Evaluate whether the '4 per 1000' soil organic carbon (SOC) initiative's annual sequestration target is achievable as a sustained, globally uniform rate, and assess whether voluntary regenerative-agriculture carbon-credit protocols are pricing credits against a decay curve the feasibility literature has already falsified.

Synthesized peer-reviewed regional feasibility assessments of the 4p1000 target (France, Bavaria, a 30-year tropical-systems projection, and a country-by-country pan-European estimate) against the original 2015 4p1000 framework.

Cross-referenced these against soil carbon saturation-dynamics literature quantifying the time horizon (roughly 10-100 years) over which SOC accumulation rates decay toward a new equilibrium, and against a climate-integrated projection of achievable sequestration under future warming and precipitation change.

Compared the resulting achievable-rate range to the flat, multi-year linear rate implicitly assumed by common regenerative-agriculture carbon-crediting logic.

I want to talk about a number that has been repeated so many times it has started to sound like a law of physics: 4 per 1000. 4% a year, compounding, and you offset the net annual increase in atmospheric CO2 from human activity. It's an elegant number. It fit on a slide at COP21 in Paris in 2015.

It is also, according to nearly every rigorous regional feasibility study done since, not a stable target -- it's a snapshot of year one of a curve that bends hard toward zero.

, feasibility case study, PMC8252610). Their finding: the 4p1000 rate is achievable in some systems for some period, but nowhere close to indefinitely, and the achievable rate depends enormously on starting carbon stock.

Soils that are already carbon-rich have far less room to gain than degraded soils -- which is the opposite of where a lot of "climate-smart agriculture" investment currently flows, because degraded soils are cheaper and more available to the farmers who need the intervention for food security, not carbon-market optics.

A separate regional assessment for Bavaria reached a harder verdict: not feasible at the modeled scale, full stop.

Then there's the tropics, where one synthesis found only 26% of agricultural land could hit the 4 per mille target over 30 years -- not because the soils can't sequester carbon, but because perennial cropping systems (coffee, cacao, oil palm, rubber) already occupy the soils with the lowest baseline SOC.

The land types that would show the biggest relative gains are precisely the ones locked into cropping systems the models don't credit.

The pan-European country-specific estimate (PMC9293132) is the most methodologically careful thing I've read on this, because it goes country by country instead of averaging Europe into one carbon-sequestering blob, and the achievable rates come in well under 4 per mille almost everywhere except a handful of degraded-soil regions in the Balkans and parts of Iberia.

The mechanism nobody puts on the slide is saturation. Soil isn't a bank account -- it's more like a bathtub with a variable-height overflow drain.

Carbon accumulates fast right after a management change (no-till, cover cropping, agroforestry), and then the rate decays as the soil approaches a new quasi-equilibrium, typically somewhere in a 10-to-100-year window depending on climate and texture.

The "dual saturation" literature (PMC13266082) is blunt about this: there's a biophysical saturation limit and a separate, less-discussed operational-capacity limit -- farmers can't sustain optimal management indefinitely, extension support lapses, land changes hands, contracts expire.

Layer in a PLOS Climate finding that future warming and altered precipitation actively erode achievable sequestration rates in many regions, and you get a target calibrated against a stationary climate baseline that no longer exists.

Here's where I get irritated rather than just academically interested: voluntary carbon-credit protocols built on "regenerative agriculture" routinely price credits as if a farm's early, steep sequestration curve continues linearly for the entire 10-to-20-year crediting period, because that's the assumption that makes the spreadsheet work and keeps credits cheap to originate.

My colleague Valentina Cruz, who does actual soil ecology fieldwork at UNAM rather than reading about it from a desk, has been telling me for two years that most MRV (measurement, reporting, verification) protocols in this space still don't budget for re-measurement past year five -- which is exactly when saturation starts eating the assumed gains.

I didn't fully believe her until I went and read the saturation modeling literature myself, which is professionally embarrassing to admit but at least honest.

None of this means soil carbon sequestration is fake or useless -- the France and pan-European studies confirm real, meaningful gains are achievable in specific soil-climate-management combinations.

It means the 4 per 1000 figure should never have been treated as a globally uniform annual target, and any carbon market still pricing credits off a flat 4 per mille assumption for a decade-plus crediting window is pricing off a curve the underlying soil science has already falsified. Fix the crediting math to the actual decay curve, or stop calling it measurement-based.