Can taking carbon dioxide from the atmosphere and splitting it into carbon and oxygen help stem the tide of rising greenhouse gases? (2024)

While CO₂ molecules can be broken down and re-used, there are some technological hurdles to clear before we can do this cheaply, put the carbon to good use, and not emit more greenhouse gases in the process.

July 11, 2022

Let’s break this question into two parts: first, pulling CO₂ out of the atmosphere, and then splitting the CO₂ molecules.

The use of technology to pull CO₂ out of the atmosphere is called direct air capture (DAC). Most commonly, machines suck in surrounding air and then chemically bind it with a non-toxic solution to separate out the CO₂. Currently, DAC comes with high energy costs: one study estimates that the energy needed to run DAC machines as a primary climate solution would be the same as the current annual energy needs of Japan, China, the US, and the EU combined.¹ To make DAC a climate solution, this energy would need to come from low- or no-carbon sources, such as wind, solar and nuclear power—which raises the question, should this massive amount of low-carbon energy be used to power DAC machines, or support our current energy needs and replace fossil fuels? Because of the complications this creates, DAC has been cited by the Intergovernmental Panel on Climate Change (IPCC) as just one of a suite of technologies and practices for limiting global warming to 2°C.²

CO₂ splitting has also gained interest among those who want to capture CO₂ before it goes into the atmosphere. A suite of technologies called carbon capture, utilization, and storage (CCUS) can be used to capture CO₂ from major land-based sources, like factories or power plants. CCUS is potentially a way to eliminate the carbon emissions of otherwise hard-to-decarbonize industries, like making chemicals and steel.

Captured CO₂—from DAC or CCUS—can be dealt with in a number of ways: compressed and stored underground, or injected into the ground to extract oil (called “enhanced oil recovery,” which isn’t a climate solution). Or it can be used for other purposes, which does, often, require the CO₂ molecules to be split.

When split, CO₂ separates into carbon monoxide and oxygen. (You don't generally get pure carbon, which is a bit of an obstacle because carbon monoxide is poisonous and a troublesome pollutant in its own right.) These molecules are not greenhouse gases, so they don’t warm the atmosphere. And they can be used as building blocks for making synthetic fuels, such as methanol, low sulfur diesel, butane, propane, and jet fuel, and even building supplies. In fact, there is a $20 million XPrize competition that funds projects that can turn CO₂ into usable products.

However, the current process for splitting CO₂ requires high temperatures (1300° F/700° C or more) to spark a series of chemical reactions that cause the molecules to separate. This process is energy-intensive, and currently accomplished by burning fuels that—guess what—emit more greenhouse gases.

Scientists are, however, exploring low-carbon ways of splitting CO₂. For example, James Miller, a chemical engineer and professor at Arizona State University’s Global Institute of Sustainability and Innovation, is part of a team that’s designing a new thermal chemical reactor that would use solar energy to split CO₂ molecules. The reactor would mimic photosynthesis by using concentrated solar thermal energy (that is, heat from the sun), water, and CO₂ captured from smokestacks (not the atmosphere), to produce liquid, energy-dense hydrocarbon fuels.

CO₂ molecules can be split through a few other processes, too. Some methods are actually low-carbon, using new chemical catalysts or photocatalysts to trigger chemical reactions that can break apart CO₂ without applying high heat.^3,4

One benefit of splitting CO₂ is that it creates a financial incentive for pulling excess CO₂ out of the atmosphere (DAC) or capturing it from smokestacks (CCUS), since the captured CO₂ can be used to create marketable products. Another benefit is that these molecules might one day be used to create cleaner high-energy fuels, as is the aim of Prof. Miller’s reactor.

But until low-carbon energy sources can be used on a large scale to both capture and split CO₂, the process won't be a major climate solution. The role that splitting CO₂ will play in the climate mitigation puzzle hinges on whether (and how) researchers overcome the energy challenges involved in DAC and CCUS, and turn the CO₂ into products that are cost-competitive on the market. At the moment, there’s no clear best solution to that problem, Miller says. “It comes down to, not only the science, but which technology develops the fastest, politics, and the ethics.”

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