Article by Adrian Reid

8 November, 2019

The transformation of CO2 into minerals and rocks is a natural process taking millions of years. Scientists are investigating whether they can speed up the process. One way is to cover fields in basaltic rock dust and let it react with the CO2 in the air. Another way is to spread ground up olivine onto beaches where it can react with the CO2 dissolved in the waves. The issue with both methods seems to be the amount of rock required and the cost of mining it.

Article by Adrian Reid

8 November, 2019

A CO2 rock lockdown

Most of the carbon on earth is locked down in the form of rock — limestone, graphite, even diamonds. A lot of it got that way with the help of a little rain.

Rain combines with CO2 in the air to become slightly acidic. Each time it rains, the rain reacts with the rock and slowly dissolves it. This reaction forms minerals which are then carried off down rivers and eventually out to sea. The carbon locked in these minerals may stay that way for hundreds of thousands of years.

This entirely natural weathering process removes about a gigaton of CO2 from the atmosphere every year. It's worked in the past to handle natural increases in CO2 from forest fires or volcanic eruptions. Could it help us again, this time to fight climate change?

Naturally weathered rock — Valley of Fire State Park, Nevada. cjarv2010, CC BY 2.0

Scientists hope so, and are working on ways to accelerate natural weathering, using "enhanced weathering". The goal is to find a way to get rock to react with CO2, and trap the carbon either underground, or deep in the ocean.

Rocks on crops

Soil is a great place to store carbon, and basalt rock could be a way to do this. This is a common kind of rock which removes CO2 from the atmosphere and transforms it into limestone. Farmers already use rock dust in their fertiliser, so the basalt could easily be added to the mix and spread over cropland.

Depending on how much basalt is applied, one study suggests this could reduce CO2 by up to 300 ppm by 2100. Converting that to gigatonnes (multiply by 7.82), it's a reduction of over 2000 gigatonnes.

Basalt rock — a common rock that traps CO2. Bryant Olsen, CC BY-NC 2.0

Hopefully we never need to remove that much CO2. To get the project started and to remove the first 50 ppm of CO2 from the atmosphere would cost an estimated US$60 - 600 trillion for mining, grinding and transporting the basalt.

Why so expensive? Removing just one gigatonne of CO2 from the atmosphere requires 3 gigatonnes of basalt. For comparison, the world's largest mining industry, coal-mining, produces 8 gigatonnes of coal a year.

We would need a mining industry much larger than coal to make a difference to CO2 levels. To return CO2 to natural, pre-industrial levels means removing hundreds or even thousands of gigatonnes of CO2 from the atmosphere. We would need a basalt mining industry many times larger than any other mining industry in existence, and we'd need to set it up fast.

Gems on beaches

Another place to store CO2 is deep in the ocean. Scientists believe this could be done using a green-coloured mineral called olivine.

Olivine can't be poured out into the ocean, it would just sink. Instead, a research group called Project Vesta is investigating if olivine could be scattered on beaches, where waves would then dissolve the olivine and capture CO2. Research is planned for an island in the Caribbean later this year, where presumably researchers will carefully watch waves crashing on the beach.

Natural olivine on Papakōlea Green Sand Beach, Hawaii. Jay, CC BY-NC-SA 2.0

It sounds like a dream job for a scientist, but it has major implications. Project Vesta believes that applying olivine to 2% of the world's beaches could remove "100% of humanity’s yearly CO2".

Independent research does suggest olivine is effective. One study shows the use of enough olivine could reduce atmospheric CO2 by more than 800 gigatonnes of carbon by the year 2100. That's close to 3000 gigatonnes of CO2 (multiplied by 3.67).

All the olivine

Adding all that olivine to the ocean is going to change things. According to Project Vesta, to remove 1.5 gigatonnes of CO2 means adding 1.2 gigatonnes of olivine to the ocean. To remove hundreds or even thousands of gigatonnes of CO2 from the atmosphere means adding similar quantities of olivine to the oceans.

Such a massive amount of olivine could change the ocean's chemistry. When olivine dissolves it leaves behind iron and silicon. This could have unpredictable consequences on plankton, which are the basis of the ocean's food chain, on which all sea life depends.

Olivine is a grand experiment. It might work, it could remove large amounts of CO2 from the atmosphere. But swapping gigatonnes of CO2 in the atmosphere for gigatonnes of olivine in the ocean may just be solving one problem to create another.