Soil Carbon Can't Fix Climate Change By Itself—But It Needs to Be Part of the Solution

September 26, 2016
Soil cross-sectionPhoto: USDA-NRCS
Marcia DeLonge
Research Director and Sr. Scientist, Food and Environment

A rigorous study just published in the prestigious journal Science argues that soil alone cannot be can be counted on to save us from climate change. Yet the stark analysis does not undermine the importance of better understanding, protecting, and building carbon in soils (“carbon farming”). In fact, the findings reinforce the need for soil carbon science and action to remain priorities, especially when it comes to agriculture.

The study in a nutshell:  Scientists from the University of California used 1-meter (3.28 ft) deep soil samples from 157 places around the world, which were analyzed with sophisticated carbon dating methods to improve the way that soil carbon is represented in some of the best Earth System Models. They found that models may have been overestimating how much carbon would likely be stored in soils under climate change, particularly in response to the so-called “CO2 fertilization effect” (the effect of higher atmospheric CO2 concentrations on fostering plant growth). The findings suggested that the size of the resulting soil carbon “sink” that will be available soon enough to effectively mitigate climate change is lower than previously estimated (by anywhere from 5.9% to 87%). They conclude that models need to represent soil carbon more accurately when simulating climate change scenarios, and emphasized the importance of emissions reduction strategies.

So what does this mean?

If you’re wondering about the implications of the study for soil carbon, climate change, and agriculture, here’s what you need to know:

  • As my colleagues have written, we are well beyond the stage where we can choose between either reducing emissions or increasing carbon sequestration. We need to act on both, and quickly.
  • To review the basics, there is a lot of carbon in the atmosphere (as CO2, the main climate change culprit today), but there is far more in soils. If you have been following “carbon farming”, you know that this feature of the carbon cycle is the basis for much enthusiasm. Since plants suck up CO2 and return carbon to the soil, there is constant movement between these two “pools.” Because the soil pool is large, small changes in soil carbon can mean relatively big changes in atmospheric carbon. This is great if soil carbon is increasing, but worrisome otherwise. Either way, understanding soil carbon is one of the keys to the climate change solution.
  • The amount of carbon in soils is altered by both nature and humans. Although all carbon atoms are created equal, the life of any given one depends a lot on circumstances.
    • In the new study, the scientists used one of the “business as usual” climate modeling scenarios to try to better understand how much carbon soils would likely soak up in that future. As they describe, one natural result of increasing atmospheric CO2 is that it stimulates plant growth, which boosts soil carbon. But another (competing) response is that warmer temperatures speed up decomposition (much like fruits and veggies go bad faster when not in the fridge). The balance of these processes needs to be well represented in models for accurate predictions, but today’s models still contain a lot of uncertainty. This is exactly what that the new study was addressing when the researchers found that models, as currently configured, seem to have been overestimating the size of the future carbon sink.
    • One thing that’s important to remember, however, is that soil carbon is also affected by humans through managementThis aspect of the carbon cycle was not a focus of this study, and the actual potential for management to increase soil carbon is not likely reflected by the historical soil samples and the large-scale models that were used. Although the full potential to increase soil carbon uptake through soil management is not yet known, this is an active area of research.
  • Critically, for farmers and eaters, soil carbon simply matters. Carbon-rich soils tend to be healthy soils, which also tend to be more resilient to droughts, floods, and other extreme events (which are expected to become more frequent and severe with climate change). So, whether or not substantial mitigation through soil carbon is on the table, carbon-rich soils need to be part of a climate change adaptation strategy.
  • Speaking of which, farmers incorporating practices that improve soil carbon and soil health often reduce emissions as well. This is because building carbon in soils is frequently achieved through healthy farming practices and insights from agroecology, which can reduce inputs such as fossil fuels and chemicals (like fertilizers and pesticides).
Food and Environment , inorganic

The science and practice of “carbon farming” is an active area of research. (photo credit: USDA-NRCS South Dakota)

We need more science, practice, and policy to help build up healthy soils

Studies like the one just published are critical to a realistic, science-based understanding of the carbon cycle and the potential role of soils, and to identifying the most promising strategies to battle climate change.

While the new research suggests that soil might not soak up quite as much carbon as large-scale modeling efforts previously projected, the findings do not indicate that soil carbon science should be given lower priority. Quite to the contrary, this work reiterates the importance of aggressive strategies that both reduce fossil fuel emissions and foster soil carbon sequestration for a slew of reasons—food security being just one.

Fortunately, many new initiatives, like the USDA’s Climate Building Blocks and a new White House Call to Action, recognize the importance of keeping soil carbon in the conversation.