Article by Adrian Reid

7 November, 2019

Tiny organisms floating in the oceans, known as phytoplankton, could be a way to bring down CO2 levels. Scientists have found that scattering a small amount of iron on the ocean surface helps the phytoplankton to thrive and take up more CO2. The effect is so dramatic that scientists believe it could have a major impact on CO2 levels and global temperatures.

Article by Adrian Reid

7 November, 2019

The fresh sea air

Nothing beats a walk on the beach and all that fresh air.

A good portion of the air could have been created not far away, in the nearby sea, by microscopic sea creatures known as phytoplankton. Scientists estimate that phytoplankton produce close to 50-80% of the oxygen we breathe.

Just like plants, phytoplankton take in CO2 and produce oxygen. The phytoplankton then dies and sinks to the sea floor where some of it stays. This cycle removes 10 gigatonnes of carbon from the atmosphere each year and is known as "the carbon pump".

That's a lot of CO2 taken out of the atmosphere and buried deep in the sea. Could this natural process be pushed to remove even more CO2 and become part of the fight against climate change?

The Iron Hypothesis

One of the first scientists to suggest this idea was Dr John Martin, late director of Moss Landing Marine Laboratories.

Dr Martin noticed that levels of phytoplankton were quite low in the oceans around the Arctic, Antarctic and the equator, even though there were plenty of nutrients for phytoplankton to feed on. The missing ingredient, Dr Martin suspected, was iron.

His curiosity provoked, Dr Martin collected samples of the phytoplankton and fed it extra amounts of iron. When the results came back, it confirmed what became known as "The Iron Hypothesis" — that iron is fertiliser for phytoplankton. Since phytoplankton breathe in CO2, this could be a way to remove CO2 from the atmosphere and cool down the planet.

Dr Martin was so confident of his results that he once said, "Give me half a tanker of iron, and I'll give you an ice age". It's a quote he became famous for, at least among climate scientists.

Phytoplankton — takes in CO2, creates oxygen, stores carbon, and sometimes puts on a show. Andrew Wallace, CC BY-NC-ND 2.0

An iron-fuelled ice age

There may have been an inkling of truth in Dr Martin's quote on creating an ice age. In a study of ice ages over the last four million years, scientists describe a "tight connection" between ice ages and increases in iron dust blown across the oceans.

Through volcanic eruptions or just strong wind, iron is carried into the air as dust. Some of the iron dust lands on the ocean where it fertilises phytoplankton on the ocean surface.

This extra iron dust could explain a reduction in CO2 of up to 40 ppm, or about 300 gigatonnes.

Could this be a way to solve climate change? If not with an actual ice age, then at least with some cooler temperatures?

So far, scientists are undecided.

Dust plumes carrying iron from the Sahara Desert.NASA Earth Observatory, CC BY 2.0

The world's largest migration

Every night, across the world's oceans, a silent migration occurs. Deep-sea dwelling fish swim upwards towards the ocean surface to feed on phytoplankton, before dropping back to the ocean's depths to rest during the day. As they do this, they bring carbon down with them, in the form of the plankton and small fish that they eat, or poop.

This is one of the main ways carbon is stored long-term deep in the ocean, and away from the atmosphere.

Could iron fertilisation enhance this process?

One scientific expedition to the Antarctic's Southern Ocean found that for every part of iron they added to the ocean, at least 13,000 parts carbon were absorbed in the ocean, with half of that eventually sinking into the deeper ocean.

UN shutdown

But scientists, governments and even the U.N. are worried about the risks of experimenting with phytoplankton. So much so that in 2008, the U.N. shut down large-scale iron fertilisation projects. The risk of experimenting with an essential part of the food chain was deemed too high.

The devil is in the detail with phytoplankton. They do much more than take in CO2 and emit oxygen. Some produce sulphur. So much that, an increase in phytoplankton could increase clouds and paradoxically reduce rainfall around the world.

Other phytoplankton produce neurotoxins, or use up oxygen to create dead zones, or even produce more potent greenhouse gases such as methane or nitrous oxide.

Also, with oceans so interconnected, if growing phytoplankton ate up nutrients, e.g., in the Southern Ocean, this might not leave enough nutrients for another part of the world, such as around the equator. The net effect of an increase in iron could end up being zero.

Falling levels of phytoplankton

But time may be running out. Since 1950, the amount of phytoplankton in the oceans has dropped by 40%. It's almost halved.

As the world's waters warm, phytoplankton find it harder to survive and produce oxygen, until they reach a cut-off point. University of Leicester mathematicians found this point is reached once oceans become on average 6 °C (10.8 °F) warmer.

Without the oxygen created by phytoplankton, study author Professor Sergeii Petrovskii said, "This would likely result in the mass mortality of animals and humans.

As well as this, without phytoplankton to remove CO2 out the atmosphere and send it to the bottom of the ocean, scientists estimate that CO2 levels could rise by 200 ppm. That's more than the CO2 we've added to the atmosphere already. In terms of gigatonnes of CO2 (multiply by 7.82), it's about 1500 gigatonnes.

At the moment, it makes sense to hold off iron-seeding phytoplankton until we understand more about them.

Time is running out though. We need phytoplankton to breathe.