Can Soils Solve Climate Change?
One other dubious claim of natural climate solutions makes the rounds
A number of months ago, I questioned a claim that planting trees could solve climate change. In keeping with some scientists, reforestation “is by far the most cost effective solution that has ever been proposed,” and for $300 billion it could sequester 200 gigatons of carbon (GtC, or 733 GtCO2). Many media outlets swooned, however the assertions were weak if not incorrect. Indeed, just this week, Science — the unique outlet — published 4 scathing critiques of the underlying scientific paper. (See the update at the underside of my original blog post for more details.)
Last week, a headline from Bloomberg boldly announced “Methods to Halt Global Warming for $300 Billion,” telling us that “UN scientists say reclaiming wasteland… would stall emissions growth for as much as 20 years.” Specifically, the concept is
to lock hundreds of thousands of tons of carbon back into an neglected and over-exploited resource: the soil…. Returning [900 million hectares of degraded land that could be restored] to pasture, food crops or trees would convert enough carbon into biomass to stabilize emissions of CO2, the largest greenhouse gas, for 15-20 years… “With political will and investment of about $300 billion, it’s doable.”
This struck me as too good to be true. So let’s break down these numbers and compare them with current leading evidence.
Assuming that “to stabilize emissions” means withdrawing enough carbon to compensate for all of our annual greenhouse gas emissions, “15-20 years” implies 200 to 275 GtC, for a median price of $1.10 to $1.50 per ton of carbon (tC, as “giga” = one billion) or $0.30 to $0.40 per GtCO2 (the common way that costs of reducing emissions are reported). This also implies 220 to 300 tC sequestered per hectare of restored land.
Yet soils have lost only 133 GtC in the midst of human history. By way of cost, a recent review of the scientific literature estimated that spending $20 per GtCO2 could allow the capture of 0.4 GtC (1.38 GtCO2) per yr and spending $100 per GtCO2 could allow the capture of 1 GtC (3.7 GtCO2) per yr. In other words, the quantity of carbon that the Bloomberg article claims could possibly be captured would cost roughly 50 to 300 times as much as reported and would take roughly 200 to 700 years.
Perhaps despite the headline, but implied within the quote, the article means sequestration in all biomass: soil, plants, animals, fungi, etc. Even then, the lack of all biomass in the midst of human history has been 379 GtC. On condition that the planet has 13 billion hectares of land, the Bloomberg article implies that greater than half of all carbon loss from biomass ever could possibly be sequestered by restoring 7% of the world’s land.
The article didn’t link to any paper or report, and I discovered no papers or reports on the web sites of the international organizations to which it referred. It appears to base its estimates of each the quantity of carbon (i.e., 15 to twenty years value) and the fee on a single quote from Rene Castro Salazar, an assistant director general on the UN Food and Agriculture Organization.*
If it sounds too good to be true, it probably is.
*Update (evening 28 October): One in every of the authors of the Bloomberg article confirms, “The $300B figure got here out of the UNCCD conference in Delhi, from interviews with delegates, including Castro-Salazar on the FAO.”