Iron Salt Aerosol

Amplifying natural greenhouse gas depletion

Artificial Iron Salt Aerosol (ISA) is FeCl3 (ferric chloride) nebulized into the troposphere. It could be produced in-situ from chemical precursors combusted in hot flues designed to lift a plume up to 1km above sea level. Adding a total of 280,000 tons of iron annually to the iron-poor ocean regions could remove 40 billion tons of carbon dioxide and equivalents per year (more than quadrupling the abatement of the whole Paris Accord over the next decade). Iron Salt Aerosol could be implemented safely, quickly and at low cost. A first order estimate based on materials cost is 45 US cents per tonne of carbon-dioxide equivalent removed. Our official estimate is currently $1 per tonne of carbon-dioxide equivalent, which incorporates a 120% uncertainty.

The strongest Iron Salt Aerosol climate restoration mechanisms are:

  • Depletion of powerful atmospheric warming agents: methane and tropospheric ozone. A sunshine-induced photo-catalytic reaction produces chlorine radicals – a powerful oxidising agent that is estimated to deplete methane at four times the natural rate, possibly much faster.
  • A reaction that renders black/brown particulates more readily washable out by rain;
  • Carbon-dioxide drawdown, from very low concentration but widely dispersed ocean fertilisation. We estimate 0.05mg per square meter per day over 4% of the ocean may be sufficient;
  • Rapid cooling by marine cloud brightening from ISA itself and the airborne excreta of phytoplankton;
  • Ocean brightening, as growth of phytoplankton turns it from black-blue to turquoise.

The ocean has been a natural carbon sink for billions of years, as seen by the existence of the 100s of millions of gigatons of continental carbonate rock. We argue it is capable of safely and permanently sequestering around 10 billion tons of carbon yearly from the atmosphere. This is equivalent to the current emission rate from human activities (2020).

Iron Salt Aerosol already exists in the troposphere from steel manufacturing, coal fired power stations (along with the undesirable mercury compounds and particulates etc) and numerous other sources, including windblown dust.

An incremental scaling approach with testing at each stage is required to measure efficacy and safety. The ideal candidates for iron fertilisation are areas of open ocean that are low in iron content. One of our initial proposals is to carry out a field trial from a remote tropical island in such an area. This would deplete atmospheric methane and CO2. The resulting cloud brightening would also have a cooling effect.

There is much more to it than that. Focused largely on chemistry, the ISA team has extensively reviewed the way carbon is transported in and out of the atmosphere, ocean, ocean crust aquifer and Earth’s mantle.

We do not accept the claim that the oceans can absorb only 1 billion tons of carbon each year owing to hypothetically large carbon-dioxide out-gassing from plankton litter oxidation. Most of the organic material produced by phytoplankton growth and its downstream food-web becomes oxidised back to bicarbonate far below the surface and stays in the ocean for centuries. Eventually it becomes permanently sequestered as carbonate and organic compounds in the ocean crust and sediments, largely by microbial life. Over geological time the sediment becomes continental landmass. This is how it has worked since life began, and why Earth has almost no carbon in its atmosphere, while Venus has nearly 100 Earth atmospheres of CO2.

Contrary to claims that Iron Salt Aerosol damages the stratospheric ozone layer we argue that ISA depletion of methyl-halides is more likely to strengthen the ozone layer.


This Working Strategy Document (4.3 MB PDF, 19 pages, 16 June, 2020) was written by Kevin Lister, Restore Our Climate President and Board Member. The paper sets out:

  • Why we need to intervene in the climate system with a large-scale technology;
  • The time scales that we must work to;
  • The testing strategy;
  • Evidence that it will work;
  • Politics and Markets; and
  • Investments needed

Scientific paper

Lead authors are Franz Oeste and Renaud de Richter, published this highly technical paper in August 2017 in the peer-reviewed journal Earth System Dynamics: Download PDF.