Soil carbon to decline from global warming negative feedback loop

By Patrick Francis

Farmers efforts to increase soil carbon to levels which can have a positive impact on reducing global warming may be in vain. Ironically global warming itself is having a negative effect on soil carbon concentration through what is called “land carbon (C) climate feedback” which could accelerate climate change.

It does not mean that on-farm management that restores soil carbon levels where it has been degraded by past farming practices should be ignored, it is important for a wide range of ecosystem functions and plant productivity. But big picture forecasts made in some farming methodology circles that soil carbon sequestration can be used to offset a large proportion of the world’s anthropogenic emissions, are doubtful.

The research revealing this feedback loop was published in a co-authored paper published in Nature by 50 scientists headed by T.W Crowther from the Netherlands Institute of Ecology in November 2016. The scientist concluded that “that global soil C stocks in the upper soil horizons will fall by 30 (± 30) to 203 (± 161) Pg C under 1 degree of warming, depending on the rate at which warming effects are realised.”

The conclusion was reached after assembling data from 49 field experiments located across North America, Europe and Asia. There is considerable uncertainty in the soil loss estimates but the scientists contend the “direction of the global soil carbon response is consistent across all scenarios”.

“This provides strong empirical support for the idea that rising temperatures will stimulate the net loss of soil C to the atmosphere, driving a positive land carbon-climate feedback that could accelerate climate change.”

Figure 1: Global warming due to anthropogenic greenhouse gas emissions will continue unless emissions can be significantly reduced.

In their research the scientists found that the effects of warming on soil C stocks were variable with positive, negative and neutral impacts.  The overriding influence being the size of the standing soil C stocks and the extent and duration of warming.  “Specifically, the impacts of warming were negligible in areas with small initial C stocks, but losses occurred beyond a threshold of 2 – 5 kg C m-2 and were considerable in soils with ≥ 7 kg C m-2”

The scientists explained the variation in extent of C loss based on initial C content.

“In ecosystems with low initial soil C stocks, minor losses that result from accelerated decomposition under warming may be offset by concurrent increases in plant growth and soil C stabilization. In contrast, in areas with larger standing soil C stocks, accelerated decomposition outpaces potential C accumulation from enhanced plant growth, driving considerable C losses into the atmosphere.”

The interesting repercussion of this research surrounds soil carbon farming advocates who contend that increasing C in farming soils has the potential to counter global warming. This research points out that the high-latitude regions have the largest standing soil C stock but are experiencing the fastest rates of warming.

Figure 2: Warming is increasing fastest in high latitude regions.

“These high-latitude C losses drastically outweigh any minor changes expected in mid and lower latitude regions (for example by carbon farming), providing additional support for the idea of Artic amplification of climate change feedbacks.”

They say that even increasing vegetation in the high latitudes will not off-set the warming induced soil C losses.

Given that rising temperature stimulates the loss of soil C into the atmosphere, driving a positive land C-climate feedback accelerating global warming, the authors contend the strategy needed to avoid the most damaging impacts of negative feedback is reducing greenhouse gas emissions.

Find out more: Crowther, T., Todd-Brown, K., Rowe, C. et al. Quantifying global soil carbon losses in response to warming. Nature 540, 104–108 (2016). https://doi.org/10.1038/nature20150

Box story:

Climate change can destabilize the global soil carbon reservoir

By Woods Hole Oceanographic Institution, March 2021

The vast reservoir of carbon that is stored in soils probably is more sensitive to destabilization from climate change than has previously been assumed, according to a new study by researchers at Woods Hole Oceanographic Institution (WHOI) and other institutions.

The study found that the biospheric carbon turnover within river basins is vulnerable to future temperature and precipitation perturbations from a changing climate.

Although many earlier, and fairly localized, studies have hinted at soil organic carbon sensitivity to climate change, the new research sampled 36 rivers from around the globe and provides evidence of sensitivity at a global scale.

“The study results indicate that at the large ecosystem scale of river basins, soil carbon is sensitive to climate variability,” said WHOI researcher Timothy Eglinton, co-lead author of the paper in the Proceedings of the National Academy of Sciences of the United States of America. “This means that changing climate, and particularly increasing temperature and an invigorated hydrological cycle, may have a positive feedback in terms of returning carbon to the atmosphere from previously stabilized pools of carbon in soils.”

The public is generally aware that climate change can potentially destabilize and release permafrost carbon into the atmosphere and exacerbate global warming. But the study shows that this is true for the entire soil carbon reservoir, said WHOI researcher Valier Galy, the other co-lead author of the study.

The soil carbon reservoir is a key component in keeping the atmosphere in check in terms of how much carbon dioxide is in the air. The amount of carbon stored in terrestrial vegetation and soils is three times more than how much the atmosphere holds, and it consumes more than a third of the anthropogenic carbon that is emitted to the atmosphere.

Find out more:

Timothy I. et al Climate control on terrestrial biospheric carbon turnover. Proceedings of the National Academy of Sciences, 2021; 118 (8): e2011585118 DOI: 10.1073/pnas.2011585118