Adding iron to oceans could increase carbon storage

Could dropping iron particles from aircraft into the oceans be a new weapon in the fight against climate change?

David Emerson from the Bigelow Laboratory for Ocean Sciences in the US likes to think so. Writing in the journal Frontiers, he suggests it as a novel but potentially effective way to “fertilise” microscopic ocean plants and ultimately lower atmospheric carbon levels.

The idea is to take advantage of minerals synthesised by iron-oxidising bacteria that feed on the tiny spark of energy they generate by transferring electrons between iron and oxygen. This process produces rust minerals as byproducts, which are of the right chemical composition to be used by the phytoplankton that help remove carbon dioxide from the atmosphere.

It’s not a completely new concept; using iron fertilisation as a climate change mitigation tool was first proposed in the 1990s.

It’s needed, Emerson says, because it’s estimated that around 30% of the world’s oceans are low in iron.

And it’s not without some basis for hope. Evidence in the geologic record indicates that the amount of iron captured by the oceans may have helped moderate global climate in the past and played an important role in controlling earlier ice ages.

When events such as volcanic eruptions add large amounts of iron to the atmosphere, they may have the effect of fertilising the ocean – increasing phytoplankton activity and ultimately carbon drawdown.

So, says Emerson, it’s time for a controlled research program as the next step in exploring the idea’s efficacy.

“At minimum, we would gain a better sense of how the ocean works,” he says. “At best, iron additions would act on a short time scale to help mitigate climate change.”

Phytoplankton grow using carbon dissolved in the upper ocean. When they die, some sink, taking carbon to the deep ocean, and more atmospheric carbon diffuses into the upper ocean.

Stimulating phytoplankton growth with iron fertilisation could ramp up this process, Emerson suggests, ultimately shuttling more of the excess atmospheric carbon into the deep ocean.

Most iron enters the ocean as dust from the world’s deserts. Using aircraft to distribute a fine iron powder over deficient ocean regions would approximate natural iron inputs, and timing flights with seasonal phytoplankton “blooms” would stimulate growth and boost populations.

Iron-oxidising bacteria live in environments as extreme as the deep ocean and as common as roadside ditches, so cultivating them in shallow ponds should be pretty simple.

“In addition to cutting carbon emissions, we need to remove more carbon from the atmosphere to limit global climate change,” Emerson says.

“These geoengineering approaches are not solutions to the whole problem, but they are potential ways we can mitigate the worst effects.”