Human endeavors continue to inject billions of tons of carbon dioxide into the environment annually, elevating global temperatures and provoking extreme climatic phenomena. While nations struggle with the repercussions of climate and seek substantial carbon emission reductions, numerous initiatives have been launched to enhance carbon dioxide removal (CDR) technologies that effectively extract carbon dioxide from the atmosphere and store it for extended durations.
In contrast to carbon capture and storage technologies that target carbon dioxide emissions from specific sources such as fossil-fuel facilities, CDR focuses on extracting carbon dioxide molecules that are already present in the atmosphere.
A recent report by the American Physical Society, spearheaded by a physicist from MIT, offers a summary of the primary experimental CDR techniques and assesses their fundamental physical limits. The report emphasizes methods that hold the greatest potential for extracting carbon dioxide on a gigaton scale annually, which would be necessary for a climate-stabilizing effect.
The latest report was commissioned by the American Physical Society’s Panel on Public Affairs and was published last week in the journal PRX. Washington Taylor, a professor of physics at MIT, chaired the report and discussed CDR’s physical constraints and its relevance alongside global initiatives to cut carbon emissions with MIT News.
Q: What inspired you to examine carbon dioxide removal systems through a physical science lens?
A: The primary driver of climate change is our extraction of carbon that has remained underground for 100 million years and releasing it into the atmosphere, resulting in warming. Recently, both government and private sectors have shown significant interest in developing technologies aimed at directly eliminating CO2 from the atmosphere.
Addressing atmospheric carbon management is the essential challenge in mitigating our influence on Earth’s climate. Thus, it is crucial to ascertain if we can manipulate carbon levels not only by altering our emission patterns but also by actively removing carbon from the air. Physics provides important insights on this matter, as the options available are significantly constrained by thermodynamics and mass considerations.
Q: Which carbon dioxide removal techniques did you assess?
A: All the techniques are in early development stages. It resembles the Wild West in terms of the diverse methods companies propose for atmospheric carbon removal. This report categorizes CDR methods into two types: cyclic and once-through.
Imagine being in a boat with a leak that is quickly filling with water. Naturally, we want to seal the leak as swiftly as possible. But even after fixing it, we must remove the water to avoid capsizing. This becomes especially critical if the leak is not entirely resolved, causing a slow infiltration. Now, picture having a couple of choices for removing the water to prevent sinking.
The first method involves a sponge that we can use to soak up the water, which we can then squeeze out and reuse. This cyclic approach implies utilizing a material repeatedly. There are cyclic CDR methods like chemical “direct air capture” (DAC), which essentially function like a sponge. You establish a large system with fans that circulate air over a material that captures carbon dioxide. When the material is saturated, you seal the system and then use energy to extract the carbon and store it in a deep reservoir. Afterward, you can reuse the material in a cyclic fashion.
The second category of methods is termed “once-through.” In the boating analogy, this would be akin to attempting to fix the leak with paper towel rolls. You allow them to absorb water fully, then discard them overboard, using each roll just once.
Once-through CDR strategies, such as enhanced rock weathering, are structured to expedite a natural process wherein specific rocks absorb atmospheric carbon when exposed to air. Globally, this natural weathering is estimated to extract approximately 1 gigaton of carbon yearly. The “enhanced rock weathering” method involves extracting a large quantity of this rock, pulverizing it to a size smaller than a human hair, to accelerate the process. The concept is to extract, spread, and absorb CO2 in one fell swoop.
The fundamental distinction between these two methods lies in the cyclic process being governed by the second law of thermodynamics, which imposes an energy constraint. You can derive an actual limit from physics indicating that any cyclic operation will require a certain amount of energy, which is unavoidable. For instance, for cyclic direct-air-capture (DAC) facilities, based on second law limits, the absolute minimum energy necessary to capture a gigaton of carbon is comparable to the total annual electrical energy used by the state of Virginia. Existing systems under development consume at least three to ten times that energy per ton and capture tens of thousands, not billions, of tons. Such systems also need to circulate substantial air; the volume of air required to pass through a DAC system to capture a gigaton of CO2 is similar to the total air traversing all cooling systems globally.
Conversely, with a once-through process, one could potentially bypass the energy constraint, but now faces a materials limitation dictated by the fundamental principles of chemistry. For once-through methods like enhanced rock weathering, this implies that capturing a gigaton of CO2 requires, roughly, a billion tons of rock.
To extract gigatons of carbon through engineered techniques necessitates enormous quantities of physical materials, airflow, and energy. Meanwhile, all the actions contributing to atmospheric CO2 emissions are extensive as well, presenting large-scale emission reductions with similarly significant challenges.
Q: What are the report’s findings regarding the feasibility and methods for removing carbon dioxide from the atmosphere?
A: Initially, we believed that CDR would demand an overwhelming amount of energy, an inevitability due to the second law of thermodynamics, irrespective of the method.
However, there is a nuance involving cyclic versus once-through systems. We navigated between two perspectives. One perspective holds that CDR is a panacea, allowing us to solely rely on CDR without addressing emissions — simply extracting all CO2 from the atmosphere. This perspective is misleading. Large-scale CDR will indeed be costly and energy-intensive. Conversely, another viewpoint dismisses CDR entirely, arguing that even contemplating it undermines emissions reduction efforts. The report arrives at a middle ground, asserting that CDR is neither a miraculous solution nor entirely out of the question.
If we are genuinely committed to addressing climate change, we will likely require considerable CDR alongside rigorous efforts to reduce emissions. The report concludes that research and development in CDR methods should be pursued judiciously and strategically despite the anticipated costs and resource needs.
At the policy level, the primary takeaway is the necessity for an economic and regulatory framework that encourages both emissions reductions and CDR within a unified approach; this would enable the market to optimize climate solutions. Since in numerous instances, it is often simpler and more affordable to minimize emissions than it may ever be to extract atmospheric carbon, a clear comprehension of CDR challenges should incentivize swift emissions cuts.
Personally, I’m optimistic because from a scientific viewpoint, we understand what is required to reduce emissions and employ CDR to lower CO2 levels slightly. The challenge now lies in societal and economic realms. I believe humanity possesses the capability to tackle these issues. I hope we can identify common ground to implement actions that will serve both humanity and the broader ecosystems on our planet, before encountering even greater challenges than those we currently face.