The Darkness at Noon
"A new plan to inject biochar into the stratosphere raises more questions than answers"
This week, the North American Biochar Conference 2025 came to Minneapolis. Biochar has finally begun to gain traction, despite being an orphan climate solution that has lacked government subsidies and incentives for many years. In this respect, biochar is not very different than solar cells, which were invented by Charles Fritts in 1883 and improved by Bell Labs in the 1950s. We all knew back in the 1970s that solar was the cheapest, safest, cleanest way to make electricity, but the government’s heavy thumb, especially the national weapons laboratories’, kept the money away until solar energy could no longer be held back. Even then, Western energy policymakers only let it compete because they feared China’s commercial hegemony.
In the greenhouse repair space, DACCS, or direct air carbon capture and storage (artificial trees), is the equivalent of tax-supported nuclear power (bomb factories) for power generation. If you can’t build enough reactors to power DACCS and also electric cars, steel plants, and data centers for A.I., there is always BECCS—biomass energy carbon capture and storage (genetically engineered monocrops burned while holding onto the smoke—kind of like smoking a joint). Wacky government market incentives aside, these charts of the present carbon capture market give us a radically different view than the government’s:
Recently my friend and mentor for all things biochar, Tom Miles, posted to a popular climate solutions forum:
I sort of agree with him that the absence of markets at the moment is what is holding back the expansion of biochar production, but as one familiar with biochar’s ramp-up into asphalt, concrete, and pyrolytic chemistry, I am also confident any lag in the markets is temporary.
Kathleen Draper and I pointed this out in our seminal book, Burn: Igniting a New Carbon Economy to End the Climate Crisis (2019). 50-plus gigatons per year could be a viable CO2 removal target for biochar were it not constrained by readily available feedstocks. Tom Miles rules out plantation forests, as well he should, and carbon wastes are ripe for pyrolyzing from municipal landfills, sewage plants and marine permaculture, among other sources, but even producing one gigaton (one billion metric tons) per year from all that is a stretch at this point. As we saw from the charts above, we are at 1 million tons per year now—0.002 percent of human greenhouse emissions in 2025. A one-thousand-fold increase is needed, and not by 2050. By 2030, if possible.
As we shift to a carbon economy, mining CO2 from the air, pyrolyzing it to inertinite, there is scant limit to the new products and services biochar can supply. Literally, the sky is the limit.
This past June, graduate student Haozhe He published a seminal article in Nature Communications Earth and Environment that may have been overlooked by most climate solutions technicians. We hear a lot about marine cloud brightening and related geoengineering interventions that will slow the heat gain on Earth’s surface a little, giving us time to stop polluting and deploy drawdown techniques like biochar and algal reforestation. Haozhe and his co-authors suggested an approach to aerosol injection that is both surprising and promising. They suggested injecting black carbon into the outer stratosphere.
Whenever you want some number to reach infinity, such as the annual budget of the Pentagon, just involve outer space.
This may be counterintuitive because up until now scientists have been exploring the effects of injecting reflective particles such as sulfate into the lower stratosphere or troposphere to reflect light back into space. Haozhe’s group said that if you move injection altitude up to 19 miles or higher (25 miles—or 40 km—for the greatest effect), the absolute cold of space increases the rate of outgoing heat (outgoing longwave radiation). By injecting black aerosols there, you absorb incoming solar radiation and allow it to convert to heat there rather than at the surface of the planet or in the ocean. That heat is quickly exhausted back to the infinite cold of space.
The study, which is still very preliminary, said:
Would biochar work as well as fly ash? That is controversial, and all we can really say is that experiments are needed, because so far, all the tests and modeling have used black carbon. While there are similarities—heat absorption, weight, optical behavior—there are also differences. Biochar has a lower skeletal density (~1400-1500 kg/m³) and much higher porosity compared to carbon black (~1800-2100 kg/m³), leading to differences in aerodynamic behavior and persistence. High porosity lowers the effective aerodynamic density of biochar particles because internal voids reduce mass without increasing drag significantly, which can decrease settling speed and potentially increase residence time.
Biochar contains various surface functional groups, residual ash, and some volatile organic compounds or heavy metals, depending on the biomass feedstock and pyrolysis conditions, potentially causing chemical instability or undesired atmospheric reactions with molecules like chlorine. Even if biochar were milled to a fine powder with high molecular carbon content, its higher porosity, irregular particle morphology, lower density, and variable surface chemistry might not be as well-suited for sunlight capture as carbon black.
Still, lofting carbon acquired from plants after photosynthesis has to be better than releasing more fossil carbon obtained from deep in the Earth. It certainly would be a whole lot cheaper.
How much would you need? Haozhe says 0.5 teragrams. That is 500,000 tons, or about a month’s global production of biochar right now, in 2025. Once it is up there, you might have to add some to keep the level replenished, but it will essentially be there for centuries. This is their chart of the temperature response when run through a climate simulation:
The red line is the temperature change from adding 5 Tg sulfate to the lower stratosphere. The blue line is the temperature change from adding 0.5 Tg biochar closer to the edge of space. The temperature is given in Kelvin but a change of 1 K is exactly equivalent to a change of 1°C. What we can see from the chart is that a one-time carbon injection drops surface temperature of Earth by about 0.8°C while an injection of SO4 drops temperature about 0.6°C. For Hansen fans, we are speaking of dropping Earth’s solar energy gain by 0.25 to 1.43 W/m2.
All of this is in the realm of idealized climate model simulations, as the team readily admits, and there are many unanswered questions.
One question is, how do you turn it off, once you have started it? The answer, so far, is you can’t. They don’t sell Dustbusters that large. Sulfate will descend back to the surface within a few weeks. Black carbon doesn’t. With SO4, that decay is helpful if you want to contain any unwanted effects, but also more expensive because you have to keep injecting if you want the cooling to continue. Biochar will descend, but much more slowly. It actually has a self-lofting effect, meaning it tends to drift up, not down.
Another question is rainfall. Even before the surface temperature begins to respond to a biochar injection, precipitation begins to drop. The authors say, “Preliminary analysis suggests that this suppression may stem from anomalous infrared absorption in the lower troposphere, possibly driven by unexpected increases in water vapor.” There is pronounced drying over the tropics and wetting over the subtropics, with stronger wetting in the Southern Hemisphere, accompanied by a southward shift of the Intertropical Convergence Zone.
Biochar is not alone in changing rain patterns when injected into the stratosphere. Looking at sulfate, the authors say:
A concern of both intervention strategies is the warming of tropopause temperatures, which can trigger a cascade of negative consequences — including increased water vapor input into the stratosphere, reductions in stratospheric ozone concentrations, and disruptions to both stratospheric and tropospheric circulations.
There are other unknowns. As they fall back to earth, biochar aerosols sediment onto snow and ice surfaces, causing the snow and ice to absorb more solar radiation rather than reflecting it. This effect triggers feedback mechanisms that could offset some of the cooling.
Biochar aerosols also absorb ultraviolet radiation which could locally reduce ultraviolet flux and mitigate ozone depletion, particularly for ozone closest below the aerosol layer.
What would it cost, particularly if continuous delivery is required to maintain the cooling? Probably a lot, and that is for only enough cooling to take us back to, say, 2010, assuming we don’t continue polluting, in which case we could hope, as soon as a few decades hence, to return to the disastrous impacts of climate change in, say, 2025.
Sources:
CDR.fyi. Biochar Market Report 2025
He, H., Soden, B. J., Vecchi, G. A. & Yang, W. (2025). "Stratospheric aerosol injection can weaken the carbon dioxide greenhouse effect” Nature Communications Earth and Environment 6:485
Data set: Zenodo, https:// doi.org/10.5281/zenodo.15616917 (2025).
Morgenstern, M. (2025). Climate engineering in The Ministry for the Future and Termination Shock. Textual Practice, 1–22.
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When humans are locked in a cage, the Earth continues to be beautiful. Therefore, the lesson for us is that human beings are not necessary. The air, soil, sky and water are still beautiful without you. So, when you step out of the cage, please remember that you are guests of the Earth, not its hosts.
We have a complete solution. We can restore whales to the ocean and bison to the plains. We can recover all the tremendous old-growth forests. We possess the knowledge and tools to rebuild savannah and wetland ecosystems. Coral reefs rebuilt with biorock build beaches faster than the seas are rising. It is not too late. All of these great works of nature are recoverable. We can have a human population sized to harmonize, not destabilize. We can have an atmosphere that heats and cools just the right amount, is easy on our lungs and sweet to our nostrils with the scent of ten thousand flowers. All of that beckons. All of that is within reach.
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