Carbon capture comes up often in policy discussions about climate change. The concept is simple – there is too much carbon dioxide in the atmosphere, so we should pull that CO₂ out of the air, lowering the CO₂ levels! While this is intuitive, it raises some obvious questions: how do you remove carbon dioxide from the atmosphere? And is this actually necessary?

To answer the second question first: carbon capture is extremely important. According to the IPCC, carbon capture will need to be part of any climate solution that keeps warming under 1.5 °C. Even the most optimistic projections predict that carbon dioxide levels in the atmosphere will lead to substantial warming. Carbon capture will be necessary to keep climate change at a reasonable level.

Now the more difficult question – how do we remove carbon dioxide from the atmosphere? More specifically, how do we sequester it in a manner that is scalable, cheap, and safe? Fortunately, there are several methods that have been developed, each with their own advantages and limitations. Learning about these technologies is useful for understanding broader challenges related to climate science.

Direct Air Capture

Image by Carbon Engineering

To understand how carbon capture technology works, it is perhaps best to start by discussing direct air capture. This technology filters CO₂ from the air by using a large fan assembly to suck up massive amounts of air. Next, this air is passed through a specialized set of filters that captures CO₂ molecules. Once these filters are filled to capacity, they can be removed, placed in a specialized location. The filter is then heated, and will release the CO₂, leaving a highly concentrated CO₂ gas.

Next, the carbon dioxide is mixed with compounds like potassium hydroxide, to produce a carbonate salt. These carbonate salts allow for the easy transport of carbon dioxide, which can then be released again by further processing. Ultimately, carbon dioxide is stored underground where it will become rock over a few years.

One of the most attractive features of carbon capture from the atmosphere is the flexibility. They can be built anywhere and they can offset CO₂ emissions from across the world. Countries with lots of low-value land like Canada or Russia could install large carbon capture plants to offset emissions in densely populated countries like the Netherlands, Bangladesh, or Japan. Direct air capture plants are also very space-efficient. Carbon Engineering’s pilot plant extracts CO₂ at a rate equivalent to 40 million trees, which would require thousands of acres of land. This versatility is far better than any other method of carbon capture.

While direct air capture is exciting, it has one major downside: cost. To sequester one ton of CO₂ using Carbon Engineering’s pilot plant in BC, it would cost between $100-250. To offset the annual emissions of a single Canadian it would cost at least $1,500, and could be substantially more. Carbon Engineering’s plant is also the most efficient in the world, earlier studies estimated the cost would be at least $200 per ton of carbon. In addition, direct air capture is incredibly energy intensive. A 2019 study from Nature predicted that carbon capture could use one quarter of global energy by the year 2100. Overall, it seems that there is opportunity for technological progress to improve the efficiency of direct air capture.

Carbon Capture and Storage

Image by Dave Navarro from Pixabay

It is great to remove carbon from the atmosphere, but one could say this is an inefficient way of tackling the problem. After all, the levels of CO₂ in the air is only a few hundred parts per million. Processing such low concentrations is difficult and expensive. It would be more economical to extract the CO₂ if it were in a more concentrated form.

Carbon capture and storage (also known as carbon capture and sequester) aims to lower atmospheric CO₂ by stopping carbon from being released. Essentially, a carbon capture facility is added to a factory or power plant, which blocks CO₂ from being emitted. Carbon capture and storage is particularly attractive because of the ability to retrofit buildings that are already in operation, making it quite versatile.

While this technology is promising, some are concerned about the cost. One of the first large scale carbon capture and storage projects ran dramatically over budget, creating the narrative that the technology is expensive and difficult to implement. Despite this, some researchers are optimistic that costs could be as low as $20 per ton of CO₂ in certain industries. Another recent analysis predicted the costs could be about $40 per ton of CO₂ avoided. While this cost per ton of CO₂ is decent, the multi-millions dollar up-front expense required to retrofit is a barrier for many companies. A substantial carbon tax would likely be necessary to incentivize this type of investment.

While carbon capture and storage does have a lot of potential use, it has important limitations. Emissions from vehicles, agriculture, and homes can’t be captured by this technology. These low-density emissions sources are simply not compatible with the technology. Carbon capture and storage also can’t impact carbon emissions that are happening in a distant location, as is the case for direct air capture. Though these limitations do restrict the application of carbon capture and storage technology, this is still a promising method for reducing CO₂ emissions.

Bioenergy with Carbon Capture

One strategy that is less well known is bioenergy with carbon capture. Bioenergy is the process of burning any biological product for the purposes of generating electricity. As the name implies, bioenergy with carbon capture just combines bioenergy with carbon capture and storage technology. Plants are cultivated, burned for generating electricity, and any emissions are sequestered.

Yes, you read that right: grow crops, burn them, then store the carbon.

In theory, this process should be carbon absorbing. Plants grow, absorbing CO₂ from the air. When the plants are burned for energy the carbon doesn’t return to the atmosphere, leading to a net drop in atmospheric CO₂. Not only are you capturing carbon, you are also generating electricity, which can be sold to the grid, which might mean you can even turn a profit!

While the cleverness of this solution may sound compelling, it relies on the efficiency of bioenergy and carbon capture. Environmentalists were once extremely enthusiastic about bioenergy, but some recent research suggests that the technology may be only slightly better than fossil fuels. The key factor is land use change. Cutting down forests to create space for farmland means that the carbon-sink capacity of those trees is lost. Once you factor that into your life cycle assessment, bioenergy does not look particularly efficient.

Cost estimates are also pessimistic. One estimate found that bioenergy with carbon capture could cost $150-350 per ton of carbon removed from the atmosphere. Another study has predicted that the overall impact of bioenergy with carbon capture would be modest. There are some pilot projects using bioenergy and carbon capture, but more research will be needed to verify that this strategy for carbon capture is actually effective.

Reforestation

Photo by Isaac Quesada on Unsplash

Sometimes the best solution is the simplest. Planting trees may seem like an absurdly low-tech strategy to remove carbon from the atmosphere, but many serious scientists feel it is viable. As trees grow, they remove carbon dioxide from the air and convert it into woody plant matter. One study estimated that tropical reforestation could remove CO₂ from the air at a cost of $20-50 per ton – far less than the cost of carbon capture from other atmosphere technologies. Tree planting also offers several auxiliary benefits, such as habitats for animals, natural beauty, and supporting local ecology.

While reforestation is very attractive, there are some limitations. First, trees do a relatively bad job at permanently sequestering the carbon. Once a tree dies any stored carbon could then be rereleased back into the atmosphere. This is particularly relevant given wildfires seen on America’s west coast, where forests that were set aside as a carbon offset were burned.

The science of trees is also extremely difficult. CO₂ absorption will vary depending on the type of tree, the region they are planted, and the density of the forest.  It is also impossible to create a closed system to quantitatively measure the exchange of gases. For mass reforestation to be an effective large-scale negative carbon strategy, it will be essential to improve the foundational science.

Finally, trees are quite land intensive. While sources vary considerably, most suggest that an acre of forest is able to absorb between 2.6 and 8.2 tons of carbon dioxide per year. The average American was responsible for about 16 tons of CO₂ in 2018, meaning each person would need between 2 and 6 acres of forest to offset their carbon footprint. To account for all the carbon of the U.S. over 25% of the landmass would need to be reforested. Young trees also absorb less carbon dioxide, since it takes decades before trees reach their peak growth rate, though there is some debate on this subject. It also wouldn’t be sufficient to use the tundra in northern Alaska or the deserts of Nevada. Good forests require good land, which means reforestation could compete with the agricultural sector.

Instead of focusing on planting new trees, it may be more impactful to stop deforestation. It is estimated that the Amazon rainforest absorbed about 2 billion tons of CO₂ per year during the 1980’s and 1990’s. Today, it is only able to absorb 1.0-1.2 billion tons of CO₂ per year due to deforestation. For a sense of scale, in 2017 the U.S. emitted 5.27 billion tons of CO₂ in total. The Rainforest Trust claims that a $10 donation will stop the destruction of 5 acres of Amazon rainforest, leading to 931 tons of CO₂ savings – hundreds of times cheaper than approaches like direct air capture. While this analysis might be overly-optimistic, it is reasonable to believe that preserving existing rainforest may be one of the most impactful measures to stop climate change.

Moral Hazard

Beyond conversations about “how” one might capture carbon, there is also a real conversation about how much carbon we “should” be trying to capture. This isn’t about climate change skepticism, but is rather about the incentives we use to structure society.

Imagine all the world governments came together and decided to implement no other changes to address global warming other than building carbon capture from atmosphere plants. To keep warming under 1.5 °C, the IPCC has said we need to reduce global CO₂ emissions from 36.6 billion tons of CO₂ per year in 2018 to 15 billion tons CO₂ per year by 2030. To achieve this, we would need to build at least 20,000 carbon capture plants, which would cost trillions of dollars. Not only that, we would need to build hundreds of additional plants per year to keep up with growth in CO₂ emissions across the world. It is simply not a sustainable strategy to implement at such a large scale.

Unless there are some astonishing breakthroughs in carbon capture technology, it seems unlikely that carbon capture alone won’t be enough to avoid global warming. This strategy will need to be paired with reduced carbon emissions. Some worry that by allocating attention and resources to carbon capture we may underinvest in decarbonizing our economy.

This may all sound hypothetical, but it is worth mentioning that some of the most active sponsors of carbon capture research are oil companies like Chevron and BP. Certain activists are concerned that these companies are trying to delay the transition away from fossil fuels, or that big oil is trying to profit off of cleaning up their own mess. Alternatively, this could be seen as a good faith effort to fix their mistakes, and find a new business model that is more sustainable. This brings in a broader challenge about how fossil fuel companies fit into the future economy, which is complicated for entirely legitimate reasons.

One way or another, we will need to utilize carbon capture initiatives to avoid substantial warming. Hopefully, regulation and policy will incentivize the right mix of emissions reductions and CO₂ offsets, allowing the carbon capture sector to play a major role in the economy in the near future.