How much carbon can kelp sequester?
Because kelp does not need to support its own weight many species have become able to achieve extremely high growth rates. Some kelp are able to draw down carbon at about 20 times the rate of typical plants that exist on land. Pine trees are one of the fastest land-based carbon sequestering plants storing in New Zealand an average of about 25 tonnes CO2 per hectare per year over their 50 year growing life. The giant kelp Macrocystis pyrifera typically draws down over 50 tonnes CO2 per hectare per year.

Why is kelp so good at carbon sequestration?
Kelp and trees sequester carbon in different ways. Trees are generally limited to storing carbon in a living form. Once a leaf or branch falls to the ground it is quickly decomposed by living organisms that quickly respire the carbon back into the atmosphere as CO2. Cutting forests down and storing the carbon as timber can extend this period of sequestration, however, most softwoods (like pine) end up as paper, packing boxes or framing and their carbon ends up back in the atmosphere within a few years. Furthermore, the process of planting, managing, harvesting and processing a pine forest emits about 30% of all the carbon stored in that forest.

Kelp holds relatively little carbon in a living form but sheds dead organic matter profusely. Some (perhaps around 12%) of this dead organic matter carbon is dispersed into deep-water where it becomes effectively locked away from the atmosphere for hundreds to thousands of years. Each year the kelp continues to contribute to this deep-water carbon accumulation. Consequently, the carbon sequestration from a hectare of kelp is not limited to 50 years, or whatever the growth cycle might be of a terrestrial tree.

How much kelp is there in the ocean now? How much more would be necessary or beneficial for carbon sequestration?
It is estimated that kelp covers about 3.5 million km² containing approximately 5.6 billion tonnes of CO2 equivalent (CO2e). This compares with the world's annual emissions of around 50 billion tonnes of CO2e. The Australian climate scientist Tim Flannery has estimated that we would need to cover 9% of the Earth's surface with kelp to offset our current emissions.

What happens to the carbon that is captured by kelp and stored 1 km deep in the ocean? What impact does it have on the ocean?
Kelp organic debris that disburses into deep water is subject to such cold, low oxygen, dark and high pressure conditions that it is not readily converted to CO2. Furthermore, even if it does break down to dissolved CO2 very little of this water mixes with surface water and so it is generally excluded from re-entering the atmosphere.

Kelp organic matter has been contributing to deep ocean carbon sequestration for many millions of years. We don't know exactly what the impact would be if we were to dramatically increase the amount of organic matter export to the deep ocean (e.g. from large-scale kelp farms), however, kelp organic matter is a very benign substance and there are relatively few lifeforms living in the very deep ocean. Most of these organisms rely upon organic matter drifting down for their nutrition, so it is possible that the increased food source may in turn increase the quantity of living organisms in the deep ocean.

Which geographical areas are suitable for growing kelp? Which not?
Kelp grows throughout the world's coastlines, however, it grows best where it has a good rocky seabed (or kelp aquaculture ropes) to attach to, and where the water is relatively fertile (from terrestrial run-off or ocean upwellings). The very fast growing Macrocystis pyrifera requires cold water and grows in the higher latitudes.