Where do we put a trillion tons of CO2?

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Earlier I posted briefly about seven techniques that can remove CO2 from the atmosphere, each of which could scale to 50 GT / year, and cost less per year than the US military budget. That rate is high enough to allow us to restore old CO2 levels within 50 years, even if the energy transition gets stalled.

Where do we put that CO2? Can it be sold?

Putting away that carbon is the field of carbon sequestration, which should be a long article, but the useful answers are brief.

The simplest sequestration method, and one with rapidly growing support, is to pump the CO2 down into basalt rock formations where the CO2 gets securely converted to carbonates, essentially limestone, within two months to two years. The key results on this were published last summer. I had conversations with the authors in November where they confirmed that it should be fairly easy to scale to 50 GT / year. More details on this later on.

The ocean is the context for the next set of sequestration methods that can be scaled to 50 GT / year. To put this in context, remember that the formation of oil deposits millions of years ago occurred by sequestering vegetation (including plankton) underwater where it anaerobically decomposed into oil and coal—so this is quite natural. In several methods, plankton and seaweed are grown, and the detritus sinks into the deep ocean where it stays, for hundreds of years up to millions of years. In another method, alkaline rocks are added to the ocean, where they combine with carbonic acid, and then fall out of solution. The carbonic acid is what CO2 turns into when it dissolves in water, so this becomes a CO2 pump from the atmosphere into the ocean, and down to the ocean floor. Greg Rau, on this list, is probably the world’s top expert on that, and can provide corrections and references.

The third set of sequestration methods is for commercial use in infrastructure. Remarkably, markets for carbon could make an impact on the 50 GT / year target. Blue Planet has successfully commercialized techniques for converting CO2 into limestone for use as the aggregate used in paving roads and in concrete for large buildings. I learned about this just two weeks ago. The global market for aggregate is the equivalent of 20 GT CO2 per year, at prices from $5-$50 / ton CO2 equivalent. There are other commercial uses for CO2 and carbon, but currently they are relatively small. The commercial advantage of using atmospheric CO2 is transportation. Carbon is cheap, but shipping heavy aggregate is expensive, so when clean aggregate is needed, creating it out of thin air is sometimes advantageous.

The purpose of all this is not to recommend one method over another, although probably every person reading this has a strong preference for one or another. The purpose is to show that there are a good number of reasonable methods to sequester the trillion tons of CO2 that we’ll remove from the atmosphere in the next several decades. I do suggest that readers expect that several, if not all of the methods will be implemented, so that you don’t get too worried before the science and engineering is done.

There are many other methods of sequestering CO2, many of which are being done already, including biochar, expanding forests, agricultural practices which increase carbon in soils, and pumping CO2 underground into sealed reservoirs in which it may or may not be converted permanently into carbonates. These methods do not appear to be scalable to 50 GT / year, so although they’re valuable, they’re unlikely to be among the critical methods.

At this stage of the Healthy Climate Project, where the world’s climate leaders are becoming fluent in the vision and narrative of restoring a healthy climate for our children, sequestering CO2 in basalt fields has been the most effective story because it is easy to visualize at scale, has been done successfully (recently and for billions of years), and it is quite inexpensive. The other methods will surely take much of the market, but each one has hurdles to cross.

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