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Comment by Pfhreak | Hacker News Reader

You need to remove 30 gigatons per year to eliminate all the CO2 humans generate. This is the "what if humans disappeared" baseline you mentioned.

If we eliminated 30 gigatons annually, yes the earth would recover at the rate you describe.

If we increase above 30 gigatons or reduce emissions, we shorten the time dramatically.

Growing trees or grasses, pyrolizing them into syngas and biochar, and sequestering the char could, in theory, supplement natural processes.

It's a process that can be run with an energy surplus, but it does require a lot of labor and land.

You'd need hundreds of thousands of square miles wholly dedicated to the effort.

DACCS facilities are expensive, but achievable. You'd need roughly three major industrial DACCS facilities for every coal fire plant worldwide, which would be incredibly challenging but not impossible.

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I think where some of these ideas run afoul of reality is that they ignore the scale of energy and resources necessary to make them happen. And the probability distribution of each idea being realizable at scale.

It's a planetary scale problem, which means the numbers are very large no matter what anyone suggests.

I have no clue how much CO2 t-shirt manufacturing produces when you consider the entire supply chain and human footprint. If you add-up t-shirt manufacturing world wide, the number likely isn't trivial. And yet, even if we stop making t-shirts the need for clothing does not evaporate. I would be willing to bet that t-shirt manufacturing is now optimized almost as far as one could go and the providing an alternative form of clothing to billions of people around the world would be dirtier and consume more resources than just making t-shirts.

This is frustrating to me because I can't find a path. The minute one includes the pesky little reality of conservation of energy things become real.

I can't think of a single problem --regardless of the domain-- that can be fixed with less energy than that which created it in the first place.

And so, if that's true --and I think it is true without dispute-- this idea of being able to control atmospheric CO2 concentration at a planetary scale is in a range between hubris and lunacy.

Another way to put it is: We cannot fix it by subtraction.

This means we cannot fix it by simply going down to zero CO2 generation, no matter what the approach might entail. That, at best, gets us to 100K years for a 100ppm reduction in CO2.

I admit, this is a defeatist view. Yet, I believe this is reality and what is being pushed out there is fantasy. My fear is that, yes, we need to start doing a number of things to make human life better (the planet will do fine without us) but none of this work will begin until we frame it from a truthful baseline.

As I have said before, there are tons of reasons to cleanup our act globally --and we should-- but let's not lie to ourselves and pretend that this is to save the planet or reverse climate change. We can't. We won't.

And we won't because, even if we go to zero CO2 the planet itself will not cooperate. If you look at the charts for the 800,000 year ice core sample CO2 data one of the questions should be: How did CO2 increase, we were not around to make it happen?

The answer is, for the most part, massive continental scale fires.

So, we go CO2 neutral and cover the planet with trees.

And then fires, massive fires, fires we cannot control, contribute more CO2 to the atmosphere than we ever contributed before with our non-carbon-neutral technologies. Just look at what happened this year in California alone:

https://news.mongabay.com/2020/09/off-the-chart-co2-from-cal...

It is hubris to think we can control this at a planetary scale. We can't control it in a US state that recently surpassed the entire United Kingdom to become something like the fifth largest economy in the world...and we can't stop massive forest fires. From the article:

"The fires have already generated more than 91 million metric tons of carbon dioxide, which is about 25% more than the state’s annual emissions from fossil fuels."

I wish I had answers. All I am able to do is point out that we are lying to ourselves in hopes that, if enough people stop to think and understand this reality we will switch tracks and empower our scientific community to look at this from a different perspective. Maybe then someone with a unique perspective might discover a way to deal with it. We don't need to fix it, I don't believe we can, we need to understand how humanity can survive the cycle we are on.

Thanks for engaging.

no, thank you for explaining it so well.

This is frustrating to me because I can't find a path. The minute one includes the pesky little reality of conservation of energy things become real.

I can't think of a single problem -- regardless of the domain -- that can be fixed with less energy than that which created it in the first place.

Accelerated silicate weathering is one approach that takes less energy than turning carbon dioxide back into carbon and oxygen [1]. Natural silicate weathering has a low thermodynamic cost but is kinetically hindered. Accelerated silicate weathering only spends energy to accelerate the kinetics of natural silicate weathering and needs much less energy than combustion-in-reverse.

Iron ocean fertilization is also possibly another case where the required human energy input for drawdown could be much smaller than reversing combustion. I say "possibly" because it has not been tested with enough rigor and scale yet.

You can see from my comment history that I agree with you about how slowly natural processes alone can bring atmospheric CO2 levels back down to pre-industrial levels: https://news.ycombinator.com/item?id=24297363

[1] e.g. Project Vesta https://projectvesta.org/

> Accelerated silicate weathering > Iron ocean fertilization

The real question is what it will cost in terms of energy and CO2 to mine, produce, prepare, deploy and manage such processes.

I have a very hard time accepting that we can use less energy to reverse something than the energy it took to create it. I have to admit it would take a lot of research on my part to fully break down these processes and quantify them from start to finish. I am just going to trust physics and say that I suspect perpetual motion machines are still impossible.

What truly scares me about ideas like iron ocean fertilization is the massive potential for causing a disaster that could damage sea life and ecosystems for hundreds of years. It's one thing to run an experiment on one beach or two. It's quite another to do this at a scale sufficient enough to affect things at planetary scale.

That's where, frankly, my brain short circuits a bit. I can't imagine some of these things done at a planetary scale without expending massive amounts of resources and producing equally massive amounts of pollution, CO2 and potential ecosystem damage.

Now, here's a twist. If the thought is that we can deploy any one of X approaches and deliver results a thousand times faster than the natural rate of change (100KY/100ppm) we have to be truly scared about what the unintended consequences might be. It's almost like that story about when they detonated the first nuclear weapon and thought there was some probability of the entire atmosphere igniting. I am not sure if the story is true, but it illustrates the point well enough.

I think we (and anyone who truly stops to look at the data and apply critical thinking) agree that this is a difficult problem that is being made far more complex by a narrative that is patently false (or distorted) all both extremes. This is a sad reality. Science should not work this way. Scientists should be free from political forces.

Accelerated silicate weathering is not a perpetual motion machine. Do you understand the difference between thermodynamics and kinetics, in the context of chemical reactions? (Not trying to condescend, just calibrating how much background to include in my next explanation.)

There is a big difference between "this approach is impossible according to physics" and "this approach might work but I'm worried about the side effects." It seems like for iron ocean fertilization you're asserting something more like the second statement than the first.

I'm interested in active atmospheric CO2 removal approaches because emissions cuts alone aren't enough to get back below 400 ppm CO2 on human time scales, as you have noted. Shying away from mitigation approaches because they could have unknown side effects at large scale is just committing to suffering the unmitigated brunt of AGW. Anything effective will have to be large scale.

Sorry, I wasn't clear enough. I was making a general comment about "solutions" being offered while completely ignoring the entire resource, energy and CO2 generation chain required to actually deploy the solution. A silly example of this is using huge "air filters" in every city...which some have actually proposed.

I need to go learn more about silicate weathering, don't know enough.

> thermodynamics and kinetics, in the context of chemical reactions

I regret not having paid more attention in university during chemistry class. Paid lots of attention during multiple years of physics (it was more interesting to me at the time).

Wait a minute...isn't chemistry just applied physics? :)

In general terms, I think we need to take this perspective on the problem:

1- We can't fix it on a human time scale (let's define that as a number between 100 and 1000 years)

2- We need to free-up our scientists (and fund them) to start thinking about and working on this implications and the solutions we will actually need

3- We need to start working on having to live with the reality of more intense weather events

4- We need to start working on mitigating effects for food supplies and other essentials

5- We need to be super careful about the potential for unintended consequences. I always think about what happened in places like Australia, New Zealand and others when we dared to think we could exercise control:

https://www.nytimes.com/2009/02/17/health/17iht-17isla.20237...

When compared with trying to produce a planetary scale effect, these ecosystems are but a rounding error. This is what worries me the most. We can't "fix" something on an island and we have the hubris to think we can actually "fix" the planet and not kill everyone on it as part of the process.

This, BTW, is why I tend to be a proponent of learning to live with it while cleaning-up our act to the extent possible without being so arrogant as to think we can do anything about it on a human time scale.

5- We violently remove politics from this. I do not mean this in terms of physical violence, I am using the term to mean "faster than fast". In other words: Go sit in the corner while the intelligent adults in the room have a conversation.

This isn't a simple problem and we need to be exceedingly careful not to be led by the nose by political and other forces into something that could destroy more life on this planet than we can possibly imagine.

The weathering of alkaline silicates naturally draws down atmospheric CO2. It's a slow acid-base neutralization reaction. CO2 dissolved in water forms carbonic acid, which reacts with alkaline rocks. Calcium silicate plus carbon dioxide turns into calcium carbonate plus silicon dioxide. The reaction is spontaneous under ambient conditions on the Earth's surface, meaning that it is thermodynamically favorable. It doesn't require any added energy beyond that naturally present in the environment. The reverse reaction that separates carbon dioxide from calcium carbonate again is thermodynamically disfavored. It requires a large energy input, as in the making of quicklime from limestone for cement production.

The geological carbon cycle based on silicate weathering is what will naturally neutralize human CO2 emissions on a time scale of hundreds of thousands of years.

The reason it takes hundreds of thousands of years is that the chemical kinetics -- rate -- of the natural reaction are very slow, being limited by the available reactant surface area. This is the same reason that e.g. a steel hammer left outside in a rainy region takes years to completely disintegrate to rust, while steel wool under the same conditions will disintegrate to rust in under a year. The thermodynamics are the same in both situations: iron oxidizes spontaneously. But the kinetics are much faster when the material has a large surface area exposed.

Most of the exposed weatherable silicates on Earth are in the form of huge chunks: boulders, mountains, and region-spanning plateaus. The idea behind accelerated silicate weathering is to crush huge chunks of silicates down to sand-size particles so that the surface area and reaction rate increase dramatically. If the crushed material is dumped into shallow ocean water near shores, wave action also provides additional "free" mechanical grinding to further accelerate the process. Using these silicates to neutralize excess soil acidity on agricultural land, where limestone would normally be used, is another way to further accelerate the chemical transformation.

The human energy input required for accelerated silicate weathering is still large in absolute terms, but much smaller than trying to turn CO2 back into carbon and oxygen. It might take 5% of a coal plant's electricity output to crush enough silicates to offset its CO2 emissions. (Though ideally you would run the process on renewables, since crushing can be scheduled flexibly and only annual throughput really matters.) The process reverses ocean acidification effects of CO2 as well as reversing warming effects from CO2 in the atmosphere. It doesn't require artificially concentrating CO2 out of the atmosphere.

I believe that accelerated silicate weathering can bring atmospheric CO2 back below 300 ppm in less than 1000 years, though still more than 100 years. That's assuming that anthropogenic emission rates decline over time, mind you.

It makes sense that the vast majority of the discussion around AGW mitigation is still about cutting emissions. While emissions are still growing, even ambitious plans like large scale accelerated silicate weathering can't offset them. Still, if you look at the IPCC reports and other scholarly literature, scientists are looking ahead beyond emissions cuts. The term they use is "negative emissions."

This is valuable insight to add to my knowledgebase.

One way I am thinking about what you are saying is the concept that a block of ice melts at a much lower rate than the same mass of ice in small cubes. The principle being that a greater exposed surface area produces a higher rate of heat transfer from warm air to ice, accelerating melting.

I am trying hard to frame this issue in the simplest possible terms so that it is easy to consume the information by those who might not have the scientific background. I don't think the effort to shift the conversation will succeed if it is framed by equations impenetrable by the average person.

What you highlight --that the limit rate of CO2 "consumption" is a function of available reactant surface area-- is a valuable tool with which to communicate the idea that this process is beyond human time scale. In other words, the natural rate of change is what it is due to physical realities of this planet. It cannot be a thousand times faster just by installing solar panels or banning IC vehicles. I can see a YouTube video using the simple example of ice melting as a way to explain this.

I'll do a bit more reading and shamefully steal some of your insight. Like I said, I regret not having paid more attention during Chemistry class in college. The good news is, it's never too late to learn.

Thanks.

I'm happy that you found this valuable.

The simplest analogy I might try to use is that table sugar crystals stirred into water dissolve in seconds while a piece of hard candy will take minutes to dissolve.

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