Earth's plants take up and release carbon dioxide each year in a seasonal cycle, a “breathing” pattern that apparently has deepened during the past 50 years.
The seasonal breathing pattern for terrestrial plants was vividly captured in a video NASA released this week, showing the rise and fall of CO2 levels as well as CO2's movement across the hemisphere over the course of 2006.
Now, two teams of researchers working independently suggest that across the northern hemisphere, farming is responsible for between 20 and 50 percent of the long-term increases in the amount of CO2 taken up during the growing season and in the amount given back during the dormant season.
Farming's influence has come not from large increases in acres planted but from enormous increases in farm productivity, the two teams note. According to one of the two studies, fertilizers, improved crop breeds, and better irrigation have been roughly twice as influential as the fertilizing effect of rising levels of CO2 in the atmosphere. Increased farm productivity also has been more influential than the longer growing seasons that have come with global warming.
The researchers say they are still trying to tease out the broader implications of their results. But at the least, the results reveal an under-appreciated human influence that isn't reflected in models that simulate the planet's carbon cycle, suggests Josh Gray, a researcher in Boston University's Department of Earth and Environment and the lead author of one of the two studies. Both are set to appear in Thursday's issue of the journal Nature.
The results also could change the way researchers interpret the increase in plant uptake and release of CO2.
Last year, a research team led by scientists from the Scripps Institution of Oceanography in La Jolla, Calif., reported the general trend in CO2 exchanges between plants and the atmosphere. They attributed the larger fluctuations to increases in the size of forests and the trees they contain, to increases in the proportion of young, fast-growing species in forests that had been heavily logged or hit by fire, as well as to changes in the distribution of plant species as trees and shrubs have migrated north as the climate has changed.
They noted that the increase in deep breathing they saw – between 30 and 60 percent since 1960 – was more intense than terrestrial ecosystem models indicated it should be. This has led the models to underestimate the “widespread ecological changes" that have taken place during the past 50 years and leave models vulnerable to under-predicting future changes, the team notes.
If the two studies' results hold up to further scrutiny, they could imply that the current generation models are closer to being correct, since a significant component of the increase in plants' deep breathing initially attributed to forests is due to farming.
Both teams were struck by the CO2 exchange trend noted in the 2013 study. Members wondered about the role farming might play.
With US corn yields now five times higher than before World War II, and with the corn harvest's precipitous plunge during the drought of 2012, “one couldn't help wonder what that might do to the Earth's 'metabolism,' ” explains Ning Zeng, a researcher at the Earth Systems Science Interdisciplinary Center at the University of Maryland in College Park and the lead author of the other paper.
For Dr. Gray and colleagues, their study was sparked by the realization that major farming areas in North America and Asia were in the same latitude range that the 2013 study identified as the areas where the trends in the intensity of the CO2 exchanges were most apparent.
“We know that crops have increased in productivity over this time period and they were in the right place to be influencing this,” he says. In a dinner conversation with a colleague and after some quick calculations on a napkin, the study was born.
Compared with the overall increase in CO2 concentrations in the atmosphere over the past 50 years, the increase in magnitude of the seasonal fluctuations due to plant activity is tiny, Gray acknowledges.
Over the past 50 years, the summertime portions of the plant-atmosphere exchange have seen CO2 shrink by two to five molecules out of every million molecules in the atmosphere, Gray notes. Over the same period, humans have pumped an average of 80 to 100 parts per million of CO2 into the atmosphere by burning fossil fuel and through land-use changes.
Gray's team used carbon-accounting tools and crop statistics to look at maize, wheat, rice, and soy beans. Improved production accounted for up to 25 percent of the increase in the size of seasonal CO2 swings. Of the four crops, corn was the heavy hitter, accounting for two-thirds of all of the increase attributed to the four crops.
The high number for the four crops was surprising in itself, he says, but the corn's influence “was really remarkable.”
Corn production has tripled globally, but it comes out of relatively small production zones in the US Midwest and in China. The amount of land given over to these four crops in the northern hemisphere has increased slightly over the years, even as the amount of land farmed for all crops has decreased slightly, he says.
Dr. Zeng and colleagues modeled the changes in farming practices and their impact on productivity and seasonal carbon swings. They found that farming accounted for 45 percent of the growth in the size of the seasonal swings, while climate change accounted for 29 percent. The fertilization effect from increasing average CO2 levels accounted for 26 percent.
Farming will have to become more productive still to feed a global population expected to continue to grow through the end of the century. And researchers are still trying to weigh the contributions various changes to land use and climate are making the seasonal carbon cycle.
These studies indicate that farming now must join the list of human activities driving seasonal swings in the northern hemisphere's terrestrial carbon cycle, write Natasha MacBean and Philippe Peylin researchers at the Science Laboratory of Climate and Environment at Gif-Sur-Yvette, France, in a commentary accompanying the two studies.