Threefold increase of dust led to -40 ppm CO2 Antarctic iron fertilisation during Pleistocene

Iron fertilisation in PleistoceneAn international team of researchers today in Nature explain the importance of dust storms for climate variability, not just for the radiative balance, but also for the Earth’s carbon cycle.

For geoengineering minds: iron fertilisation at least seems to have worked in the past.

The increase of dust particles in the second half of the Pleistocene, the epoch of the ice ages, has played a role in increasing the Earth´s albedo, reflecting more sunlight and thus lowering mean temperatures. The real surprise is in the CO2 lowering potential that the researchers have found though – as the extra deposition of nutrients like iron in the Southern Ocean alone may have led atmospheric CO2 concentrations to decline by 40* parts per million (ppm), thanks to an algae bloom.

[*) For comparison: under current levels it would take our combined fossil fuel consuming and deforestating effort around 15 years to ‘compensate’ such a CO2 decline.]

The dust storms – which in turn may have been provoked by the glaciations – may have changed the character of the late Pleistocene ice ages, which kicked off suddenly, instead of the slower glaciation periods during the early Pleistocene.

Iron fertilisation geoengineering

The idea has been raised to try and mimic the Pleistocene iron fertilization in the waters around Antarctica or elsewhere where iron is a growth-limiting nutrient to phytoplankton.

The authors in their Nature publication warn though that ancient results hold little promise for such CDR geoengineering schemes as the Earth’s thermohaline circulation during the late Pleistocene was notably different from today’s. We don’t want all that iron we ship down south to get flushed straight to the bottom of the ocean, without ever being digested by any plankton – although perhaps we may get offered an unexpected helping hand by Antarctic krill to bring that iron back to the surface waters.

Small-scale iron fertilisation monitoring projects in the Southern Ocean show there is little chance we can simply increase plankton without affecting other parts of local ecosystems.

Where does the carbon go?

It may be down to unanswered practical questions: where does the extra carbonate go if the plankton don’t simply die and accumulate on the sea floor [as we had hoped], but are for instance eaten by fish instead? That may be a growth limiter that prevents plankton populations to reach their ‘iron optimum’. We also don’t want any extra carbonate to linger for too long in the upper ocean. That’s because of ocean acidification too: as the CCD line lowers more and more carbonate dissolves, rereleasing the CO2 back to the water and the air.

Still, it’s tempting to think of a tool to lower the atmospheric CO2 concentrations by 40 ppm. It would bring us from today’s value of 390 ppm straight to James Hansen’s safe limit of 350 ppm. Of course we’d still have to do something about the continued growth of CO2 emissions – but that goes for all scenarios anyway. Meanwhile we have to remember the oceans are working in the opposite direction, with natural CO2 uptake decreasing – leaving more, not less, for the atmosphere.

© Rolf Schuttenhelm |

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