Agricultural Soils and Climate Change

Agriculture’s role in the fight against global warming just got a bit trickier. A new study published in Nature raises a potentially devastating conundrum by suggesting that agricultural soils may emit higher concentrations of potent greenhouse gases in a future CO2-rich atmosphere.

For years, scientists have expounded upon the value of soils as potential ‘carbon sinks.’ This means that—with the right management practices—soils can actually store increased amounts of carbon dioxide, thereby preventing it from circulating in the atmosphere and increasing warming through the greenhouse effect. Study after study has evidenced the importance of soil as an important tool in the fight against global warming.

CalCAN Science Advisor and UC Davis Professor Johan Six has conducted extensive research on the complex ways in which soils interact with greenhouse gases and other atmospheric components. His work has played a crucial role in developing ecologically beneficial soil management practices and guiding our evolving knowledge of biogeochemical systems.

The Chicago Climate Exchange, which was North America’s only voluntary, legally binding carbon trading market until it closed last year, actually allowed farmers to bank on the idea of soil as a carbon sink. Specifically, farmers who implemented select management practices such as no-till and strip-till were given ‘carbon credits’ that could then be sold and traded to industrial polluters to ‘offset’ factories’ greenhouse gas emissions. Millions of tons of ‘carbon dioxide equivalent’ (CO2e) were traded on the market as farmers earned credits by changing their soil management practices.

The use of soils to sequester carbon could also feature prominently in California’s new cap-and-trade program, which is slated to start next year under the state’s Global Warming Solutions Act of 2006 (AB 32). The California system includes state-approved ‘offset protocols’ that specify which projects can produce tradable carbon credits for sale to capped entities (e.g. fossil-fuel-powered electricity plants). If the agricultural offset protocols currently being developed by the Climate Action Reserve are approved for use in the cap-and-trade system, it seems highly likely that soil sequestration would become an accepted practice for farmers to implement.

For the time being, this strategy seems to make sense. Farmers have an opportunity to practice smart soil conservation while reducing atmospheric carbon dioxide concentrations and receiving financial incentives.

But according to the study in Nature, this strategy could backfire in the future. By simulating scenarios in which the earth’s atmospheric CO2 concentrations creep higher and higher, the researchers found that various future atmospheres with CO2 concentrations above 463 parts per million might actually cause soil-sequestered carbon to be re-emitted as nitrous oxide (N20) and methane (CH4), two greenhouse gases with many times more warming potential than carbon dioxide.

In essence, soils would be sequestering CO2 only to spit out by-products that are significantly worse for the climate.

According to co2now.org, the current concentration of carbon dioxide in the atmosphere is at 392.39 parts per million and rising. If atmospheric carbon dioxide levels continue to rise due to fossil fuel emissions, the study suggests, we are in danger of seeing soils’ mitigation potential negated by at least 16.6 percent.

The study comes at a time when planning for California’s cap-and-trade system has kicked into high gear. If the results of this study are heeded, they will raise just another of many misgivings around the use of offsets in the state’s program. Environmental justice groups have already filed a lawsuit against the state, claiming that offsets would allow capped entities to continue emitting the vast majority of the greenhouse gases the cap-and-trade system is designed to prevent.

The study’s authors have expressed some surprise at the results of their analysis, but stand by their conclusions. Above all, this finding is a reminder that—as scores of researchers and policymakers are already well aware—agricultural systems are enormously complex beasts to successfully manage and understand. They call for patience, care, perspective—and perhaps a big heaping spoonful of humility.

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