Caldeira Lab Research:Ocean acidification and ocean carbon cycle

Can ocean iron fertilization mitigate ocean acidification?

Long Cao & Ken Caldeira

It has been suggested that adding iron to the ocean could mitigate climate change and even ocean acidification. Using ocean carbon cycle model simulations, we show that ocean iron fertilization has negligible benefit to upper ocean chemistry, and would lead to greater acidification of the deep ocean.

Cao, L. and K. Caldeira. 2010. Can ocean iron fertilization mitigate ocean acidification? Climatic Change, Springer Netherlands. Volume 99, 1-2. March 2010. DOI: 10.1007/s10584-010-9799-4




















Fig. 3: Simulated surface ocean pH and saturation state of aragonite (aragonite) in pre-industrial time and year 2100 for the simulation with/without iron fertilization. By comparing surface ocean chemistry at the end of this century with that at pre-industrial time, it is shown that ocean iron fertilization, even implemented to achieve the upper bound of its maximum effect, has a minor effect on mitigating surface ocean acidification caused by anthropogenic CO2 emissions.


Ocean iron fertilization has been proposed as a method to mitigate anthropogenic climate change, and there is continued commercial interest in using iron fertilization to generate carbon credits. It has been further speculated that ocean iron fertilization could help mitigate ocean acidification. Here, using a global ocean carbon cycle model, we performed idealized ocean iron fertilization simulations to place an upper bound on the effect of iron fertilization on atmospheric CO2 and ocean acidification. Under the IPCC A2 CO2 emission scenario, at year 2100 the model simulates an atmospheric CO2 concentration of 965 ppm with the mean surface ocean pH 0.44 units less than its pre-industrial value of 8.18. A globally sustained ocean iron fertilization could not diminish CO2 concentrations below 833 ppm or reduce the mean surface ocean pH change to less than 0.38 units. This maximum of 0.06 unit mitigation in surface pH change by the end of this century is achieved at the cost of storing more anthropogenic CO2 in the ocean interior, furthering acidifying the deep ocean. If the amount of net carbon storage in the deep ocean by iron fertilization produces an equivalent amount of emission credits, ocean iron fertilization further acidifies the deep ocean without conferring any chemical benefit to the surface ocean.