Soil Carbon Sequestration Goals of Paris Climate Agreement Are Unrealistic, Say Scientists

Soil Carbon Sequestration Goals of Paris Climate Agreement Are Unrealistic, Say Scientists

Newswise — The goal to offset rises in atmospheric greenhouse gas concentrations by increasing soil carbon storage by 4 per mille (0.4%) per year is unrealistic, say scientists from The Netherlands, The United Kingdom and the United States in an opinion piece in the journal Environmental Science and Technology.

The plans to store more carbon in the soil (“4P1000”) were launched at the 2015 United Nations Framework Convention on Climate Change (UNFCC) in Paris. The soils of the world contain approximately three times the amount of carbon in organic matter as currently held in carbon dioxide in the atmosphere. Increasing this soil organic matter stock at a rate of 4 per mille per year could, in theory, fully compensate the rise in atmospheric CO₂. Such an increase could come about by changes in land management, or by using different crop rotations.

“Building soil organic matter is a good idea, so the 4 per mille goal is on the right track,” said Bruce Hungate, Director of the Center for Ecosystem Science and Society at Northern Arizona University and co-author on the paper. “More soil carbon slows climate change and improves soil fertility.”

The nitrogen problem

Storing additional carbon in the soil, notes Hungate, requires other nutrients, like nitrogen. “The problem is, soils are not made of carbon alone. It would be nice if we could just beam up some nitrogen, but the nitrogen cycle doesn’t work that way,” said Hungate. “The amount of nitrogen required is equivalent to boosting N fertilizer application by 75%. Producing the extra fertilizer is not the answer, because fertilizer production causes even more greenhouse gas emissions. And, much of the N applied in fertilizer creates other problems, like polluting groundwater, lakes and rivers.”

Does that mean that we should abandon the 4P1000 goals? “Absolutely not,” said Jan Willem van Groenigen, Professor at Wageningen University and lead author of the paper. “Let’s not throw away the baby with the bathwater. The 4P1000 goals are a great inspiration to do everything we can as farmers, soil scientists, agronomists and policy makers to help fight global warming and at the same time improve our soils.”

Instead, the authors appeal to the scientific community to think about the role of nutrients in reaching the 4P1000 goals. “For instance, this could mean that additional soil carbon should be stored in areas where nutrients are also available, and focusing on minimizing emissions of other greenhouse gases such as nitrous oxide and methane,” van Groenigen explained.

More broadly, the authors urge caution about expecting a single solution to slowing climate change, according to Hungate. “Soil management can contribute to climate change mitigation, but to expect a sole solution in soil is unrealistic and misguided. We need to focus on reducing emissions across all sectors of the economy.”

The opinion piece was written with colleagues from the Wageningen University, University of California at Davis, Northern Arizona University (USA) and Rothamsted Research (UK) and can be read here).