4.2 Ocean fertilisation
Trees and grasses are, of course, not the only type of plant life. Another important example are the green blooms of algae, a type of phytoplankton (plant plankton).
Figure 11 shows an image of phytoplankton blooms. These plants take their CO2 from the water around them, not directly from the atmosphere. The nearly invisible ‘forests’ of phytoplankton in the world’s oceans remove (we say “fix”) almost as much carbon as all land plants, and that has a profound indirect effect on atmospheric CO2.
Phytoplankton are consumed by zooplankton (their microscopic animal counterparts) and other animals, or die in a matter of days and are colonised by bacteria that decompose them. All these marine organisms respire, so most of the dissolved CO2 taken up by phytoplankton photosynthesis is returned to the surface ocean and may therefore be outgassed to the atmosphere.
But about a quarter of the carbon escapes this recycling system: particles that are large enough to overcome the buoyancy of sea water sink, taking carbon down into the deep ocean.
In many parts of the world, such as the Southern Ocean, the growth of phytoplankton is controlled by the availability of dissolved nutrients such as iron, nitrogen and phosphorus.
Ocean fertilisation proposes adding these nutrients to the water to enhance plankton growth, with the aim of increasing the transfer of atmospheric carbon into the deep ocean water. In other words, biological capture at sea, or a ‘carbon sink’.
As the planet’s ocean circulation patterns are so vast, it can take hundreds or thousands of years for deep water to re-emerge at the surface: easily enough for this to be considered long-term sequestration. Lenton and Vaughan (2009) estimate iron fertilisation could offset around 8% of a doubling of CO2 by 2100.
Enhancing plankton growth could also provide extra food for krill and, in turn, whales.