Mixing iron into the north Pacific stirs geo-engineering controversy

A British Columbian fishing community has drawn almost universal condemnation after dumping 100 tonnes of iron rich dust into the ocean to stimulate a plankton bloom, in an effort to restore salmon numbers.

When the story broke in the Guardian, the New York Times, CBC, Nature and Science, many of the commentators claimed that ocean iron fertilisation on such a scale violated international protocols and had potentially dangerous impacts on the marine ecosystem.

But it is equally possible that the bloom was natural.

To understand why this may be the case and why the community took this approach to improving fish stocks, it is important to understand the science around iron fertilisation.

Oceanic iron fertilisation (OIF) has come to prominence as a potential geo-engineering approach to global warming. The idea is that iron dissolved in the ocean could accelerate the growth of massive plankton blooms. This would increase the uptake of anthropogenic CO₂ from the atmosphere and sequester it deep in the ocean.

The natural process by which atmospheric CO₂ enters the ocean and is sequestered on the ocean floor is known as the biological pump. It relies on large numbers of plankton in massive blooms capturing carbon through photosynthesis.

When the blooms die they sink to the bottom of the ocean, remaining there for thousands to millions of years and taking the CO₂ with them.

However, in vast areas of the global ocean – the north Pacific, tropical Pacific and Southern Ocean – the efficiency of the biological pump is limited by extremely low bio-availability of dissolved iron. Without sufficient dissolved iron, large plankton blooms cannot form.

At least nine OIF experiments have been performed since 1993 in each of the major iron-limited regions of the global ocean, mostly to understand changes in ocean productivity and atmospheric CO₂ concentrations over glacial-interglacial cycles (~20,000 years).

Until recently, the export of carbon into the deep ocean had been confirmed in only two of these experiments. Even then, the efficiency of the transport was far less than expected.

But in 2004, an export efficiency (beyond 1000m depth) of greater than 50% was measured for an iron-fertilised bloom in a clockwise-rotating eddy in the Southern Ocean. This result has to some extent revived the debate surrounding iron fertilisation as a geo-engineering technique.

There are concerns about the impact of OIF on several aspects of ocean health. Fertilising with one nutrient will likely cause scarcity of the next least-available nutrient, probably phosphorous or nitrogen. When the blooms die and decompose, they consume oxygen and may lead to larger oxygen deficient zones. The phytoplankton favoured by iron addition could be the kind that produce potent neurotoxins, which accumulate up the food chain and cause illness in marine mammals and humans. These concerns have led to moratoria on ocean iron fertilisation. But it’s also important to realise that these are only potential impacts. With the possible exception of depletion of other nutrients, none of these processes have been observed in any experiments to date.

When the Haida Salmon Restoration Corporation (HSRC) announced it had dumped 100 tonnes of iron-rich dirt into the north Pacific from the fishing vessel Ocean Pearl during mid 2012, people suggested it had potentially violated the international moratoria. And there were worries about the scale of the experiment and the potential for unintended negative environmental impacts.

All of the earlier OIF experiments deployed about 1 tonne of iron as iron sulphate solution. If the dirt used by the HSRC was any more than 1% iron by weight (it was likely more than 5%), this would qualify as the largest deliberate iron addition ever performed.

Some news reports of the iron addition have shown a single satellite chlorophyll image indicating what appears to be a bloom. However, this region of the ocean is populated by eddies that move productive, high-iron coastal waters into the low-productivity open north Pacific.

Is it possible to determine whether the bloom in the satellite image below was due entirely to deliberate iron fertilisation by the HSRC?

The image above is probably an average of about two weeks of satellite data. Individual daily images may be more useful for determining whether the circled bloom was natural or iron fertilised.

Many of the news reports say the iron fertilisation occurred in July this year, but this does not seem to be true. The Ocean Pearl deployed 20 drifters provided by the National Oceanic and Atmospheric Administration (NOAA). The deployment information for these drifters gives an approximate location of the ship.

The two images below tell us a little about the pre-existing chlorophyll concentrations in this region of the North Pacific and the Ocean Pearl’s expedition.

The first is a snapshot from August 8 (before the ship left port). It shows some high chlorophyll near 139°W, 53°N, where the bloom was eventually seen in the first relatively clear satellite image.

In the August 25 image, the magenta circles are the deployment locations of the drifters (indicating the ship’s track). The white circles are the location of the drifters on August 25 and the black lines are the drifter trajectories.

Based on the drifter data and some images from August 14 and 17 (not shown here for brevity), the Ocean Pearl arrived in the location of the bloom on August 14 at the earliest but probably started fertilising no earlier than August 16 or 17.

The high chlorophyll observed on August 25 (no more than eight or nine days after fertilisation) is approximately 4.5 mg m^-3 and is probably an underestimate because of known problems with satellite chlorophyll retrievals at high latitudes.

In the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES) experiment in 2002, chlorophyll concentrations of this magnitude were not observed until about 14 days after fertilisation.

All of this evidence strongly suggests the Ocean Pearl/HSRC did not solely generate the bloom observed on August 25. At best they may have slightly enhanced an already high chlorophyll area, as suggested in an article in the Vancouver Sun. They did not create a bloom in an ocean desert.

If it is their intention to sell carbon credits to help offset the cost of the experiment, how will they document the amount of carbon sequestered? Measuring the amount of sequestered carbon has been one of the biggest challenges from every OIF experiment. How would they determine how much of that carbon sequestration was due to the addition of iron and how much was natural?

It is easy to understand why this experiment has drawn such heavy criticism. However the over-emphasis on potentially negative but undocumented impacts does not help the debate. No deleterious impacts have been observed for any OIFs thus far. Risks may well be minimal and mitigated by good project design.

The scientific community, in its concern to show respect for international rules, should be careful to avoid hyperbole if it wants the public to support future research into this important aspect of global climate.