Since 1961, there has been an ever-increasing seasonal change in the Earth's carbon dioxide levels. The reason for this may be the increased activity of global agriculture, a group of researchers say.
This study was published in the journal Nature.
From 1961 to 2010, when the improvement in agricultural technology known as the "green revolution" began, something curious happened. The Earth's carbon dioxide levels began fluctuating seasonally.
This NASA-funded study linked the change to farming activity. When farm activity picks up in the spring and summer, plants absorb large amounts of carbon dioxide as they grow. Then, in the fall and winter, they release carbon dioxide when they die.
Such seasonal activity is distinct from global warming, the scientists said. Although farming activity does contribute to the overall increase of carbon dioxide in the atmosphere, the seasonal fluctuation itself does not affect global warming.
You can see the changes on a graph called the Keeling curve, which shows carbon dioxide levels by month over a 50-year time span. Previously, people attributed this seasonal change to the way plants were adapting to climate change. However, agriculture plays a part as well, accounting for nearly a quarter of the seasonal swings.
"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," said Eric Kort, co-author of this study.
In the time since 1961, the amount of land on Earth used as farmland increased by 20 percent, and overall production of crops grew three times. There was a 240 percent increase in the production of crops in the Northern Hemisphere alone.
Josh Grey, lead author of this paper, called this overproduction an "ecosystem on steroids."
Another way to think about this is that normally, a small seasonal fluctuation in carbon dioxide levels caused by plants would be normal, but the present fluctuation we are seeing is directly caused by the human production of plants.
The researchers predicted that as the population grows, and farm production continues to increase, humans will have an even bigger effect on the environment than we do now.