The daunting physics of carbon removal

Anthropologists believe our ancestors first used fire as a tool nearly two million years ago. Eventually, fire became a necessity for cooking and warmth. Then, 4,000 years ago, dwellers in modern-day northern China discovered a black rock that burned better than wood: coal.

Today, we mine and consume an estimated 8.8 billion metric tons (tonnes) of coal every year, among other fossil fuels, freeing into Earth’s atmosphere billions of tonnes of carbon that had been locked away in Earth’s crust for hundreds of millions of years. That carbon dioxide, we now know, is blanketing our planet — trapping heat, supercharging hurricanes and heat waves, and melting vast expanses of sea ice and glaciers.

As countries race to drive their annual greenhouse gas emissions to net zero by 2050, some are contemplating a different question: What can we do about the 1.5 trillion tonnes of carbon dioxide we’ve already added to our atmosphere?

On Jan. 27, APS released a new report, “Atmospheric Carbon Dioxide Removal: A Physical Science Perspective,” that aims to answer this question. The four authors of the report — which was commissioned by the APS Panel on Public Affairs — are Washington Taylor of the Massachusetts Institute of Technology, Jonathan Wurtele of the University of California, Berkeley, APS Past President Bob Rosner of the University of Chicago, and APS President-elect Brad Marston of Brown University.

The report summarizes the current state of available carbon dioxide removal (CDR) technologies and outlines recommendations for policymakers. Above all, the report emphasizes that in most cases, cutting current carbon emissions is easier and less costly than large-scale, engineered carbon dioxide removal efforts may ever be.

Major findings of the report

Human activity emits a total of 35 billion tonnes (35 gigatons) of carbon dioxide every year. Given the scale-up effort needed, removing just 1 gigaton would be the equivalent of a baseball batter “getting on first base,” says Marston. Yet at a time when the world is trying to bring as many new renewable energy producers online and drive down annual emissions, diverting clean electricity to carbon capture efforts would be “a huge ask,” he says.

Even so, the report provides a summary of the carbon dioxide removal technologies in the pipeline, distinguishing between once-through and cyclic approaches, in part because the categories have distinct energy and material needs.

Taylor uses a simple metaphor to distinguish between these two approaches. “Imagine we are in a boat full of water,” he says, and “we are in danger of sinking.” To get the water out, we could either run through a bunch of paper towels, or we could use one sponge, and put in the extra effort — energy — to squeeze out the sponge before reusing it.

Paper towels represent a once-through process: We use a paper towel and discard it. But a sponge is cyclic, because we could use it “over and over without using up any materials,” he says.

One cyclic removal technology is chemical direct air capture, which relies on a solvent or solid sorbent to ‘capture’ carbon dioxide from air that fans pull through the system. Because the carbon dioxide in the atmosphere is dilute — just 420 molecules of every million molecules in the air — “a lot of energy has to be expended just to concentrate the carbon dioxide in the air,” says Marston. It’s an example of “basic physics at work.”

Compressing the carbon dioxide into a liquid and injecting it into the ground requires even more energy. Plus, to remove 1 gigaton of carbon dioxide — just 3% of what humans add every year — these systems would need to process the same amount of air that all the air conditioners in the world currently process in one year.

And to have a significant impact, Marston says these systems would have to consume power “comparable to a large fraction of the total electric power output of the United States” — one of the highest energy producers and consumers, per capita, in the world.

The report explores two once-through approaches: enhanced rock weathering and ocean alkalinity enhancement, both of which rely on finely ground minerals. The minerals could be scattered across a land surface to suck carbon dioxide out of the air or mixed into ocean water, to drive up its alkalinity and enable carbon dioxide absorption from the air above.

“The reactions are generally exothermic,” meaning they don’t require energy inputs to capture the carbon dioxide, Marston says. This reduces the total amount of energy needed to capture a comparable amount of carbon dioxide as a cyclic process, but it also requires more material. To have a significant impact, we would need to quarry and process a similar amount of rock as we do for the global production of cement — the binder used in concrete, the most-consumed material on the planet.

The report also considers ecosystem-based approaches, which allow the natural environment, like trees, grasses, and soil, to capture and store carbon. In many cases, Taylor says, these approaches are “more economical than engineered approaches and can have a variety of co-benefits, such as improving water and air quality and helping with the biodiversity crisis,” though he cautions that ecosystems are vulnerable to carbon-releasing events like wildfires.

Recommendations for policymakers

The report’s authors acknowledge that carbon dioxide removal may one day be necessary, despite its energy costs. Thus, the report recommends that R&D investments in CDR technologies still be pursued — but only “selectively and prudently.”

“These kinds of technologies are something that we need to explore and have ready if necessary,” says Marston. “But whether they can be scaled up is a big question,” which is why the report also emphasizes that reducing carbon emissions today is the most direct way to decrease future carbon dioxide levels.

Marston gives an example of a geothermally powered direct air capture plant recently brought online by Climeworks in Iceland. The plant made splashy headlines, billed as the first “large-scale” CDR plant in the world — “but you would need a million of those plants to absorb all of our annual carbon emissions,” says Marston. Scaling up enough to have a meaningful impact would require a “mind-boggling” amount of effort and energy, he says.

This is why the report also suggests ecosystem-based approaches, like reforestation and shifts in agricultural practices, which can be cheaper and help reverse the disruption of human activity.

“Certain ecosystem-based CDR approaches could be our chance to get some part of this right,” Taylor says, despite factors that limit their potential, like conflict over land usage and difficulty guaranteeing long-term durability. For example, wildfires are unpredictable — and increasingly common.

The report further cautions that effective planning for carbon dioxide removal will require extensive new generation of carbon-free power, like solar or nuclear, and that once-through approaches still need their effectiveness confirmed before being considered for wide-scale deployment.

“The main drawback is you’re dealing with this open system” — an ocean with currents or a field subject to runoff into a river during rain — that makes it “very hard to quantify how much carbon dioxide is actually being absorbed,” says Marston. “How to quantify this measurement is very important and much less straightforward than it is for direct air capture.”

For this reason, the report underscores the need to develop reliable systems of measurement, reporting, and verification, so that scientists will know if efforts are working and whether countries are actually meeting targets. “That will require rigorous standards,” says Marston — akin to international standards for methane emissions and fluorocarbons.

Lastly, the report recommends that policymakers develop economic and policy frameworks for carbon management, and weigh the benefits of implementing large-scale carbon dioxide removal technologies against approaches for reducing emissions.

Can we solve climate change?

Taylor, a physicist who specializes in energy systems, says that as recently as 2006, “the real story” of climate change still wasn’t clear to him. He wanted to learn more, and in 2021, his interest in climate change led him to POPA. Marston joined the panel a year later.

Although APS had previously published a report on direct carbon capture from the atmosphere, it didn’t take much for another POPA member, Bill Collins, to convince Taylor to revisit the topic. “The field has been advancing rapidly,” says Marston. The timing was right.

Marston’s own interest in climate physics dates to his graduate studies. “It was around the time that Jim Hansen, the NASA scientist, testified to Congress that there was a sign of global warming,” he says.

In his 1988 testimony, Hansen reported that air temperature data from several meteorological stations indicated 0.5-0.7 degrees Celsius of warming, on average. With records indicating warming across both hemispheres, Hansen told Congress he was “99 percent sure” the evidence pointed to a global warming phenomenon. He also noted that 1987 was one of the two warmest years in the entire historical record.

In 1989, the United Nations established the Intergovernmental Panel on Climate Change to provide a deeper scientific perspective. Predictions given in the IPCC’s first report, published in 1990, included more severe droughts and heat waves, more powerful hurricanes and typhoons, and a sea level rise of 11 to 38 inches.

With the IPCC’s Sixth Assessment Report on the topic published just three years ago, in 2022, humanity has now witnessed every major prediction of its first report.

“The evidence is that people are not acting quickly enough and that the impacts of climate change will get worse and worse,” says Marston.

That’s why climate science research is vital, says Taylor. “We cannot make intelligent decisions as a society about how to move forward without understanding what is happening with [our] climate … [and] physicists can play an important role in this effort.”

While the POPA report focuses on the science of carbon dioxide removal from the atmosphere, Taylor says the core message is that “it will take a lot of energy and material to do CDR at the gigaton scale” needed to have an appreciable impact. Hence, the clear need for “a system of policies that balances the challenges of CDR with the challenges of curbing carbon emissions,” he says.

“This is a complex problem, but at this point, climate and carbon management is really a social and economic challenge,” says Taylor. In other words, scientists know how to solve the problem. But can world leaders work together and devote the necessary resources to put effective solutions in place?

Until then, Marston cautions that the dramatic weather events we’ve witnessed in recent years are “just a taste of what’s to come.”

To learn more, read the APS report or join the report authors online for a moderated panel discussion on Feb. 27.