Southern Ocean feedbacks put a floor under atmospheric CO2 in a carbon-removal model

The two feedbacks are what separate this from a restatement of gas solubility. Gruber's piston image covers the simple part: press it into the ocean and the water takes up CO2, pull it back up and the CO2 comes out again ^[6]. The first amplifier is heat. The Southern Ocean has been carrying heat as well as carbon into the deep, and warm water holds less dissolved gas, so as that heat works back to the surface the region loses more CO2 than the concentration gradient alone would explain ^[17]. The second runs through alkalinity. Plankton build shells of calcium carbonate, die and sink, carrying alkalinity downward ^[18]. Warmer surface water is less dense and mixes less with the cold, alkaline water beneath it, and a surface layer starved of alkalinity absorbs less CO2 ^[18]. Both mechanisms are driven by heat already in the water, which is why the timescale here is centuries rather than decades.

The flip itself is a swing of 11 to 12 grams of carbon per square metre per year, from about 3 absorbed to 8 or 9 emitted ^[1]. That is an area-normalised figure, so it does not convert to gigatonnes without an area to multiply by. The removal run is where the concentrations bite: from the model's 700 ppm starting point, drawdown covers 275 ppm and then stalls at 425 ^[2]. The gap between the two runs is 165 ppm ^[3], which is what centuries of aggressive removal buys over doing nothing extra, and the endpoint is roughly where the atmosphere sits today ^[11].

Those ppm values are properties of the scenario, not projections. Net zero in 2125 is 75 years later than the 2050 date the UK and EU members are working towards ^[4], and the 2400 flux figure sits 275 years beyond that modelled net zero ^[5]. Removal today is minuscule next to emissions that are still rising ^[13], so a run that assumes drastic removal has already granted itself the harder half of the problem.

The thing this doesn't tell you is what the same physics produces from a lower peak. Shin says the sooner net zero arrives and the heating of the Southern Ocean stops, the less CO2 these waters ultimately emit ^[19], which follows from both feedbacks but is not a number the published scenarios supply. Nor does a single model settle the alkalinity leg, which depends entirely on how strongly the surface layer stratifies. What survives those caveats is the direction of the lever: the size of the eventual outgassing is set by how much heat goes into the Southern Ocean before net zero, not by the removal capacity anyone hopes to build afterwards.