Soil carbon feedback

• 1. Introduction

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  Soil carbon feedback begins with a striking imbalance: the world's soils hold roughly two to three times as much carbon as the entire atmosphere. Warming disturbs that reservoir: measurements show 4 °C of warming can raise annual soil respiration by up to 37%. That released carbon adds to the atmosphere, which then drives further warming, the definition of a positive climate feedback. An increased rate of soil respiration is the main driver behind this cycle. What actually triggers this rise in respiration, where does the released carbon concentrate, and why do scientists studying it still argue over its ultimate scale?

• 2. Impact On Climate Change

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  Since 1991, researchers have tracked this feedback through an observation-based study conducted at Harvard. That research projects a release of about 190 petagrams of carbon from the top 1 meter of Earth's soils by the year 2100. The projected amount matches roughly two decades of greenhouse gas emissions from burning fossil fuels. Researchers tie this projected release to shifting microbial communities responding to elevated temperatures.

  A 2018 study reached a blunt conclusion: "Climate-driven losses of soil carbon are currently occurring across many ecosystems, with a detectable and sustained trend emerging at the global scale." That detectable global trend says nothing yet about the far north, where an entirely different set of frozen soils waits.

• 3. Permafrost

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  Permafrost is frozen ground found across the Arctic and sub-Arctic regions, at higher latitudes than the soils described so far. Observational evidence tied to this thawing points to a steady, ongoing release of greenhouse gases rather than a single abrupt event. Researchers describe this pattern as linear and chronic, tracking alongside the pace of ongoing climate change. That steady, chronic pattern stands apart from a very different kind of release found elsewhere, one description of which calls it explosive.

• 4. Tipping Point

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  A 2011 study identified what its authors called a compost-bomb instability, a tipping point behind explosive soil carbon releases from peatlands. The same authors described a unique stable soil carbon equilibrium that exists for any single fixed atmospheric temperature. Researchers predict that peatland ecosystems will flip from a carbon sink into a carbon source sometime this century.

  Despite that prediction, peatland ecosystems are still left out of the major Earth system models and integrated assessment models used to project climate change. That same gap between prediction and modeling practice reaches further than peatlands alone.

• 5. Uncertainties

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  Climate models built to project soil carbon feedback leave out the biochemical heat released during microbial decomposition, warmth the standard equations do not capture. A separate limitation involves the animals living in soil rather than the microbes alone. Insects and worms interact with microbial communities in ways that current global decomposition models do not sufficiently incorporate. Accounting for how worms and insects reshape decomposition chemistry would mean rebuilding models around soil life that has, so far, gone largely uncounted.