by Sandra Steingraber, SEHN senior scientist and writer in residence

In 1822, a free Black man in South Carolina, Denmark Vesey, attempted to lead a revolution. The plot included the mass execution of slave owners and the emancipation of enslaved people from the city of Charleston. Thousands secretly pledged to participate. However, before they could carry out this uprising, the plan was leaked and Vesey was hanged—along with 34 of his co-conspirators.

At the same moment, across the ocean in France, another man was doing something else with the barrel of a gun.

Baron Charles Cagniard de la Tour dropped a flint ball down its chute, filled it with various liquids, sealed it up, exposed it to heat, spun it around and noticed that at a certain temperature, the splashing sounds inside the barrel entirely disappeared.

Thus was discovered, in the same year a massive slave revolt was put down, the supercritical state of fluids.

***

Somewhere in middle school science class, we all learned about the three phases of matter: solid, liquid, and gas.

Water turns to humidity via evaporation. And the humidity of our exhaled breath becomes droplets on the windshield via the phase change called condensation. Water in the freezer becomes ice cubes. Snow in the sunlight vanishes via sublimation—a physical transformation that skips the liquid phase altogether and sends a solid into the air as a vapor.

A liquid, by definition, is something that has a volume but no shape. It flows. Its shape is whatever container it is placed within. You can leave off the lid, and there it is. A liquid, critically, can dissolve things. Like sugar in tea.

By contrast, a gas has neither shape nor volume. It’s the holy ghost of the trinity. The molecules of a gas bounce around and, if you open the lid, they leave. Whereas water splashes on the floor, steam rises, whistling as it goes, and escapes the tea kettle. Also, critically, a gas has the power to effuse through solids.

Supercritical fluids blur the distinction between liquid and gas. They flow like a liquid and can serve as a solvent, but they can also pass through solid materials like a gas. Supercriticality is created when a substance is brought above a certain temperature and pressure—the critical point—and its phase boundary disappears.

Flowing like a liquid and effusing like a gas means that the chimeras called supercritical fluids are used to extract things.

***

A half century after the gun barrel discovery, in 1869, physicist Thomas Andrews figured out the critical temperature (88o Fahrenheit) and pressure (1070 pounds/square inch) to compel carbon dioxide to exist in a state exactly poised between liquid and gas.

And 100 years after that, in 1970, the coffee industry figured out how to use the hybrid powers of supercritical CO2 for creating decaf.

Its vaporous qualities allow the CO2 to slip freely through the tiny pores within coffee beans. Meanwhile, its fluid properties allow it to dissolve and carry out the molecules of caffeine as it wafts through—and without stripping the beans of the chemicals that give it its signature aroma when roasted.

Soon after, the flavor and fragrance industries began deploying supercritical CO2 technology to extract flavors and aromatic oils from plants. More recently, it’s been used as a non-toxic, dry-cleaning solvent to clean fabrics.

Here’s how one leading company that makes extraction technology for foods and pharmaceuticals extols the miracle of supercritical CO2:

By the late 1970s, carbon dioxide had become the most sought-after fluid due to its low critical temperature and relatively low critical pressure. In addition, it is a nontoxic, nonflammable, noncorrosive, inexpensive and widely available gas that is easy to handle. As a result, it is recognized as a green solvent. If properly purified, it has a minimal effect on the quality of the extracted biocomponents, thus preserving their medicinal and functional properties.

***

More quietly in the 1970s, the fossil fuel industry began using supercritical CO2 to dissolve and extract crude oil from the pores of rocks that would otherwise refuse to flow the surface. And now, that technique—carbon dioxide enhanced oil recovery (CO2 EOR)— enables the entire fossil fuel industry to keep drilling even as their wells become depleted.

It works like this: molecules of gaseous CO2 from naturally occurring underground deposits—or, more recently, molecules of CO2 captured from a CO2-generating industry, such as power plant or an ethanol distillery—are pressurized into supercritical form. Next, they are piped to oil wells and pumped into depleted oil fields where they effuse through the microscopic pores of rocks, dissolve the residual oil still glued to their walls, and carry it up the surface. Ready to be piped, refined, and ignited

Which, at this stage of the climate crisis, is the opposite of a miracle.

***

Carbon capture and storage for the purpose of enhanced oil recovery also just seems like a hairbrained idea on the face of it. But, in the early 1920s, when the oil industry first began separating naturally occurring CO2 from the methane gas in natural gas reservoirs, oil drillers had ready supplies of CO2 on hand. Decades later, as more oil wells faltered, CO2 EOR became a standard practice for spiking production.

But it’s an expensive technology, and naturally occurring CO2 is a limited resource. As we document in our August 2024 report, The False Promise and Potential Health Harms of Carbon Dioxide Enhanced Oil Recovery (CO2 EOR) as a Tool of Climate Mitigation, the cost of buying CO2 now likely accounts for 25-50 percent of all CO2 EOR project costs, making it perhaps the single largest operational cost.

Enter the re-branding of CO2 EOR as a tool for mitigating the climate crisis. With subsidies in the form of tax credits overseen by the U.S. Department of the Treasury, the fossil fuel industry can essentially get paid for burying supercritical CO2 in the ground—even if the purpose is to extract more oil out of the ground, which perpetuates oil and gas extraction and generates more heat-trapping gases. Which we also document in our report.

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There are many reasons to suspect that the shapeshifter and all-around Houdini, supercritical CO2, will not stay imprisoned, forevermore, in depleted oil fields pin-cushioned with old and abandoned wells. Indeed, there are many reasons to predict that its eventual escape is inevitable. It tends to self-extract.

The most recent reason comes from Decatur, Illinois where the ethanol-maker Archer Daniels Midland (ADM) has operated the nation’s first commercial-scale carbon capture and storage operation since 2017. In this project, supercritical CO2 is simply injected into the ground in a nearby site specifically chosen to serve a high-security penitentiary for CO2 because it is overlain by an impermeable cap rock. Carbon sequestration, not oil recovery, is its sole purpose.

And yet, as recently revealed by investigative reporters, the operation has, on at least two occasions, sprung a leak. The first malfunction was detected in September 2020. By November 2021, a monitoring well had developed a surface leak and by January 2022 was entirely dysfunctional. A subsurface leak of CO2 followed shortly thereafter. And by August 2024, the U.S. Environmental Protection Agency had issued a notice of violation.

No one knows exactly where the missing 8,000 metric tons of CO2 are. No one knows if the caprock cracked, but there are signs that briny water is now moving between geological formations.

In October 2024, ADM suspended the injection of CO2 at its storage site.

***

Before he was executed, Denmark Vesey was a community leader and a co-founder of the African Methodist Episcopal Church, also known as Mother Emanuel. It was the nation’s first independent Black church.

He was never able to buy his wife and children out of slavery. Emancipation was another 43 years away.

After the Civil War, Denmark’s son, Robert Vesey, rebuilt the church to replace the original structure, which had been burned down. In 1962, Reverend Martin Luther King spoke at the church, urging members to register and vote. In 2015, the 21-year-old white man, Dylann Roof, shot and killed nine parishioners inside Mother Emanuel.

Denmark Vesey’s contemporary, Baron Charles Cagniard de la Tour, not only discovered the slippery properties of supercritical fluids inside the barrel of a gun—which are now being used to extract fossil fuels—he was also the inventor of the emergency siren, which he named after the mythological bird-woman who lured sailors to their deaths.

The siren that now warn us of extreme weather events.

The weather events that are more and more frequent and more and more intense because of a rapidly destabilizing climate.

The climate that is destabilizing because of fossil fuels.

The fossil fuels that are increasingly extracted with a gas-liquid called supercritical CO2.

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RePercussion Section: Effusing the Oil Fields—A Short History of Supercritical CO2