Guest essay by Eric Worrall

The world has a CO2 problem – there is not enough free CO2 in the atmosphere, to maximise food production, alleviate world hunger, green deserts, and to attempt to hold off the next ice age. But if my calculation is correct, raising CO2 to a safer level would be surprisingly affordable.

Although a lot of nonsense has been written about CO2 harming plant growth, the reality is commercial greenhouse growers maintain elevated CO2 levels of around 1000ppm, because one of the most effective means of promoting plant growth is to make sure plants get enough CO2 – enough being defined as a significantly higher concentration than is currently available in the atmosphere.

The world is also almost certainly teetering on the brink of the next glaciation. I’m not suggesting it will start tomorrow, but interglacials, of the kind we are currently experiencing, typically only last 10-15,000 years. We are well past the Holocene Climatic Optimum, the peak of our current interglacial. Without serious anthropogenic intervention, it is all downhill from here. There is no guarantee raising CO2 will prevent or mitigate the slide into the next glaciation period, but given the catastrophic consequences the coming ice age will have on human civilisation, it has to be worth a try.

How much energy would be required to raise atmospheric CO2 to 1000ppm?

According to Wikipedia, cooking a kilogram of Limestone in a regenerative kiln takes around 3.6MJ / Kg.

Calcium has an atomic weight of 40, Carbon has an atomic weight of 12, Oxygen has a molecular weight of 16. Burning Limestone produces Calcium Oxide (Quicklime) and Carbon Dioxide.

CaCO3 + heat => CaO + CO2.

So burning 1Kg of Limestone releases:

(12 + 16 + 16) / (40 + 12 + 16 + 16 + 16) * 1Kg = 0.44Kg of CO2.

The atmosphere, at 400ppm of CO2, contains 400ppm x 2.3Gt / ppm = 920Gt of CO2.

To raise CO2 to 1000ppm, we need another 600ppm * 2.3Gt / ppm = 1380Gt of CO2.

This will require burning 1380Gt / 0.44Kg CO2 per Kg Limestone = 3136Gt of limestone.

This would require the expenditure of 3.6MJ / Kg * 3136Gt or limestone = 1.1289 x 10^19 joules of energy.

The total global energy budget is 3.89 x 10^20 joules per annum, so if the energy expenditure was spread out across say a decade, we’re talking about 1.1289 x 10^19 / 3.89 x 10^20 = 0.3% of global energy expenditure.

At around $30 / MWh (source Wikipedia), or $30 per 1,000,000 x 3600 joules = 3.6×10^9 joules, the total cost would be:

1.1289 x 10^19 / 3.6 x 10^9 * $30 = around $100 billion dollars.

Obviously there are additional costs for building the kilns and mining the limestone, but even if these additional costs drive the price up to $300 billion, the return on investment would be tremendous – slightly milder winters and substantially improved farm productivity on a global scale. Spread over 10 years, a cost of $300 billion is $30 billion per year – a lot of money, but in the context of previous vast expenditures such as President Obama’s Trillion dollar Stimulus Package, $300 billion would be affordable, for all the good it would deliver.

In addition, I haven’t considered that a lot of the heat for cooking limestone would likely be delivered using fossil fuel – so the amount of limestone which would have to be cooked to achieve this goal would likely be less than the amount indicated by the calculation.

One final issue would be what to do with the approx. 1500Gt of Quicklime which would be produced by burning the limestone. The obvious solution would be to dump it into the ocean, where as Calcium Hydroxide it could counter any ocean acidification caused by the rise in atmospheric CO2 levels, and would hopefully not promote rapid re-absorption of the released CO2.

Update (EW) – h/t daveburton, Leonard Weinstein – unfortunately my calculation was way off, so this scheme is currently impractical. However in a hundred years, let alone a millennium, mankind’s engineering capability will be far greater than we currently enjoy (think Wright Brothers to Apollo Moon Landing). Engineering projects such as this one should become feasible well before our civilisation is endangered by the coming glaciation.

Update 2 (EW) – higley7 and Miso Alkalaj pointed out that rapid ocean absorption of the released CO2 would make it difficult to maintain the desired atmospheric concentration.

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