Global ChangeIntersection of Nature and Culture

High Arctic soil carbon underestimated

Saturday, March 13th, 2010

Most people have heard about the potential positive feedback of soil carbon on climate: As temperatures warm, soil microbes are more active and permafrost begins to thaw–both of which can hasten decomposition and the release of CO₂ to the atmosphere.  This, in turn, has the potential to accelerate warming.

A lot of us who study climate warming impacts in boreal and Arctic ecosystems are interested in this problem.  There are a few things we keep an eye on:

• How fast are temperatures warming in the Arctic?
• How fast is permafrost thawing?
• How much soil carbon is there?

All of these questions are active areas of research.  Increase any of them, and you have the possibility of strengthening the positive feedback.  The third one is particularly interesting.   The more we study and inventory soil carbon at high latitudes, the more we revise upwards the estimate of soil carbon.

Here’s an example:  The atmosphere contains about 750 gigatons of carbon.  When I was in grad school back in the early 90′s, we thought that boreal and arctic soils might have stored around 350 gigatons—about half the atmospheric content.   With the discovery of extremely carbon-rich yedoma soils in Siberia, we learned that this number might be a serious underestimate.   And as we learn more about soil carbon stored in deeper, harder-to-sample permafrost soils, we are coming to the realization that high-latitude soils may store between 1000-1700 gigatons—substantially more than the atmosphere (here’s one example).

Let’s say for illustration that the real number is 1500 gigatons.  This means that warming would only need to cause a loss of 1/2 of 1% of this soil carbon to release 7.5 gigatons—roughly the total amount of fossil fuel carbon released worldwide each year.   Thus, small changes in decomposition of a huge soil carbon pool can lead to carbon releases that rival anthropogenic emissions.

In a forthcoming issue of Global Biogeochemical Cycles,¹ Jennifer Howarth Burnham and Ronald Sletten further illustrate that the more we sample, the more this soil carbon number goes up.

Focusing on Greenland, they dug 55 soil pits and measured soil carbon.  Then, they extrapolated these estimates to the rest of the circum high Arctic by (1) linking soil carbon to certain vegetation types and (2) using satellite imagery estimates of the area of each vegetation type to estimate soil carbon for a much larger region.

Although the new number they produced is not large (12 gigatons), it is five times the previous estimate for High Arctic soils.   It’s important to note that much of the High Arctic is a polar desert with little plant growth that could contribute to soil carbon, so it’s not surprising that more of the soil carbon is farther south—in boreal and subarctic regions.

One important caveat is that they only sampled surface soils that thaw during summer and are easy to sample.  By omitting deeper permafrost soils, they probably underestimated the total.

And so we keep sampling…

¹Burnham, J. H., and R. S. Sletten (2010). Spatial Distribution of Soil Organic Carbon in Northwest Greenland and Underestimates of High Arctic Carbon Stores Global Biogeochemical Cycles : 10.1029/2009GB003660

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Photo credit:  One of my photos from the Canadian Arctic that you can view on my flickr site