Understanding Carbon Capture
How does carbon capture work? And is it necessary to avoid climate change?
The concept behind carbon capture is simple — there is too much carbon dioxide in the atmosphere, so if we remove the CO2 from the air, we can stop global warming! While this is intuitive, it raises some obvious questions: how do you remove carbon dioxide from the atmosphere? And is this necessary?
To answer the second question first: carbon capture is essential. 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 expected carbon dioxide emissions will lead to over 2 °C of warming. Some carbon capture will be necessary to stop climate change.
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, each with advantages and limitations. Understanding these technologies also provides insight into broader challenges related to climate science and more profound ethical questions about how we should structure society.
Direct Air Capture
Understanding direct air capture is foundational to many carbon capture technologies. First, a large fan assembly sucks up massive amounts of air. This air passes through a specialized set of filters that captures CO2 molecules. Once these filters reach capacity, they can be removed and placed in a specialized location. The filter is then heated, releasing the CO2 bound to the filters, leaving a highly concentrated CO2 gas. The filters can then be reused.
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 re-released by further processing. Ultimately, carbon dioxide is stored underground, where it will form into rock over a few years.
One of the most attractive features of carbon capture from the atmosphere is flexibility. Plants can be built anywhere and can offset CO2 emissions from across the world. Countries with lots of low-value lands, such as Canada or Russia, could install carbon capture facilities to offset the emissions from 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. It would cost at least $1,500 to offset the annual emissions of an average Canadian. Carbon Engineering’s plant is also the most efficient in the world. Earlier studies estimated the cost of direct air capture to be at least $200 per ton of carbon. Much of this cost comes from energy usage. A 2019 study from Nature predicted that carbon capture could use one-quarter of global energy by 2100. Overall, there is room to improve the efficiency of direct air capture.
Carbon Capture and Storage
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 sequestration) aims to lower atmospheric CO2 by stopping carbon from being released. Essentially, CO2 emissions are blocked by a carbon capture facility that is built on site. Theoretically, any building could be retrofit to include a carbon capture and storage unit, making this a flexible strategy.
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, giving a reputation that the technology is expensive and challenging 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-million 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 significant limitations. This technology can’t efficiently capture emissions from vehicles, agriculture, and homes. Carbon capture and storage also can’t offset carbon emissions from a distant location, as is the case for direct air capture. Though these limitations 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 to generate electricity. As the name implies, bioenergy with carbon capture 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, but you are also generating electricity, which is used or sold to the grid. Maybe you can even turn a profit!
While this solution’s cleverness may sound compelling, it is limited by the efficiency of bioenergy and carbon capture. Environmentalists were once enthusiastic about this technology, but some recent research suggests that bioenergy may be only slightly better than fossil fuels. The critical 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 account for land-use in your life cycle assessment, bioenergy does not appear to be efficient.
Cost estimates are also uninspiring. 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. Some bioenergy and carbon capture pilot projects are in operation, but more research will be needed to verify that this carbon capture strategy is useful.
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 the cost of $20–50 per ton — far less than the price of other carbon capture technologies. Tree planting also offers several additional 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 lousy job at permanently sequestering the carbon. Once a tree dies, any stored carbon can re-release back into the atmosphere. This storage issue is particularly relevant given that forests set aside as carbon offsets were burned this summer during the wildfires.
The science of trees is also complicated. CO2 absorption will vary depending on the type of tree, the region they planted, and the forest’s density. It is also impossible to create a closed system to quantitatively measure the exchange of all gases in the wild. Optimizing mass reforestation as a large-scale negative carbon strategy requires improvement in the underlying science.
Finally, trees are quite land-intensive. While sources vary considerably, most suggest that an acre of forest can 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 U.S. carbon emissions, over 25% of the landmass would need to be reforested. Young trees also absorb less carbon dioxide since it takes decades to reach their peak growth rate (though some research suggests young forests absorb more CO2 than previously thought). Vibrant forests also require good quality land, meaning reforestation could compete with the agricultural sector. It wouldn’t be possible to “reforest” the tundra of northern Alaska or Nevada’s deserts.
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 1980s and 1990s. Today, it can only 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 direct air capture approaches. While this analysis might be overly-optimistic, it is reasonable to believe that preserving the existing rainforest may be one of the most impactful measures to stop climate change.
Beyond conversations about “how” one might capture carbon, there is also a real debate about how much carbon we “should” be trying to capture. This isn’t about climate change skepticism, but the incentives we use to structure the economy.
Imagine all the world governments decided to implement no 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, costing trillions of dollars. Not only that, we would need to construct 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 on such a large scale.
Unless there are some astonishing breakthroughs in the technology, it seems unlikely that carbon capture alone will be enough to stop global warming. This strategy will need to happen alongside reduced carbon emissions. Some worry that attention and resources used on carbon capture will lead to underinvestment 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. Some activists are concerned that these companies are using carbon capture to delay the transition away from fossil fuels or that big oil is attempting to profit from 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. There are broader questions 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 soon.