observed in our research] is being driven by corn production in the Midwestern US and Northern China, and those croplands have certainly benefitted from fertilization, farm mechanization, and improved genetics."