Science1 publisher2 min readPublished
Carbon-removal simulations end with the Southern Ocean releasing three times what it absorbed in 2001
A Science Advances team ran two idealized mitigation scenarios. Southern Ocean uptake weakened and then reversed in both, and in the negative-emissions run the globally averaged flux turned from sink to source in 2154.
The Scientist · Science desk
What happened
- Plans to hold warming to 1.5C depend on reaching net-zero emissions and, in many cases, on actively removing CO2 from the air. A new Science Advances study simulates how the Southern Ocean responds to that.
- The world's oceans have absorbed roughly 30 percent of human-caused CO2 emissions, and the Southern Ocean alone accounts for about 40 to 50 percent of that absorption.
- The team ran two idealized long-term scenarios, one reaching net zero and one adding sustained direct air capture, compared 10 simulations of each, and checked modeled recent uptake against observation-based estimates.
- Once emissions are reduced in the runs, Southern Ocean uptake declines rapidly and then gradually turns into a net release of CO2.
- Both scenarios end with the Southern Ocean emitting carbon: about 8.6 grams per square meter per year in the net-zero run and 8.5 grams in the negative-emissions run.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- contradiction Models broadly agree that Southern Ocean uptake continues while emissions rise, and they part company on removal pathways, so how much sink a budget can assume depends on which scenario family produced the projection.
- decision Anyone treating direct air capture as insurance for the ocean sink gets no protection from it in these runs, because the negative-emissions ending lands within 0.1 grams a square meter of the plain net-zero ending.
- constraint A sink that becomes a source has to be paid for twice over. Removal on land would need to cover residual emissions plus the outgassing, and to keep doing so while atmospheric CO2 is already falling.
In 2001 the Southern Ocean absorbed about 2.8 grams of carbon per square meter per year [11]. The simulated flux at the end of these runs is a release roughly three times that size [2], a swing of 11.4 grams per square meter per year between the two states [1].
The trigger the authors put first is persistent warming of Southern Ocean surface waters, with the broad decline in seawater alkalinity second [13]. Warm water holds less CO2, and lower alkalinity weakens the water's chemical capacity to take it up [14]. Both raise the partial pressure of CO2 in the water, and once that pressure passes the partial pressure in the air above, the gradient reverses and the water gives CO2 back [15].
On timing, the authors wrote: "The temporal evolution of the globally averaged air-sea CO2 flux is characterized by enhanced oceanic CO2 uptake until the emission peak is reached (approximately year 2050), followed by weakened uptake as the emissions decline. In the NEG experiment, this weakening continues, and in the year 2154, the ocean transitions from a net sink to a net source of CO2." [12] That date is for the global ocean, not the Southern Ocean alone [12]. It falls 104 years after the emissions peak in the same run [4]. The direct air capture in that scenario keeps going for another 42 years, until 2196 [5].
Sustained negative emissions did not produce a different endpoint. The net-zero and negative-emissions runs finish 0.1 grams per square meter per year apart, about 1 percent of either figure [3]. The reversal also holds while atmospheric CO2 is falling, which the authors describe as a delayed response [8].
Ten simulations per scenario measures the spread inside one model. The phys.org account does not name the model or the area needed to convert these per-square-meter fluxes into tonnes a year. Krystal Kasal wrote in that account: "There are still many unknowns, and these simulations often lack details that might affect the outcome." [17] Climate models generally project continued Southern Ocean uptake while emissions keep rising. The open disagreement is about what happens on carbon-removal pathways [5].
In my view the question this opens is whether removal budgets assume a Southern Ocean sink that keeps working indefinitely. The authors' own claim is narrower. They wrote that the nonlinear physical response "could eventually generate delayed and multifaceted impacts on oceanic carbon dynamics, introducing substantial uncertainty into projections of future CO2 flux and the overall impact of climate mitigation" [6].
What to watch
- Whether other modeling groups reproduce the flip with different ocean models, since 10 runs per scenario measure one model's internal spread.
- Whether the reversal survives realistic removal pathways and this-century horizons, instead of the idealized ZEC and NEG runs used here.
- Whether the published paper reports Southern Ocean totals in tonnes per year alongside the per-square-meter fluxes.