Plants on land and algae in the sea obtain energy using photosynthesis. In the process, they absorb CO2 from the air. Oceanic phytoplankton alone account for about half of global photosynthesis and thus absorb large quantities of CO2 from the atmosphere. The stored CO2 is then transferred via the food chain to other marine organisms such as small crustaceans, fish and whales. These organisms sink when they die. The majority of their biomass is decomposed by bacteria, whereupon the CO2 stored in the organisms is released back into the surrounding water and eventually comes into contact with the atmosphere again as a result of ocean circulation. A small portion of their biomass sinks to the ocean depths where it is sequestered in sediment on the sea floor. Part of the CO2 originally captured in plankton is consequently removed from the atmosphere for a long time.
Since a quarter of the world’s oceans are naturally deficient in plant nutrients and in particular iron, experts began work some years ago on developing the idea of artificial iron fertilisation. Fertilising with relatively small quantities of iron would stimulate plankton growth significantly, thus increasing the absorption of CO2 from the atmosphere. This would ultimately lead to more CO2 being transported in dead biomass to the ocean depths. Experiments in the laboratory and at sea have shown that phytoplankton indeed do grow vigorously after the introduction of iron powder.
Potential and scale
The experiments show that iron fertilisation increases plankton growth and CO2 uptake. However, only a small proportion of plankton actually sink to the depths and remove CO2 from the atmosphere for the long term. According to various scientific studies, were iron fertilisation to be developed on a large scale in the years ahead, between 200 million and two billion tonnes of CO2 a year could be removed from the atmosphere worldwide from 2050. To have a global effect, however, at least the entire Southern Ocean would have to be constantly fertilised with iron.
Application readiness and research needs
After several field experiments, many scientists are now abandoning the idea of iron fertilisation as a CDR method, partly because of the unpredictable side effects on marine ecological communities. It could thus have effects in the oceans that are familiar from coastal areas over-fertilised with nutrients today. The excess nutrients could cause large plankton blooms whose oxygen-depleting bacterial degradation leaves areas that are low in oxygen or even anoxic. A further side effect could be the increased formation of nitrous oxide, a potent greenhouse gas that can rise from the sea into the atmosphere. This would partly reverse the effect of the CDR method. A further mechanism would also be counterproductive: Modelling shows that once fertilisation ceases, it is highly likely that much of the CO2 absorbed by the oceans would return to the atmosphere over timescales of decades to centuries. The atmospheric CO2 concentration could consequently rise again after the measure comes to an end.