What it would take to suck more carbon dioxide out of the air than we put in

Two of the six plants we have left announced last month they wouldn't be bothering going to the 2025 coal elimination date, and will close in 2020. Leaves 4 I think.

Same month the lovely government approved a new deep coal mine in Cumbria. huh?

China is adding millions of new drivers to the roads, in fossil-fuel powered vehicles with fossil fuel (and tree) derived tires, annually.

What about the fact that China is actively constructing hundreds of coal power plants? Or the fact their middle-class is rapidly growing which will mean more energy demands (which will mean more fossil fuel consumption) and more consumer goods (more plastics, from fossil fuels)?

>Since the early 2000s, China's middle class has been among the fastest growing in the world, swelling from 29 million in 1999 (2 percent of population) to roughly 531 million in 2013 (39 percent of population).

https://chinapower.csis.org/china-middle-class/

China has hundreds of millions of people that will, presumably, continue to be added to their middle class resulting in more cars, more electricity, more gadgets, more plastics, etc.

Unless China says "no, only the people that have stuff now can have stuff" they're going to consume more fossil fuels, construct more things with concrete, etc.

What I'm not sure I follow is the relationship you seem to believe exists between per-capita emissions and responsibility to curb them. Emissions need to be curbed in proportion to their percentage of overall global emissions, not in proportion to per-capita consumption. You can vary the per-capita statistics however you like, and as long as the absolute emissions are the same, the damage to the planet is the same. Those emissions could be coming from one person or a trillion. If China had ten billion people, or fifty billion or a hundred it would still be just as incumbent upon them to curb such emissions as they are producing.

Everything else about "implicit beliefs", the "leaps" you believe people to be taking, the questions of who should or shouldn't be blamed are abstract, highly subjective, and while important, should be considered without letting them derail the conversations around needed next steps to curb emissions.

So it's a terrible devil's advocate argument. I'd advise Old Scratch to fire you and see if he can get a public defender instead.

First, let's correct the calculation results. You've calculated that shipping 3 billion tonnes of iron powder 1000 miles from shore would burn almost 5000 tonnes of bunker fuel. But that is incorrect. It would burn almost 5 million tonnes of bunker fuel, according to your numbers:

    You have: (3 billion tonnes / 80000 tonnes) (1000 miles / 12 knots) * 44 tonnes/day
    You want: tonnes
            * 4978509.7
            / 2.0086332e-07

Second, let's tackle the implications of this calculation, if it were correct. 4.98 million tonnes of bunker fuel, taking your figure, is 4.23 million tonnes of carbon, which is 0.004 billion tonnes. Since we're stipulating that this is the cost of removing 1 billion tonnes of carbon (and a larger amount of carbon dioxide) from the atmosphere, this is 0.996 billion tonnes of carbon dioxide removed, net. Your hypothetical fleet of ships used less than 0.5% of the carbon dioxide removal budget. According to your calculation error, it would have been 0.0005%, entirely insignificant. If you're trying to advocate for Old Nick, you're doing a terrible job.

Third, let's tackle some of the underlying misconceptions behind your argument, although I have neither the time, nor the patience, nor the knowledge to refute all of them. Iron fertilization is not carried out with iron powder; it is carried out with ferrous sulfate, which is water-soluble and consequently does not sink to the bottom as you suggest.

Ferrous sulfate solution is commonly carried in tankships, which are in fact "a specific thing": https://en.wikipedia.org/wiki/Tankship. Ferrous sulfate is enormously cheaper to prepare from rock than iron is, and in particular produces enormously less carbon dioxide emissions, because it does not require reducing the iron to its metallic state, much less "grinding [iron] to a powder". In fact, in many cases, it does not require reducing the iron at all, because it's already in the ferrous state in rocks. It is industrially produced in vast quantities as an industrial byproduct of steel pickling and titanium refining (mostly for pigment production, not for the titanium metal), and, where required, by burning fool's gold. According to news reports, it's about a billion-dollar-a-year industry; it costs about US$2 per kg at the garden store, which puts a floor of 500,000 tonnes produced at present per year; however, a lot of it is currently treated as merely a slightly hazardous chemical waste.

Iron ores are mostly not, as you incorrectly assume, trucked across land, because they are sufficiently low in value that it is not profitable to do so. Instead they are transported by ship or train. But this is irrelevant, because no iron ore would be mined for iron fertilization.

However, even if iron were to be mined and reduced to obtain the iron required, this would be an insignificantly small impact. 3 million tonnes of iron per year are required to sequester a billion tonnes of carbon; current global steel production is about 1600 million tonnes, so this would be about 0.2% of current global steel production. Again, though, this is irrelevant because ferrous sulfate can be produced directly from iron pyrite without reducing it.

You assume that iron fertilization ships would be propelled by burning fossil-fuel carbon, thus adding to the atmospheric carbon dioxide burden, although the amount added is insignificant compared to the amount removed. But we're talking about a future in which serious measures are being taken to combat climate change. So it's likely that ships will be driven by Fischer-Tropsch synfuel, not fossil fuel, or, in many cases, by kites, or even by solar panels onboard. (It seems likely that the depreciation costs of ships will drive ships to continue to go fast, though, and that probably can't be done with solar panels.)

The 16 tankship trips per year we're talking about here are insignificantly small compared to the current total global shipping industry, so their total environmental impact --- aside from the iron fertilization itself --- is also insignificantly small. No purpose-built ships would be needed, and indeed the amount of shipping needed is so small that it would not justify the cost of building even a single purpose-built ship.

As for the overall environmental impact of iron fertilization, it is true that it is somewhat unknown. But some experiments have been done, and results are promising; the high-nutrient, low-chlorophyll "desolate zones" of the ocean where this would be done, amounting to 20% of the world ocean, though not entirely devoid of marine life that could be harmed, tend to become much more fertile. That is, the total ecological impact is unknown, but the balance of the evidence suggests that it's positive. The impact of the iron on ice formation is of course nil because the quantities of iron we're talking about are tiny traces; although presumably algal growth could affect ice formation, it would promote ice formation, because algae strongly reflect infrared light, while desolate waters are almost perfect radiation absorbers (albedo around 10-20%). The larger concern is that there isn't enough ocean that could be fertilized to absorb more than a fraction of atmospheric carbon. I suggest that you read https://en.wikipedia.org/wiki/Iron_fertilization and https://en.wikipedia.org/wiki/HNLC before commenting further.

In short, you have not done your homework and appear to be merely making up random arguments with no basis in reality. I regret the need to state this so bluntly, but my previous polite and encouraging comment on the subject has been both downvoted and flagged into oblivion.