The concentration of carbon dioxide in the world’s atmosphere is now very close to 429 parts per million.1 That’s not just the highest level ever directly measured, it’s the highest in more than three million years, higher than humans have ever experienced.
That’s a direct result of carbon dioxide emissions from fossil fuels and industry, which reached a record 37.4 billion metric tons in 2024.2 The world’s oceans, plants, and soils absorbed more than half of that, but the CO2 that stayed in the air increased the total by about 15 billion metric tons, further fracturing the global carbon cycle and intensifying what the UN Secretary-General says should now be called the era of global boiling.3
If all anthropogenic emissions were to stop tomorrow, natural processes would gradually reduce the amount of CO2 in the atmosphere to safer levels, but the key word is gradually. As a leading climate scientist writes,
The lifetime of fossil fuel CO2 in the atmosphere is a few centuries, plus 25 percent that lasts essentially forever.
The climatic impacts of releasing fossil fuel CO2 to the atmosphere will last longer than Stonehenge. Longer than time capsules, longer than nuclear waste, far longer than the age of human civilization so far. Each ton of coal that we burn leaves CO2 gas in the atmosphere. The CO2 coming from a quarter of that ton will still be affecting the climate one thousand years from now, at the start of the next millennium.4
Other scientists put the very long-lasting fraction at 20 percent, but that is a trivial difference given that the amount of CO2 in the atmosphere is now a trillion tons greater than when direct measurements started in 1958. Many centuries from now, that CO2 will still be keeping the Earth’s temperatures well above pre-industrial levels.
The most widely promoted solution is variously called Carbon Dioxide Removal (CDR) or Negative Emissions Technology (NET)—synonyms for using technology to remove carbon dioxide from the atmosphere. Indeed, most plans advanced for keeping warming below 1.5 degrees include CDR as a necessary component. Climate policy planners in every country have accepted the fossil fuel industry’s argument that it is not practical to reduce emissions rapidly, so we must find some means of pulling CO2 back out of the air faster than it is emitted.
Most plans assume that global temperatures will rise higher than the target and later be pulled back by carbon dioxide removal technology. How much warmer will it get? How much longer will CO2 concentrations be unsafely high? How much irreversible damage will be done to Earth and its inhabitants in the meantime? No one offers encouraging answers to those questions.
The Economist, authoritative voice of free market capitalism, tells us that passing 1.5° “does not doom the planet … but it is a death sentence for some people, ways of life, ecosystems, even countries.” And for that reason,
Technologies to suck carbon dioxide out of the atmosphere, now in their infancy, need a lot of attention.5
The editors of The CDR Primer argue that while priority should be given to cutting emissions, CDR must be deployed at the same time, because some industries will be unable to get to zero,
even with a massive global effort to cut climate pollution.
The scale of such emissions requires a massive investment in CDR, likely on the order of gigatons of CO2 removed per year by mid-century. Even larger amounts would have to be removed to draw down atmospheric concentrations from their peak after we reach net-zero global emissions.6
That may be what we need, but what is actually possible?
If anthropogenic greenhouse gas emissions continue, temperatures will rise unless concentration increases are matched by removals, which would require removing billions of tons of gas every year. Setting aside the obvious barriers of politics and cost, what physical requirements would have to be met for CDR to stop or (better) reverse global boiling?
Atmospheric Carbon Dioxide Removal: A Physical Science Perspective, a peer-reviewed scientific study published in January by the American Physical Society (APS), provides an authoritative response to that question, and it isn’t encouraging.7 Quite apart from the problems of storing captured CO2 for thousands of years, which the study doesn’t address, the scale of the problem is forbidding.
Carbon dioxide capture at oil refineries, where the gas is highly concentrated, has existed on a limited scale for decades. The CO2 is most often used for “enhanced oil recovery,” forcing still more carbon-emitting oil out of the ground.
Technologies for removing carbon dioxide from the open air, on the other hand, are new and untested. There are only a few dozen operating systems, and all are tiny, compared to the task at hand. The authors of the APS report examine a dozen proposed technologies, focusing in particular on scaling—how big can they get? While none is adequate today, three appear to have promise: direct air capture, biological carbon capture, and enhanced rock weathering.
Direct Air Capture (DAC)
DAC gets the most press. You’ve likely seen pictures of demonstration plants, with giant fans blowing air through chemical filters that absorb carbon dioxide. When the filters are saturated, the CO2 is extracted, and the chemical is reused.
The best-publicized DAC installation is the Climeworks project in Iceland, whose founder predicted that it would capture 1% of the world’s annual CO2 emissions by 2025. It has sold thousands of $250/month “carbon removal credits” each supposedly representing a quarter of a ton (250 kilograms) of captured CO2.8 In fact, as an investigative report in the Icelandic newspaper Heimildin showed in May 2025, it has captured only 2,400 tons in total since 2021. Its own emissions are much higher than that.9
To be fair, the technical challenge facing DAC is enormous. A CO2 concentration of 429 parts per million is enough to significantly change the world’s climate, but in absolute terms it is a tiny fraction of the atmosphere—carbon dioxide comprises only 0.062% of the air’s weight, only 0.04% of its volume. To remove one ton of CO2, a perfectly efficient Direct Air Capture system would have to process 2,000 tons of air.10
As the APS report comments, if we outfitted every air conditioning unit in the world with devices enabling complete capture of all CO2 in all the air flowing through them, they would remove less than a billion tons a year.11 Just stabilizing the global temperature would require over 15 times as much equipment; turning down the heat would require much more.
We’re told that the technology will improve over time—just look at the exponential improvements in semiconductors, for example—but DAC faces a limitation that computers don’t: the second law of thermodynamics. First stated by German physicist Rudolf Clausius in 1850, it says that all systems become more disordered over time. Entropy (disorder) increases and the only way to reverse it is by adding energy from outside. That’s not speculation—it is as solid a physical law as any ever discovered. There are no exceptions.
CO2 molecules in the air are highly disordered, scattered at random among vastly more molecules of nitrogen, oxygen and other gases. The second law of thermodynamics allows physicists to calculate, very accurately, how much energy would be needed to capture all the CO2 molecules in a given amount of air and consolidate them in one place.
The calculation applies no matter what technology is used, because we’re only measuring how much more energy there is in the consolidated output than in disordered atmospheric CO2. That’s the minimum amount of outside energy needed in a perfectly efficient process—real-world processes will require more, usually much more.
Fortunately for the equation-averse, the APS authors have done the calculation for us. Removing one ton of CO2 from the atmosphere, in a perfect system, requires 120 kilowatt hours of energy. Someone once said that the second law doesn’t tell you what you can do, it tells you what can’t be done. In this case it isn’t saying that you can capture a ton of CO2 with 120 kilowatts—it is saying that you can’t do it with less, no matter what technology you use.
Scaling up, that means that removing one billion tons a year would require a minimum of 14 billion watts of electricity, 24 hours a day, all year, which today would mostly come from CO2 emitting power plants. Realistically, less-than-perfect CDR would require three to ten times that much energy to remove one billion tons—enough to power New York City several times over, while actually adding tons of CO2 to the atmosphere.
Removing the hundreds of billions of tons actually needed to get Earth’s temperature back to preindustrial levels would require a very large fraction of the world’s energy supply—to the point where it would limit our ability to carry out other energy-hungry programs, including feeding children and combatting pandemic diseases.
For Direct Air Capture’s contribution to reducing atmospheric CO2 levels to be more than marginal, there would have to be massive improvements in DAC technology itself, and qualitative leaps in the capacity and efficiency of solar energy systems, so that every DAC installation can have its own high output zero carbon solar power plant. Since neither is likely to occur in time to prevent dangerous global warming, neither belongs in today’s climate change prevention plans.
Bioenergy with Carbon Capture and Storage (BECCS)
Can we get around the Second Law limit by using natural energy over a longer time? For example, there is enthusiasm in some circles about Bioenergy with Carbon Capture and Storage, which would involve growing crops or trees, then burning them to produce electricity while capturing and burying the released CO2. Trees naturally remove atmospheric CO2 using energy from the sun (essentially a free and unlimited source), and the CO2 at the point of burning would be easier to capture because it would be highly concentrated.
Clouds of emissions pour from Drax Power Station in Yorkshire, England. (NRDC)
The world’s largest BECCS operation, the Drax Power Station in UK, says it will capture eight million tons of CO2 a year by 2030, about 3% of the UK’s annual emissions, but that is misleading. Drax doesn’t burn trees that were purpose-grown in Britain—it imports and burns wood pellets manufactured overseas, including hundreds of thousands of tons a year made from old growth forests in central British Columbia.12 In other words, Drax is “capturing” CO2 that was already stored in some of the world’s most magnificent forests.
What’s more, the Drax plant itself is the largest CO2 emitter in Britain, pumping 12 million tons into the atmosphere every year. International carbon accounting rules give it a free pass, on the specious grounds that the forests will regrow, so their wood is a renewable fuel.
Leaving such scams aside, the physicists’ report points out that for a BECCS system that actually grows new trees,
the low efficiency of photosynthesis in converting energy to biomass… means that a lot of land would be required… This land use would compete with existing agriculture and biodiversity efforts.13
In IPCC scenarios for keeping the global temperature increase below 1.5°C that include BECCS, energy crops would occupy least 20% of all the world’s arable land by 2050, and much more if BECSS is the primary method used. Environmentalists have pointed out that such a massive change in land use would risk “compromising planetary health in areas besides climate change.”14 At each step—planting, fertilizing, harvesting, transporting, burning and burying—BECCS is emits its own greenhouse gases. The National Resources Defense Council calculates that if current plans go ahead, by 2040 “emissions from BECCS alone are projected to surpass the U.K.’s total emissions from all other sources.”15
In short, BECCS will do more environmental harm than good. The European Academies’ Science Advisory Council recently told the European Union that:
The role of bioenergy with carbon capture and storage (BECCS) remains associated with substantial risks and uncertainties, both over its environmental impact and ability to achieve net removal of CO2 from the atmosphere. The large negative emissions capability given to BECCS in climate scenarios limiting warming to 1.5°C or 2°C is not supported by recent analyses …
Deployment of BECCS at the scale in IPCC models could potentially help mitigate climate change, but at the expense of further exceeding the planetary boundaries related to biosphere integrity, land use and biogeochemical flows, while bringing freshwater use closer to its boundary… BECCS remains associated with substantial risks and uncertainties, both over its environmental impact and ability to achieve net removal of CO2 from the atmosphere.16
Enhanced Rock Weathering (ERW)
In the slow carbon cycle, a key part of the Earth System’s metabolism for hundreds of millions of years, exposed rocks combine with atmospheric carbon dioxide to form stable minerals that don’t affect the climate. That weathering process naturally removes about a billion tons of CO2 a year—it is one of the factors that has kept global warming lower than total emissions could otherwise cause.
Proposals have been made to accelerate the process by grinding tons of rock into tiny particles, about one-fifth the width of an average human hair, and spreading them over large areas of land. That would greatly increase the exposed rock surfaces and—some scientists hope—greatly increase the amount of CO2 removed. Since the U.S. and other countries already mine and transport very large quantities of ground rock for other purposes, no new technology would be needed and the energy cost would be much lower than for DAC or BECCS.
It would require a lot of land—an estimated one million square kilometers to remove a billion tons of CO2 a year—but unlike BECSS, it isn’t exclusive use. Ground rock can have fertilizing effects, so it could be spread on existing farmland, depending on local soil characteristics.
It’s important to note that the ground rock can only be used once: to keep the removal process working, billions of tons of additional rock will have to be extracted, ground, transported and spread, every year. Nothing even close to that scale has ever been done, and even if it were tried, no one knows how long removal might take, or how to measure the results.
Dead End
In October 2018, responding to a request from the UN climate conference that adopted the Paris agreement, the Intergovernmental Panel on Climate Change (IPCC) published a Special Report on Global Warming of 1.5°C. The report concluded that emissions to that date were not enough to cause a 1.5°C increase, but, “lack of global cooperation, lack of governance of the required energy and land transformation, and increases in resource-intensive consumption” mean that emissions will continue, inexorably driving up the temperature.18
Accepting that immediate or very rapid reductions won’t happen, the authors developed a series of alternative scenarios, all of which require some use of carbon dioxide removal to make up for the failure.
Most of the scenarios assume overshoot—that the global temperature rise will exceed 1.5°C or 2.0°C for some time and must be brought down by “CDR at a speculatively large scale.” To completely prevent overshoot, the report’s Summary for Policy Makers says, CDR would have to remove between 100 and 1,000 billion tons of CO2 by 2100. Looking at the report itself, it appears that the actual requirement would likely be close to the high end of that range.
This dependence on very large scale carbon dioxide removal calls the entire scenario exercise into question. As the report admits:
There is uncertainty in the future deployment of CCS given the limited pace of current deployment… No proposed technology is close to deployment at scale, and regulatory frameworks are not established. This limits how they can be realistically implemented… There is substantial uncertainty about the adverse effects of large-scale CDR deployment on the environment and societal sustainable development goals.19
That was in 2018. Seven years later, there are no working DAC operations, and the parallel emission reductions that the IPCC scenarios included haven’t begun and aren’t in sight. The IPCC report’s preferred option, BECSS, has lost what credibility it then had, and no one has volunteered to grind and spread rock dust.
What’s more, even if larger volumes of carbon dioxide can be captured and buried, there is no foolproof method of preventing it from leaking back into the atmosphere, either through gradual seepage or sudden disruption.20 Another IPCC report stresses that “CO2 storage is not necessarily permanent.”
CO2 stored in the terrestrial biosphere is subject to potetial future release if, for example, there is a wildfire, change in land management practices, or climate change renders the vegetative cover unsustainable. Although the risks of CO2 loss from well-chosen geological reservoirs are very different, such risks do exist.21
The danger of leaks is higher when CO2 is injected into old wells to force out the remaining oil and then left underground —not in “well-chosen geological reservoirs.” Bear in mind the oil industry’s long history of walking away from depleted wells without plugging them or monitoring ongoing leaks and emissions.
+ + + +
For fossil fuel corporations, keeping CDR on the agenda as a credible climate change solution is a Get Out of Jail Free card. Instead of stopping emissions, they promise to capture and bury them. Not now, but someday. As the CEO of Occidental Petroleum told a conference of her peers in 2023,
We believe that our direct capture technology is going to be the technology that helps to preserve our industry over time. This gives our industry a license to continue to operate for the 60, 70, 80 years that I think it’s going to be very much needed.22
A few months ago, the United Nations Environment Program warned that global emissions must be cut by 42% by 2030 and 57 per cent by 2035 to keep warming under 1.5 degrees.23 Achieving that, or anything close to it, would require an emergency action program to stop all new extraction of fossil fuels, and rapidly phase out major sources of emissions. That’s where our focus must be now, not on speculative technologies that might work one day but for now only give polluters an excuse to continue polluting.
Notes:
1. https://keelingcurve.ucsd.edu/
2. Global Carbon Project, “Briefing on key messages Global Carbon Budget 2024,” News Release, November 13, 2024. Agriculture, land use change and cement manufacture added another 8 billion tons or so.
3. United Nations, “Hottest July ever signals ‘era of global boiling has arrived’ says UN chief,” News Release 27 July 2023, https://news.un.org/en/story/2023/07/1139162
4. David Archer, The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth’s Climate, (Princeton University Press, 2016), 1.
5. “The world is missing its lofty climate targets. Time for some realism.” Editorial, The Economist, November 3, 2022.
6. J. Wilcox, B. Kolosz, and J. Freeman, editors, The CDR Primer, 2021, https://cdrprimer.org/
7. Washington Taylor, Robert Rosner, Brad Marston, and Jonathan S. Wurtele, Atmospheric Carbon Dioxide Removal: A Physical Science Perspective, (American Physical Society, 2025)
8. https://climeworks.com/subscriptions-co2-removal. Consulted June 25, 2025.
9. Bjartmar Oddur Þeyr Alexandersson and Bjartmar Oddur Þeyr Alexandersson, “Climeworks’ capture fails to cover its own emissions.” Heimildin, May 15, 2025
10. Calculation by Aatish Bhatia in the excellent Rate of Change blog, October 28, 2020.
11. American Physical Society, “APS Releases Report on Atmospheric Carbon Dioxide Removal,” News Release, January 27, 2025.
12. Joe Crowley, “Drax: UK power station still burning rare forest wood,” BBC, February 28, 2024.
13. Taylor et al, Atmospheric CDR, 14.
14. Felix Creutzig et al., “Considering sustainability thresholds for BECCS in IPCC and biodiversity assessments,” GCB Bioenergy, February 2021.
15. Matt Williams and Elly Pepper, The BECCS Hoax, National Resources Defense Council, 2024, 4.
16. European Academies’ Science Advisory Council, “Forest bioenergy, carbon capture and storage, and carbon dioxide removal: an update,” EASAC Commentary, February 2019, 2, 6.
17. Helen S. Findlay et al., “Ocean Acidification: Another Planetary Boundary Crossed,” Global Change Biology, April 2025.
18. IPCC, Special Report: Global Warming of 1.5°C (2018), 95.
19. Ibid, 158.
20. It has been suggested that CO2 buried in ultramafic rock would form solid carbonate minerals that can’t escape. That’s encouraging, but it hasn’t gone beyond small tests.
21. IPCC, Special Report: Carbon Dioxide Capture and Storage (2005), 373
22. Quoted in Corbin Hiar, “Oil companies want to remove carbon from the air—using taxpayer dollars,” E&E News, July 13, 2023.
23. United Nations, ‘Climate crunch time is here,’ new UN report warns, News Release, October 24, 2024.