The overall amount of carbon dioxide in the atmosphere has been steadily increasing, a clear trend linked to human activities and climate change. Less concerning but more mysterious, the difference between the highest and lowest amounts of carbon dioxide in the atmosphere each year also has been increasing.
This widening disparity between carbon dioxide peaks and dips was thought to be caused by warming temperatures and more carbon dioxide in the atmosphere, but a new study led by Colorado State University has found that agriculture is the primary cause of seasonal carbon cycle swings. This discovery adds to scientific understanding of the carbon cycle and could help inform climate change mitigation strategies.
While climate and carbon dioxide concentrations do contribute to annual carbon cycle highs and lows, the research found that agricultural nitrogen fertilizer is the biggest contributor to fluctuations, highlighting the impact of human actions and land management decisions on Earth system processes.
"These findings are important because we have undervalued the role of agriculture in carbon cycle fluxes," said lead author Danica Lombardozzi, an assistant professor of ecosystem science and sustainability. "A lot of people recognize that agriculture can help mitigate climate change, but because it's not represented in most Earth system models, it's not considered in climate change projections the way it should be."
Annual carbon cycle fluctuations indicate how much the biosphere is growing every year, Lombardozzi explained. As plants grow in the spring, they draw carbon dioxide from the atmosphere. After crops are harvested and other plants go dormant in the fall, carbon dioxide in the atmosphere increases. Plants need nitrogen to absorb carbon dioxide and grow. Most crops around the world are fertilized with nitrogen, which is necessary for food production and causes more carbon to be drawn from the atmosphere to fuel greater growth.
The study, published in Nature Communications , found that agricultural nitrogen is responsible for 45% of the fluctuation increase in the annual carbon cycle. More carbon dioxide in the atmosphere and warmer temperatures contribute 40% and 18%, respectively.
The increasing fluctuation driven by crops doesn't necessarily impact carbon storage, Lombardozzi said, because crops are harvested every year and the carbon they absorb is returned to the atmosphere. However, agricultural management can be adapted to store more carbon in the soil long term, which would help to mitigate climate change.
"Agricultural management practices are very important to shaping the world we live in," said co-author Gretchen Keppel-Aleks, an atmospheric scientist at the University of Michigan. "At a time when many people feel like climate change has had profound and negative impacts on their lives through wildfire, flooding or droughts, we can use the fact that this study shows that agricultural management has a profound impact on carbon fluxes to think about how we can use agricultural management to our advantage."
Guided by emerging science, many farmers are adopting practices collectively known as regenerative agriculture to improve soil health and crop production while also mitigating climate change.
Earth system models need to include agriculture
Previous studies have shown that the seasonal difference between high and low concentrations of carbon dioxide in the atmosphere was increasing but overlooked agriculture's important role because past versions of Earth system models did not include agricultural processes. Lombardozzi and her co-authors used the Community Earth System Model, developed by the National Center for Atmospheric Research in collaboration with researchers from other institutions, to quantify the sources causing the fluctuation increase and discover that agricultural nitrogen is the most significant factor.
All Earth system models are complex because they encompass global atmospheric, land and ocean processes, including chemistry, biology and physics for the entire planet. The Community Earth System Model is unique because it also incorporates agricultural processes.
Lombardozzi said that Earth system models need to represent agriculture, in particular agricultural nitrogen, to accurately assess the carbon cycle, but most of them currently do not. "It is really hard to represent human decisions in an Earth system model, but we need to tackle it," she said.
