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Comment by vosper | Hacker News Reader

This study is about whether the rock weathering we’ve seen posted here before (crushed olivine etc) works in dry climates like California. The answer appears to be “yes, but more slowly”

> The study found the plots with crushed rock stored 0.15 tons of carbon dioxide per hectare (2.47 acres) during the study compared to plots without crushed rock. Though researchers expect different weathering rates in different environments, if this amount of carbon was removed across all California cropland, it would be equivalent to taking 350,000 cars off the road every year.

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They don't say how much rock they added to each field, but note that the 0.15 tons of CO2 they remove is only 300 lbs. A gallon of gasoline emits 20 lbs of CO2 when burnt.

So, if they use more than 15 gallons of gasoline to mine, crush, transport and spread the rock over 2.47 acres, then this process emits more CO2 than it captures.

I think they'd need to use electric high-tonnage dump trucks and electric tractors to make this process actually capture carbon. Electric tractors are stating to become available, but I don't know of any high-tonnage electric truck manufacturers.

Depends on how high “high tonnage” is. Kennworth is offering a 680hp full-electric with a 3 hour range. Volvo offers an electric VNR with identical performance to its standard VNR. Freightliner sells the eCascadia, matching its regular Cascadia.

These vehicles would need to be charged on fully renewables or non emitting sources (like hydro) to offset this. Even if Natgas or Gas plants are much more efficient than engines, the calculation still needs to account for amortized emissions.

Yes? Obviously yes? Idk why everyone is assuming we’re going to keep dragging our feet on something that, at this point, is merely a matter of buying a few batteries and setting up a few more solar panels. Provinces have done it; heck, entire countries have done it. Switch to renewables already.

Because we're still not there yet. In the US, California's grid hits near-100% solar only around midday for an hour and then at night becomes mostly natgas, and can't handle peak demand on hotter days. If you're talking about the load of adding BEV trucks then that's adding a whole different dimension to the equation. I'd like to be optimistic about these things but we're far from it in the US.

Those are semi-trucks. You'd need to attach a compatible dump trailer to them. (Those might exist, but I don't think I've ever seen one).

They exist, its how animal manures are applied to fields. Likely this would be shipped via trains long distance and last mile with semi.

The spreading is ok but the transport is high. The key thing is: how many years of weathering do you get out of one load of 47 tons (or 27t)?

The paper provides a very course estimate that what they measured over the year was only 0.15% of total weathering potential, which if we handwave a little turns into 6,000 lbs CO2 over 20 years, or 300 gallons of gas offset, which is probably a net gain. Though there are a ton of "it depends"es in there and it probably only makes sense with further electrification.

(Obviously, the weathering would go on for a lot longer than 20 years, but the unknowns get large there and I'm assuming we're mostly interested in a shorter time frame)

OK, with something something like the F-550 (on the small side for this use case, but also a reasonable choice for an ICE dump truck that's driving to/from agricultural fields), it could work:

https://www.ford.com/commercial-trucks/chassis-cab/models/f5...

That can move 14,870 lbs (minus the weight of the driver and the dump truck bed) per trip, so it would take more than 6.7 trips to deliver rock to treat an acre. At 6.7 trips per acre it would have to use less than 44 gallons of gasoline per trip for breakeven. It should use significantly less than that for fields that are somewhat close to an appropriate quarry / rockery.

The lightning can only move 7000 lbs per trip, so it would have to make 13.5 trips per acre.

Remember that if you're already distributing something else (eg. fertilizer) over the field, the added money and CO2 costs of distributing this might be negligible.

You could just mix the fertilizer and the rock together at the fertilizer factory, and automatically a large chunk of the worlds farmland gets this added.

It's only negligible if the rock is a negligible amount of extra weight. Otherwise, the energy to transport will still come from somewhere & not be negligible unless all that energy is from a non-carbon fuel source (bulk transportation is nowhere near decarbonized).

How does 6lb of gas release 20lbs of CO2?

I found this[1] link from the EPA with a number similar to yours, 8.8kg per gallon of gas, but they say it "creates" rather than "emits." I'm still struggling with this creation of mass out of thin air...

[1] https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-t...

It's because the majority of the mass of CO2 comes from the oxygen from the air used to burn the carbon. So in this case, it's quite literally creating mass out of thin air.

Specifically, the atomic mass of carbon is 12 and oxygen is 16. So for CO2, 12/44 of the mass is carbon, or about 27%. So of that 20 pounds of CO2, about 5 1/2 pounds are carbon, the other 15 pounds is oxygen sucked out of the atmosphere.

"Out of thin air" is right :-)

The CO2 contains mass from the atmospheric oxygen consumed during combustion. 12 grams of carbon becomes roughly 44 grams of carbon dioxide when it burns.

"produce" is probably a better verb than "release" in this case, for the common person to understand what's going on.

The metric system is really handy for calculations like these.

> but note that the 0.15 tons of CO2 they remove is only 300 lbs

Your point stands, but it was 600 lbs.

They quoted figure is a rate, not an absolute amount removed:

> 0.15 tons of carbon dioxide per hectare

Per hectare. They spread it over 5 acres, or roughly 2 hectares, for a total of 0.3 tons, or 600 lbs.

Taking only 3% of California's cars off the road seems way easier.

It is, but consider the future. We need net negative emissions to stabilize the climate because we didn't decarbonize early/fast enough to achieve this goal with emissions cuts alone. The vast majority of climate progress must come from energy system decarbonization, but active carbon dioxide removal approaches like this one will be necessary to draw down the excess atmospheric CO2 accumulated so far.

"IPCC Report: Carbon Removal is Now Required to Meet Climate Mitigation Targets"

https://www.carbon-direct.com/insights/ipcc-report-carbon-re...

Also see this IPCC fact sheet about carbon dioxide removal, which includes enhanced rock weathering (the technique under discussion here):

https://www.ipcc.ch/report/ar6/wg3/downloads/outreach/IPCC_A...

Without human actives the earth is slightly net negative. We need to be more net negative than that in the short term to fix some problems (ice melt), but we don't need that forever.

> Without human actives the earth is slightly net negative

Source? AFAIK greenhouse warming has the capacity to be a runaway effect. It's not immediately obvious that if you remove all current human carbon emissions, that temperatures would level off & start declining vs continue increasing because of the feedback loop.

The Earth slowly depletes excess atmospheric CO2 due to the geological carbon cycle of silicate weathering. This is the slow, natural counterpart of the accelerated silicate weathering discussed above. It is too slow to reverse global warming on human time scales, but it means that if anthropogenic emissions ceased Earth would reach its peak temperature in less than a thousand years (ice melting and other reactions to this change would go on for several thousands of years) and gradually decline back to the pre-industrial atmospheric CO2 and climate within about 100,000 years.

My source for this recollection is David Archer's 2009 book The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate.

Humans are currently on track for around 4 degrees of warming by 2100 assuming we hit all our current emission reduction targets (i.e. reaching net zero when governments have said they would). So removals are quite necessary.

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