Each year in the Northern Hemisphere, levels of atmospheric carbon dioxide drop in the summer as plants inhale, and then climb again as they exhale and decompose after their growing season.
Over the past five decades, the size of this seasonal swing has increased by as much as half, for reasons that aren’t fully understood.
“In the Northern Hemisphere, there is a strong seasonal cycle of vegetation. 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,” said study senior author Prof Mark Friedl of Boston University.
Prof Friedl and his colleagues gathered global production statistics for four leading crops – corn, wheat, rice and soybeans – that represent about 64 percent of all calories consumed worldwide.
They found that production of these crops in the Northern Hemisphere has more than doubled since 1961, and translates to about a billion metric tons of carbon captured and released each year.
“These croplands are ecosystems on steroids. They occupy about 6 percent of the vegetated land area in the Northern Hemisphere but are responsible for up to a quarter of the total increase in seasonal carbon exchange of atmospheric carbon dioxide, and possibly more,” said lead author Dr Josh Gray of Boston University.
“That’s a very large, significant contribution, and 2/3 of that contribution is attributed to corn. Corn once again is king, this time demonstrating its strong influence on the seasonal cycle of atmospheric carbon dioxide,” said study co-author Prof Christopher Kucharik of the University of Wisconsin-Madison.
This growth in seasonal variation doesn’t have a huge impact on global terrestrial carbon uptake and release, since essentially all carbon in the harvested crops is released each year.
However, with global food productivity expected to double over the next 50 years, the findings should be used to improve climate models and better understand the atmospheric carbon dioxide buffering capacity of ecosystems, particularly as climate change may continue to perturb the greenhouse gas budget.
“Global climate models don’t represent the important details of agroecosystems and their management very well,” Prof Kucharik said.
“This study highlighted the extraordinary increases in crop production in recent decades,” Prof Friedl said.
“It’s a remarkable story of what we’ve done in agriculture in general. And in particular corn, which is one crop that’s just exploded.”
“Over the last 50 years, the area of croplands in the Northern Hemisphere has been relatively stable, but production has intensified enormously. 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,” he concluded.
Josh M. Gray et al. 2014. Direct human influence on atmospheric CO2 seasonality from increased cropland productivity. Nature 515, 398–401; doi: 10.1038/nature13957
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