Hugely important new research from the United States has shown that modern arable agriculture is contributing to rises in atmospheric carbon dioxide, and growing seasonal fluctuations in levels atmospheric carbon.

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The research shows that crop cycles contribute to seasonal rises in CO2 levels. Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer when plants 'inhale' as they grow, then climb again as they exhale after the growing season or as they decompose. Over the last 50 years, the size of this seasonal swing has increased by 50 percent.

The reasons behind the increase in the swing aren't yet fully understood, though the effects of higher temperatures driven by climate change are thought to be implicated. The North also has a greater continental landmass than the Southern Hemisphere, and therefore has more plant life, which could also have an impact.

However, researchers publishing their findings in the journal Science have this week revealed that agricultural production could contribute up to a quarter of the observed increase in this seasonal carbon cycle, with maize in particular playing a significant part. Maize crops are thought to be behind up to two thirds of arable crops' contribution to the fluctuating atmospheric carbon cycle.  

The scientists behind the research said theirs is the latest in a series of studies to uncover "direct fingerprints" of human activity on Earth and its systems.

Professor Eric Kort from the University of Michigan was a contributing author to the study. He explained, "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."

"While it's been continually increasing, it wiggles up and down a bit each year, and that's this seasonal breathing of the biosphere," Kort said. "It turns out we can explain about 25 percent of the increase in seasonal swings with croplands, which are not a natural system… It's a different direct human fingerprint."

Kort added that one way to look at the fluctuations in emissions is to consider the upward trend as a result of the inputs from human fossil fuel consumption, and the 'wiggles' as the natural system.

"But humans are changing the wiggles, too," he said. "And not just indirectly, but directly through crops."

'Ecosystems on steroids'

To reach their conclusions, the researchers, who represented a number of institutions in the US and whose work was funded by NASA, gathered global production statistics for maize, wheat, rice and soybeans, which together represent about 64 percent of calories consumed worldwide. They then estimated the amount of carbon that was taken up and released by each crop over time and compared these figures with the seasonal swing in carbon exchange in the Northern Hemisphere.

The researchers found that crop production in the Northern Hemisphere increased by more than 240 percent between 1961 – when the 'Green Revolution' was underway – and 2008, though the corresponding land area grew by just 18 percent.

Although the researchers still suggest that the majority of fluctuations in the carbon cycle are the result of the effects of global warming due to climate change – including longer growing seasons, quicker uptake of carbon by vegetation and the 'greening' of higher latitudes with more vegetation – they said the role played by agriculture is still hugely significant.

Increases in crop production and yield exacerbated the seasonal carbon dioxide swings by one third of a petagram over the observed period; to compare, the world's total fossil fuel emissions are currently approximately 10 petagrams per year.

The discovery led Prof Josh Gray, the study's lead author, to remark that croplands are effectively "ecosystems on steroids." University of Maryland professor Ning Zeng added, "What we are seeing is the effect of the Green Revolution on Earth's metabolism; changes in the way we manage the land can literally alter the breathing of the biosphere."

"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," Added Boston University's Prof Mark Friedl.

The researchers suggested that population increases and the resultant lurch in food production or lands converted to agriculture will strengthen the trend they identified. Prof Gray suggested that understanding the effects of agricultural production will help improve models of global climate, and could play a part in investigating how well ecosystems will buffer rising levels of carbon dioxide in the future.