Liming the oceans

It's time to move onto Carbon Dioxide Removal (CDR) geoengineering: Part Two.

This time the focus again revolves around the oceans, this time: Liming the oceans.

This method mirrors that of my previous post on ocean iron fertilisation, as both methods focus on utilising the ocean's ability to store copious amounts of carbon; a storage greater than any other carbon sink on the planet.

Source: wattsupwiththat

Yet, ocean liming differs from ocean iron fertilisation as it is a chemical geoengineering process using chemical reactions to increase ocean carbon storage capacity whereas iron fertilisation used biological means of enhancing carbon dioxide removal from the atmosphere. Liming was first suggested by Haroon Kheshgi, and it is the sprinkling of lime (otherwise known as Calcium Oxide (CaO)) into the ocean as a means (according to Renforth et al. 2013) of manipulating the carbonate system through the addition of CaO to improve net sequestration of atmospheric carbon into the deep ocean.

Here follows a key background of why and how liming works...

So as established in my previous post, colder waters are more optimal than warmer waters at absorbing greater volumes of carbon dioxide concentrations. Yes? Right, however anthropogenic activity in recent years has both increased carbon dioxide emissions to the atmosphere and induced a decrease in ocean pH by 0.1 units. Continuation of this pattern of behaviour could result in a 0.4 unit pH decrease over the next 100 years (Renforth et al. 2013) resulting in....

Acidification of ocean surface waters.

The climatic impacts of ocean acidification results in biodiversity loss as microorganisms such as phytoplankton within the surface waters cannot form strong shells, which is crucial as these microorganisms are greatly responsible for carbon dioxide uptake via photosynthesis.

Source: Just only

How does liming stop this?

Liming is the addition of calcium oxide, an alkaline compound. By sprinkling this compound into the oceans, it increases the oceans pH to mitigate acidification, and optimise carbon dioxide uptake. When proposed by Keshgi, hydroxide particles were added to the ocean incurring the following dissolution reactions to take place:

CaCO_3 --> CaO + CO2
CaO + 2CO₂ + H₂O --> Ca²⁺ + 2HCO₃^-

On addition to seawater, calcium oxide reacts with carbon dioxide to produce calcium bicarbonate solution. The net effect of this series of reactions is beneficial in terms of carbon sequestration as 1 mole of calcium oxide absorbs 2 moles of dilute carbon dioxide, and the production of calcium oxide only required 1 mole of pure carbon dioxide for formation (Cquestrate, 2008).

The prevalence of limestone (CaCO3) making up 10% of the Earth's land surface, makes it an ideal and abundant material to use, especially as projected input into the ocean on a global scale should only subject a minor change in ocean chemistry which suggests minimal impact on the environment.

This all sounds good, but what if too much alkalinity is added to the ocean? What happens then?

Upon initial addition of lime into the ocean, a localised elevation in pH around the point of addition is induced, which in small increments has little environmental impact, however over an increased period of time, higher pH levels are detrimental to some living organisms, and could result in carbonate precipitation, which hinders the effect of ocean liming (Renforth et al. 2013).

Also, research has discovered that the benefit of liming derived from limestone could take decades before it takes effect due to the slow process of deep ocean upwelling, and is therefore unlikely to be an emergency response to global warming. If 4 billion tonnes of powdered limestone was added to the water yearly from 2020 onwards, 1 billion tonnes of carbon dioxide could be taken up by the oceans, reducing atmospheric concentrations by 30 parts per million by 2200 (Hamilton, 2013). But can we wait this long?

The extraction of lime from limestone is both costly and emits carbon dioxide through the process, (which requires coal power)- so why not forget liming and just concentrate on reducing use of coal power plants to prevent pollution of carbon dioxide in the first place? Unless lime can be extracted using renewable energy, the scheme surely is not worth the pollution and costs of producing lime in the first place.

Similar ethical questions can be raised such as, who owns rights to the ocean? Should anyone be allowed to experiment with the ocean's chemistry? Can we account for unexpected changes?

With all of this in consideration, should it go ahead? I think as proposed geoengineering schemes go, this one makes sense, as it utilises the abundance and renewal of limestone and has little known impact on biological life. However, until this process can be derived from alternative, carbon-free means, and there is a way to increase the rate of progress, the proposal still requires considerable development.

Any thoughts?
See you next time!


S xx