Natural Iron Fertilisation and Geo-Engineering
28 January 2009
The efficacy of geo-engineering the oceans through iron fertilization so as to mitigate the effects of climate change is evaluated by results published this week in the prestigious scientific journal Nature. The research was conducted by an international team led by Professor Raymond Pollard of the National Oceanography Centre, Southampton.
Large oceanic regions are high in nutrients, and yet have relatively low biological productivity. This is because such ‘high-nutrient low-chlorophyll' (HNLC) regions are deficient in iron, which is needed to support the growth of phytoplankton - the free-floating, microscopic plant-like organisms that dominate new production in the world's oceans.
Phytoplankton use sunlight to make their food through the process of photosynthesis, and sit at the base of the marine food chain. Through photosynthesis, they also draw large amounts of the greenhouse gas carbon dioxide down out of the atmosphere, thereby influencing climate. A proportion of this carbon sinks down out of the surface layer and is sequestered (‘locked away') by the deep ocean and bottom sediments.
Artificial augmentation of this so-called ‘biological carbon pump' through ocean geo-engineering has been proposed as a potential way of removing carbon dioxide from the atmosphere, thereby ameliorating global warming. One possible way to do this is through ‘iron fertilization', where iron is artificially added to the oceans so as to induce greatly increased phytoplankton population growth.
Experiments in the Southern Ocean, a large and important HNLC region, have shown that the addition of iron allows phytoplankton to exploit other available nutrients, leading to the development of large algal blooms.
Although these algal blooms take up increased amounts of carbon, much of it is released back into the atmosphere as carbon dioxide. Exactly how much carbon is exported from the surface layer, and how long it remains out of circulation, is unknown.
For iron fertilization to be useful in the battle against global warming, the oceans and bottom sediments would need to hold on to the sequestered carbon for many decades, effectively stopping it from returning to the atmosphere, at least until carbon dioxide emissions from the burning of hydrocarbons (oil, coal and gas) are reduced sufficiently to halt or reverse the seemingly relentless increase in the concentration of atmospheric carbon dioxide.
As part of the CROZEX experiment, Pollard and his team focused on the seas around the Crozet Islands and Plateau (hereafter Crozet) at the northern boundary of the Southern Ocean, about 1,400 miles (2,200 kilometres) southeast of South Africa.
The seas around Crozet are naturally supplied with iron from the islands, which have a volcanic origin, and the surrounding plateau. Ocean currents flow northward past Crozet, so that iron is not carried south of Crozet and HNLC conditions prevail. But north of Crozet, the iron accumulates over the dark winter, and each spring, once there is enough light, an enormous phytoplankton bloom develops. This annual bloom contains billions of individual phytoplankton, and covers 120,000 square kilometres (the size of Ireland).
The researchers observed significant differences in the magnitude, timing, duration and community structure of plankton blooms north and south of Crozet. South of Crozet, in the region deficient in iron, phytoplankton peaked in early December and the bloom was short-lived. But north of Crozet, phytoplankton peaked in October, and the bloom lasted for many weeks.
They show that natural iron fertilization enhanced phytoplankton growth and productivity and the amount of carbon exported from the surface layer (100 metres) by 2-3 fold. Moreover, they present the first evidence that carbon fluxes at 3000 m and the sediment were similarly 2-3 times higher beneath the natural fertilized region than for the nearby HNLC region. In addition, an examination of sediment cores taken from the sea bottom shows that this has been so throughout the Holocene (about 10,000 years ago until present).
However, the amount of carbon sequestered by the deep ocean for a given input of iron falls far short of some previous geo-engineering estimates, “with significant implications for proposals to mitigate the effects of climate change through purposeful addition of iron to the ocean.”