Atmospheric CO2 Levels Affected By Increased Crop Growth

Chuck Bednar for – Your Universe Online

Decomposing crops could be responsible for one-fourth of the post-summer increase in atmospheric carbon dioxide levels, and this discovery could help scientists better understand and predict how the planet’s vegetation will react to warming temperatures, researchers from Boston University and elsewhere report in a new study.

Scientists have long known that atmospheric CO2 levels drop in the Northern Hemisphere every summer as plants inhale the substance, then climb again when they exhale following their growing season, but the exact reasons for this phenomenon have remained unclear. Now, however, they report in the journal Nature that agricultural production may play a vital role.

“In the Northern Hemisphere, there is a strong seasonal cycle of vegetation,” Mark Friedl, a professor in the Boston University (BU) Department of Earth and Environment and senior author of the new paper, said in a statement Wednesday. “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.”

While the majority of the annual change is attributed to the impact of higher temperatures being driven by global climate change – an impact which includes longer growing seasons, a more rapid uptake of carbon by crops, and the so-called greening of higher latitude regions with increasing amounts of vegetation – lead author and BU assistant professor Josh Gray explains that this is “not the whole story,” and that human factors need to be considered.

Friedl, Gray and their colleagues collected global production data for corn, wheat, rice and soybeans – four crops that together account for nearly two-thirds of all calories consumed worldwide. They discovered that the production of these crops in the Northern Hemisphere above the tropics has more than doubled since 1961, and that the increase growth effectively translates into one billion tons of carbon captured and released each year.

These crop areas are “ecosystems on steroids,” Gray said, explaining that while they only occupy approximately six percent of the vegetated land area in the Northern Hemisphere, they are responsible for at least 25 percent of the total increase in seasonal carbon exchange of atmospheric carbon dioxide. However, the authors noted that these seasonal effects are separate and distinct from the overall upward trend in CO2 that is driving climate change.

Image Above: The Keeling curve plots atmospheric carbon dioxide. The upward slope represents increases in CO2 levels caused by burning fossil fuels. The wiggles represent seasonal swings in atmospheric CO2 levels that occur as plants bloom, grow, die and decay every year. Image credit: Scripps Institution of Oceanography

“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,” explained co-author Eric Kort, an assistant professor at the University of Michigan. “While it’s been continually increasing, it wiggles up and down a bit each year, and that’s this seasonal breathing of the biosphere.”

Those wiggles have been growing larger over the past five decades, increasing by as much as 50 percent at half at higher northern latitudes, and scientists have not fully understood why. Much of the change is believed to be caused by the natural system reacting to an altered climate, with changes in photosynthesis, respiration and expansion of woody vegetation potentially involved. Again, though, the authors said that mankind was also likely involved.

“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,” said co-author Chris Kucharik, a professor in the University of Wisconsin College of Agricultural and Life Sciences Department of Agronomy and the Nelson Institute for Environmental Studies.

“This study shows the power of modeling and data mining in addressing potential sources contributing to seasonal changes in carbon dioxide,” added Liz Blood, program director for the National Science Foundation’s MacroSystems Biology Program, which funded the recently-published study. “It points to the role of basic research in finding answers to complex problems.”

A separate study, also published this week in Nature, researchers at the University of Maryland (UMD) used a new atmospheric model developed at the university to determine that the intense farming practices of the so-called Green Revolution have been strong enough to boost the seasonal amplitude in atmospheric carbon dioxide to about 15 percent over the past five decades. The amplitude of season CO2 oscillation is increasing at a rate of 0.3 percent each year, they added.

“What we are seeing is the effect of the Green Revolution on Earth’s metabolism,” Ning Zeng, a UMD Atmospheric and Ocean Science Professor and the lead developer of VEGAS, a terrestrial carbon cycle model that factors in changes in 20th and 21st century farming practices, said in a statement. “Changes in the way we manage the land can literally alter the breathing of the biosphere.”


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