“Drastically reduce emissions first, or carbon dioxide removal will be next to useless”
David Ho’s recent paper in Nature prioritises eliminating polluting activities.
David T. Ho is a climate scientist. He has spent his career studying the natural carbon cycle. In recent years, he’s focussed on developing methods for checking whether carbon dioxide removal (CDR) works.
He was a reviewer for the US$100 million XPRIZE carbon removal competition funded by Elon Musk and the Musk Foundation.
“I don’t deny the need to develop CDR methods over the longer term,” he writes in a recent article published in Nature. “And I welcome governments committing much-needed resources to this area.”
He notes that the US Bipartisan Infrastructure Law includes US$3.5 billion to develop four large-scale, regional direct air capture hubs. “But it’s clear to me,” Ho adds, “that deploying them to remove CO₂ from the atmosphere is pointless until society has almost completely eliminated its polluting activities.”
Ho suggests thinking of CO₂ removal as a time machine. In 2022, global CO₂ emissions reached 40.5 billion tonnes. Ho uses the example of the US direct air capture hubs. Each facility is expected to remove one million tonnes of CO₂ each year. Ho writes,
“At that rate, for every year of operation at its full potential, each hub would take the atmosphere back in time by almost 13 minutes, but in the time it took to remove those 13 minutes of CO₂, the world would have spewed another full year of CO2 into the atmosphere.”
Ho also looks at tree planting. If everyone planted a tree that would be an additional eight billion trees. Once those trees had matured (which means waiting for several decades) that would take us back by about 43 hours every year.
When Ho introduced the time machine idea on Twitter in February 2022, Jacob J. Bukoski, Assistant Professor, Forest Ecosystems and Society, at Oregon State University calculated how far back in time planting one trillion trees would take us. One trillion mature trees would take us back by 229 days.
“For the record, I am unconvinced we can (or should) grow 1 trillion trees,” Bukoski adds. “But I do think combined efforts have potential for substantial impact.”
The trouble with the one trillion tree proposal is that it is a massive and dangerous distraction from the need to leave fossil fuels in the ground. It would require a vast area of land - land that is in many cases already in use. The trees would have to survive - Ho describes failed tree planting projects as a “broken time machine”.
Ho writes that,
“The time-machine analogy reveals just how futile CDR currently is.
“We have to shift the narrative as a matter of urgency. Money is going to flood into climate solutions over the next few years, and we need to direct it well. We must stop talking about deploying CDR as a solution today, when emissions remain high — as if it somehow replaces radical, immediate emission cuts.”
Ho points out that it’s a different story if we reduce emissions to about 18% of the current level over the next 20 to 30 years. We would then need 7,290 direct air capture hubs, or we could use other CO₂ removal technology.
“In the meantime,” Ho writes, “research is needed to seek CDR methods that minimize land use and energy consumption, and can be scaled up rapidly and cheaply.” We need to carry out this research now so that the technology is available in the future.
Some techniques will work in the laboratory but may not work in the real world. Others may damage biodiversity and the environment. “Developing methods to verify that CDR works is a major challenge,” Ho writes. “It will be many years before we have the science to tell us which methods work and whether they harm or benefit the environment.”
Ho is clear that the key to addressing the climate crisis is rapid decarbonisation.
Ho writes that,
“Humanity has never removed an atmospheric pollutant at a global, continental or, even, regional scale — we have only ever shut down the source and let nature do the clearing up. This is the case for chlorofluorocarbons and stratospheric ozone destruction, for sulfur dioxide and acid rain, and for sulfur and nitrogen oxides and photochemical smog. We must be prepared for CDR to be a failure, leaving us to rely on the environment to stabilize atmospheric CO₂ over thousands of years. This is another argument for rapid decarbonization.”
Thank you for posting this important article.
Several points to note:
Carbon dioxide removal (CDR) is problematic because it is also taking away the oxygen. The oxygen MUST go back to the atmosphere, since we are killing off oxygen-producers everywhere. But if you try separating CO2 into carbon and oxygen, the laws of thermodynamics will still apply and more input energy will be required than you obtained when first burning your “fuels.” Plus, of course, entropy.
The fallacy of planting trees- A trillion trees would be very many forests. A forest is an ecological climax community, being the end-point of ecological succession of many species involved in soil and biodiversity development in that region. Often, a successful climax tree species will have found its root pathways to follow the dead and rotting root paths of an historical predecessor which also followed the old roots of a yet previous occupier of that spot; each tree generation making a bit more progress in working its root deeper into the soil. Just sticking a tree into bare ground does not a forest make.
As for direct CO2 capture, recent work has shown that recovery from water is more efficient than recovery from air.
Yet, we are still running about 2500 coal plants on this planet with hundreds more in planning. What’s wrong with this picture?