Why iron can't solve the climate crisis

CTD. (Fiona Preston-Whyte)

CTD. (Fiona Preston-Whyte)

A continuous current of raging seas, storms and icebergs, albatrosses and killer whales circle Antarctica. Looking into the still mirrored water, protected between ice floes, a stark contrast to the open ocean, you look into a delicate world, the sun penetrating depths we are only beginning to understand. This is one of the last wildernesses on Earth and it holds the keys to understanding our future. This ocean, known as the Southern Ocean, contains the secrets of long-term climate change.

A changing climate has brought the Southern Ocean into sharp focus, not only due to the physical changes we are observing in the ice levels and sea surface temperatures around Antarctica, and their effect on currents, but on the biology and life within the oceans.  

When you sail across the oceans, you notice their depths change from greens to deep blues. This changing mirage is a reflection of the life beneath the surface, determined in part by the “drifting plants” or phytoplankton. These microscopic organisms are the foundation of the ocean ecosystem. They also form the basis of the biological pump, one of two processes responsible for the absorption and storage of carbon by the oceans. 

The biological pump is driven by the growth, death and subsequent sinking of living matter such as phytoplankton to the ocean floor. The second process is a physical pump: when ice forms at the poles from the sea water, the salt is expelled from it so that the ice is frozen fresh water, and the surface waters surrounding the forming ice become very cold and salty. This cold, salty water sinks to the ocean bottom, carrying with it dissolved inorganic carbon. This carbon — manmade and naturally occuring — has been absorbed by the surface waters from the atmosphere. The physical pump within the Southern Ocean is known to be key in absorbing the increased manmade carbon and heat within our atmosphere.

On land, light and nutrients are needed for plants to grow, and it is no different in the oceans. Due to the high cloud density and its proximity to the South Pole, the Southern Ocean receives very little light for phytoplankton growth. Due to a lack of land above the water, the Southern Ocean is also deficient in iron, a nutrient key for photosynthesis — the process used by plants to convert sun light into chemical energy that moves through the plant. Despite this lack, there are more than 200 known species of phytoplankton in the Southern Ocean.

Iron is a major limiting factor in the Southern Ocean’s biological pump. But increasing desertification on the globe means that there is more dust within the atmosphere, and this is a major source of iron for the ocean. Because of the increase in iron, there is more of this nutrient available for photosynthesis, and it reduces the iron constraints on the biological pump. If the ocean absorbs more carbon, it could further reduce the man-made greenhouse gases in the atmosphere and slow the rate of climate change. This is why some people have proposed fertilising the oceans with iron, to geoengineer our Earth system. But we do not know if this is a feasible  and safe option. 

Research in the Southern Ocean is a yearly operation by the Southern Ocean Carbon and Climate Observatory, a branch of the Council of Science and Industrial Research. Through the South African National Antarctic Programme resupply and research vessels, the MV SA Agulhas II and its predecessor the MV SA Agulhas, oceanographers have an opportunity to study the ocean between South Africa, Antarctica and South Georgia.

There have been notable iron fertilisation experiments in the oceans: scientists have conducted small-scale open-ocean iron fertilisation experiments in the equatorial Pacific Ocean and the Southern Ocean, as well as ship-based iron enrichment experiments in the Southern Indian Ocean. 

Our team, based on the MV SA Agulhas, studied iron fertilisation in six different areas of the Atlantic sector of the Southern Ocean over one summer, ranging from the Antarctic ice shelf to land masses (South Georgia) and the open ocean. According to both biological and chemical indicators, the former two areas show natural “iron alleviation”. Phytoplankton need iron to photosynthesise, and become stressed without it. 

Where iron alleviation occurs, phytoplankton become less stressed because of an increase in the availability of iron. Iron is found naturally in rocks, and enters the ecosystems through the weathering of rocks. In the case of the Southern Ocean, the dust from increasing desertification in Africa, Australia and South America is travelling through the atmosphere, and being blown into the ocean as well as landing on the ice, which is melting.

The aim of such experiments is to observe the response of phytoplankton — as a group and as individuals — to iron fertilisation. We determine how healthy the individual phytoplankton are by measuring their photosynthetic efficiency — how well it uses the light available to it. The effect on the community can be seen in the formation of a “bloom”, which is when phytoplankton reproduce at a rapid rate, in a short time period. In the Southern Ocean, a bloom occurs when the weather is calm and there is sunshine. Blooms are a natural phenomenon in the oceans and are so large and colourful that they can be seen from space. It is during these events that the biological pump is at its strongest.

However, dumping large quantities of iron into the oceans will not solve the planet’s climate-change problems. Recent experiments by researchers at the University of Cape Town show that with the addition of iron, the ability of phytoplankton to use light efficiently increases, even in naturally iron alleviated areas, such as at the ice shelf and around South Georgia. But an increase in efficiency, caused by iron fertilisation, does not mean that there will be a bloom, fundamental to the ocean’s ability to absorb carbon. 

This research shows that, in the Southern Ocean, the addition of iron will always lead to a decrease in iron stress (an increase in photosynthetic efficiency) but that whether or not a bloom develops is complex, depending on the initial species present, light environment and grazing rates 

The increasing iron within our atmosphere will cause changes within our phytoplankton communities, such as an increase in photosynthetic health, but unfortunately the Southern Ocean cannot be relied upon to combat climate change through an increased absorption of carbon biologically, through iron fertilisation. Iron fertilisation, though a fascinating concept, is not a magical tool to solve the climate crisis.

Fiona Preston-Whyte is a MSc candidate at the University of Cape Town.

This publication is the culmination of a six-month-long Mail & Guardian project, called Science Voices, to teach postgraduate science students how to turn their academic writing into something the public can read and enjoy.