When it comes to discussing climate change, knowing the role of carbon is important. It pops up everywhere: carbon dioxide, carbon sequestration, carbon emissions, and carbon markets. A big driver of the shifting climate is the change in carbon’s location. More and more of it is being stored in the atmosphere, instead of in Earth’s crust and vegetation.
Many solutions for righting this imbalance suggest reducing emissions and recapturing the carbon we pumped into the air. I recently read a blog by the Institute for Agriculture and Trade Policy (IATP) about one such solution: a soil carbon market. The author described the debate about such markets among international climate change policy makers at their October meeting in Panama. That meeting was a primer for the UN’s climate change conference in Durban, South Africa, which began on Monday and finishes December 9th.
It seems that there was contentious discussion around soil carbon markets in Panama. Some people argued that targeting soil carbon is a “mirage” or diversion from action that will really make a difference. Whether a market mediating soil carbon storage and release is fast or effective is beyond my scope of expertise. But I did take issue with a couple of the author’s statements about the state of soil carbon itself. I work in a biogeochemistry lab where soil organic carbon is a major research subject and the science of carbon is our specialty.
There were two statements in IATP’s blog with which I disagreed. I will list each and explain why, as a soil scientist, I find them empirically false.
Statement 1: “Soil also just can’t hold a lot of carbon. Most scientific studies say that it can range from zero tons in dry and warm regions to one ton in humid and cool zones, making it an even less enticing option for smallholder farmers.”
In fact, soil can hold a huge amount of carbon, and does. Soil contains double the amount of carbon stored in the atmosphere, and stores more than vegetation and the atmosphere combined (Houghton 2007, Jobbagy and Jackson 2000, Trumbore 2009). The amount of carbon in soil varies widely across different types of soils and different places but on the whole, there is a lot of carbon in soil.
To put some numbers on it, I’ll use the estimates from Balancing the Global Carbon Budget, an article by R. A. Houghton published in the Annual Review of Earth and Planetary Sciences in 2007. Houghton uses petagrams of carbon (PgC), which may not be a familiar unit but the numbers are still easy to compare (For the mathy types: 1 PgC = 10^15 g = 10^9 metric tons). Below is a table with Houghton’s estimates of the total carbon stored in each category:
|Fossil fuel||7500 PgC|
While soil holds less than the oceans, it is still a major player in the carbon game. Additionally, it is difficult to evaluate the blog’s statement that soil carbon ranges from zero to one tons without a unit of scale attached. But it is safe to say that, with one petagram equaling one billion metric tons, the unit the author was referring to must have been quite small.
Statement 2: “There are legitimate concerns about how to measure the carbon sequestered in the soil and how to be sure the carbon would stay in the soil, both of which make it unlikely that the carbon markets would embrace it anytime soon.”
It may be challenging to efficiently measure soil carbon with a standardized and cost effective method for a market with international scope. However, I am not sure if this statement is referring to that challenge or actually doubting the validity of current measurement techniques. To this point, I will refer to a wonderfully relevant article titled “Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation.” The lead author is Richard Conant of Colorado State University and it was published in Frontiers in Ecology and the Environment this year. Conant et al. agree that quick, cost-effective soil carbon measurements might be a challenge for such things as soil carbon markets. But they point out that “existing methods for quantifying soil organic carbon (SOC) concentration in samples are well established and have a high analytical precision.”
There are a lot of interesting intersections between soil science and policy decisions, climate change and otherwise. Policy developers and scientists have different goals and foci in some cases, but the more communication between the two groups, the better!
Referenced in this post:
Contant, Richard, Stephen Ogle, Eldor Paul and Keith Paustian. 2011. Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation. Frontiers of Ecology and the Environment. 9(3):169-173.
Houghton, R. A. 2007. Balancing the global carbon budget. Annual Review of Earth and Planetary Sciences. 35:313-347.
Jobbagy, Esteban and Robert Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecological Applications. 10(2):423-436.
Trumbore, Susan. 2009. Radiocarbon and soil carbon dynamics. Annual Review of Earth and Planetary Sciences. 37:47-66.