Crops are major factor in carbon dioxide swings, climate change

Ever the hot topic, climate change is on the forefront of both public policy and research. In a recent study, researchers report that increasing agricultural production has a unique role, increasing carbon dioxide swings, and potentially contributing to climate change.

The study, published in Nature, was a collaborative effort including scientists from Boston University, the University of New Hampshire, the University of Michigan, the University of Minnesota, the University of Wisconsin-Madison and McGill University.

A natural cycling of carbon dioxide (CO2) levels has been observed and attributed to photosynthetic activity. In the spring and summer, plants serve as CO2 sponges, rapidly absorbing carbon dioxide which they convert to sugar. When fall and winter come around, the photosynthetic rate drops and respiration, a process which releases CO2, dominates. Thus, during the warmer months CO2 levels drop, only to rise again by the same margin in the colder months.

The cycling of CO2 is independent of the overall increase of CO2 levels observed. However, there has also been an increase in the magnitude of the cycling, or rise and fall, or carbon dioxide levels over time. In the present study, researchers set out to determine why there has been a 50 percent increase in the amount of CO2 cycling over the last 50 years. The team analyzed global production statistics for corn, wheat, rice, and soybeans to estimate the amount of carbon absorbed by the leaves, stems, roots and food-portion of crops.

The team found that in the Northern Hemisphere, production of these crops has increased by 240 percent between 1961 and 2008. This increase equates to almost a billion metric tons of carbon dioxide captured and released each year. Interestingly, the total area used for production had not changed significantly. The authors explain that extensive agricultural management and improvements in plant breeding have increased productivity. “You get more bang for your buck, more crop per drop,” said co-author Chris Kucharik, of UW-Madison.

Of all of the crops, corn had the greatest impact on the magnitude of carbon dioxide cycling. With “ 2/3 of that contribution is attributed to corn,” explains Kucharik.

“Corn once again is king, this time demonstrating its strong influence on the seasonal cycle of atmospheric CO2.”

The authors explain that understanding the effect that agriculture has on seasonal carbon dioxide levels is key to accurately modeling global climate change. This information will be especially useful in assessing how well natural ecosystems can buffer rising levels of CO2.

In addition to providing information to help model CO2, the study also sheds light on the substantial agricultural improvements over the last five decades. “Over the last 50 years, the area of croplands in the Northern Hemisphere has been relatively stable, but production has intensified enormously,” said co-author Mark A. Fiedl. “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.”

Sources:

Gray, J.M. et al. Direct human influence on atmospheric CO2seasonality from increased cropland productivityNature5(15) 398–401

University of Wisconsin-Madison, News Release

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