49. Expanding the case for irrigation
This installment was originally published on June 26, 2022.
The basic premise of irrigation as a strategy for removing carbon dioxide from the atmosphere is correct, at least directionally—you can measure the carbon retained in the soil. Irrigation creates economic value, creating a scenario where carbon capture can be profitable without an artificial “price on carbon,” at least in theory. The main question is reduced to accounting: How much carbon, and at what cost?
The original installment was inspired by my friend and former colleague Howard Branz, an expert in solid-state physics who challenged me over dinner about the permanence of biologically captured carbon. His main objection was that photosynthetically captured carbon is used for energy, which, if absolute, would negate the benefit. While he’s correct that, between respiration and combustion, most of the carbon returns to the atmosphere, not all of it does, and that’s the key.
This ends one chapter and opens another. Next week, I unearth a serious case of scientific malpractice associated with the effects of land use, land use change, and forestry (LULUCF in IPCC-speak). Some of my colleagues have chosen to go well beyond the data, creating models that reach faulty conclusions to match their beliefs, conflating the science of ecology and the religion of environmentalism.
What I found most incredible is that the scientific community knew these conclusions were questionable forty years ago! It is the most shocking thing I’ve learned since starting this series!
If you think about it, as a general rule, carbon captured photosynthetically is also biodegradable. In other words, plants capture carbon for their use, but some of it is also appropriated by other biological systems for energy. This process releases carbon dioxide back into the air. If capture and release are equal, then the effects cancel out—there’s only a benefit to the atmosphere if more carbon is captured than is released.
Before humans began to burn geologic carbon for energy, the capture & release of carbon dioxide must have been balanced for thousands of years. If not, the atmosphere would change until the global ecosystem achieved a balance.
Apropos of the last installment that covered irrigation, we need to measure how much carbon is retained when previously unirrigated land becomes irrigated. That’s not precisely bleeding-edge research. It’s mundane work that depends on many variables like the nature of the soil, the specific crop being grown, etc. There are also different places where the carbon can end up—the principal value is the creation of soil with large amounts of “soil organic carbon” (or SOC). This isn’t the only place (other than being released) where carbon can end up: Some root-associated microbes create water-soluble acids that help extract minerals from rock and form soil. The carbon in these acids is easily washed away rather than retained.
Whatever we come up with, it’s not going to be a hard-and-fast number to be treated as Gospel. The best we hope for is to get an idea of whether the math supports the conclusion. Let’s look at some data:
I chose this obscure reference because it reported the primary data that I needed. As in the earlier installment, the authors looked at the Nile Delta, a typically arid region that is only useful for agriculture when irrigated. They looked at three crop types (clover, sugar beet, and rice) and reported the SOC content (as a carbon sequestration rate). They reported measurements from soils of different ages, starting when cultivation began, versus a baseline of uncultivated land.
It’s evident that irrigation of arid lands captures more carbon than is released, but quantifying the amount can be challenging. What’s remarkable (and also apparent, if you think about it) is that when soil is first irrigated, it tends to retain more carbon than it does as it ages. As the ecosystem becomes more stable and more topsoil forms, the effectiveness of irrigation for carbon capture decreases. But, even after 50 years, the net effect remains.
But does this primary data support the desalination-for-irrigation thread? Earlier, I assumed (without proof) that half of the carbon retained by a sugarcane crop would be eligible for a carbon credit in Europe because the soil would retain it. This calculation worked out to 5 tons of carbon dioxide captured per acre, worth about $450 annually at current prices. Now we can see if that assumption is close to the truth: What is observed is, over the first five years of cultivation, about 10 tons of CO2 per hectare (4 tons per acre) is retained as soil organic carbon every year. Biochemically, the crops considered in the above chart are significantly less productive than sugarcane, so that could account for the difference, but it’s really unknowable.
Bottom line: I’m pleased with the guesswork!