Two regions, two opposite outcomes
The same intervention runs in opposite directions in two oceans. When 0.35 percent of the ocean area in the equatorial Pacific is fertilised, macrozooplankton biomass falls by at least 10 percent. When a smaller area in the Southern Ocean, 0.2 percent, is fertilised, that same biomass rises by 10 percent.[1]
On the carbon side the number is this: net removal reaches up to 5.3 parts per million, that is 0.70 gigatonnes of carbon dioxide a year. Jun Yu, Adam Martiny and colleagues at the University of California derive these values from a computer model drawing on more than six decades of fertilisation data, and conclude that the Southern Ocean offers the best balance between ecological disturbance and carbon removal.[1]
What makes the difference
The opposite behaviour of the two regions appears to depend on what the food web meeting the added iron does with it: in the Southern Ocean, iron scarcity is the binding constraint on production, so added iron pulls production upward and carries food to the levels above, while in the equatorial Pacific the direction of growth shifts the composition of the community zooplankton feed on against them.[1]
This is an inference, and other accounts stand beside it. The regional ranking could also arise less from how the oceans actually behave than from how each region is represented in the model's food-web parameters; if the plankton communities of the two regions entered the model at different levels of detail, the ranking carries the trace of that detail. The authors' own caution should be read in the same direction: ocean iron fertilisation has downstream effects, and any localised intervention inherently produces nonlocal consequences.[1]
The levers we actually hold
What this study hands over is two dials: where, and over how much area. In the Southern Ocean 0.2 percent makes plankton biomass rise by 10 percent, while in the equatorial Pacific 0.35 percent makes it fall by at least 10 percent; the choice of place alone flips the sign of the outcome. None of this has been tested at sea yet. What we hold is a modelling analysis, and the first job of any field trial would be to measure whether the model's prediction about zooplankton holds.[1]