Thats a pretty good answer and it brings up a lot more detailed questions.
Just keep asking more questions. Sooner or later you’ll Jedi-mindwave everyone into your “…returning carbon to the atmosphere is the most important contribution that mankind will have made…” thesis of ecological balance.
Stranger
Well, it’s clearly a true statement, in the sense that stage IV pancreatic cancer is a major contributor toward the health of an individual. Not entirely sure he meant it that way, though…
If you look at the carbon in the total bio-mass on this planet and compare it to the carbon in the atmosphere and the carbon in oil reserves .The bio mass carbon dwarfs them. So a relatively small increase in bio mass could easily absorb any excess carbon. Increased and carefully observed introduction of nitrogen and water could have substantial effects on bio mass. There are plants that can fix nitrogen into the soil and I know there are several industrial methods. Would increasing nitrogen supplies be more effective than trying to scrub carbon. Sequestering the carbon would not be the issue as the increase in bio mass could easily handle it.
Let’s see:
Total biomass on Earth is 550 billion tonnes of carbon, according to Wikipedia. That’s 5.5e14 kg.
Earth’s atmosphere weighs 5.15e18 kg. Current CO2 levels are ~420 ppm, bringing the number down to 2.16e15 kg of CO2. And the carbon content of CO2 is 27%, giving 5.8e14 kg of carbon in the atmosphere.
Where do you get the idea that biomass carbon dwarfs atmospheric?
Thank you for interpreting that for me. I misread the atmospheric carbon values, So what would be the ratio of bio mass to atmospheric carbon. I just don’t know how to interpet some of those expressions.
Looks like it’s around 1:1 – 5.5x10^14 is about equal to 5.8x10^14.
That is interesting that they are about the same. I wonder what kind of accuracy the bio mass estimate is expected.
5.5e14 is a shortcut for 5.5 x 1014, or 5.5 with the decimal point moved right 14 places. As RitterSport noted, 5.5e14 and 5.8e14 are roughly the same value. In fact you only have to look at the exponent (the 14) to see that they’re roughly equal.
People underestimate the mass of the atmosphere. But every single square foot of Earth has over a ton of atmosphere above it. So even trace gases like CO2 are present in immense quantities.
That’s why it’s so hard to get up in the morning.
If that’s your strategy, then you will have to permanently dedicate land to carbon sequestration. Only as long as you do that will the carbon remain sequestered - as soon as you do anything with the crops (be it eating, or burning, or letting them rot), the carbon will be re-released.
An acre of forest captures about 80 metric tons of carbon dioxide (https://fpr.vermont.gov/sites/fpr/files/Forest_and_Forestry/Forest%20Carbon-Nov2016.pdf). So if you want to sequester only one per cent of the 5.8 times 10^14 kg of carbon in the atmosphere, you’ll have to permanently set aside about 7 times 10^7 acres to this. That’s about the area of Nevada. Doable, but remember but that this would be forest and only forest, not to be used for anything else, and you would have to keep reforesting permanently to replace dead and decaying biomass with newly planted trees.
If you want to sequester more than one per cent of atmospheric carbon, scale accordingly.
My calculations indicate that humankind only has to reduce the amount of CO2 in the atmosphere by 50%, so we must switch Nevada for Texas, Arizona, Louisiana, Florida, Kansas, Arkansas, Missouri, Montana, Wyoning, Utah, Alaska, Idaho, Tenessee, Oregon, Colorado, Nebraska, Indiana, Illinois, Washington (State and DC), New Mexico, Wisconsin, Michigan, Both Carolinas, Canada and Mexico. Europe would have to assume its fair share too, with France, the United Kingdom, Germany, Spain, Italy, Ukraine, Bielorussia, Luxemburg, Sweden, Finland, San Marino, Norway and Austria. The whole of Australia too, if we can get anything other than shrubs to grow there (Make Australia Wet Again! Terraforming is easy!). Sounds doable. 
Do I understand you right, are you saying that the most important contribution that mankind will have made to this planet is to eliminate itself? If you think it through the end, yes, that is a bit nihilistic, but probably true. My personal contribution was a vasectomy. What is yours?
Both the production of nitrogen fertilizer and the use of it cause significant environmental problems, including but not limited to greenhouse gas emissions. I strongly suspect the OP’s suggested technique would backfire and make matters worse.
I have heard people flout the idea of growing biomass, then chopping it down and burying it underground and safely covered so that the carbon from the decomposing biomatter wouldn’t be released intoo the atmosphere. If workable, that would allow you to use the same area over and over for fast-growing crops, safely sequestering carbon in each round. But of course it would, in a way, be strange to first dig up decomposed biomass in the form of fossil fuels, only to bury freshly grown biomass again. It would be better not to dig up and burn the fossil fuels in the first place.
Which is a roundabout way of saying that I don’t think carbon capture is a viable approach to global warming. Also keep in mind that all carbon capture technologies require energy. That’s a law of physics - hydrocarbons store chemical energy which is released when the hydrocarbon is burned, aka oxydised into CO2 (which is the very reason why we’re burning them in the first place). So carbon capture would require us to have a plentiful greenhouse-neutral source of energy. But if we have such a source of energy, we would be putting it to better use by replacing other energy sources and stop burning fossil fuels in the first place.
Perhaps I have not expressed myself clearly, but for the record I want to state that I agree with what you write: CCS is not a realistic method to stop global warming, among other reasons, because
Which brings to mind a question that has been in my head for some time. Burning carbon heats the planet by way of CO2 emmissions, which trap infrared radiation. Say a given amount of carbon generates Y calories on the long term through this mechanism, and those calories heat the planet up. But that very burning of carbon generates heat directly, say X calories. X is generated in an instant, Y is cummulative, the gift that keeps on heating (until CO2 is removed from the atmosphere). Does anybody know how long it takes for Y to become bigger that X?
You’d have a very difficult time breathing without it.
The radiative balance of the Earth is very complicated but the amount of energy impinging upon it from the Sun dwarfs all industrial waste heat production by many orders of magnitude, so even though only a fraction of solar insolation actually contributes to heating (which mostly goes into the oceans and drives the hydrologic cycle) anthropic heat production is a rounding error in comparison. If we were producing sufficient waste heat to upset the energy balance, we’d be in immediate trouble.
Strangers
Very roughly calculated:
An increase of CO2 from 270 ppmv to 370 ppmv causes an increase in radiative forcing by 1.5 W/m^2.
That’s a 100 ppmv increase, or a 153 ppm (mass) increase.
A square meter of atmosphere weighs 10,328 kg. 153 ppm of that is 1.58 kg. That’s CO2: the raw carbon content is 0.43 kg. Coal (for instance) is ~85% carbon, so that’s 0.51 kg of coal equivalent.
Coal produces 24 MJ of thermal energy per kg. So that’s 12,240,000 J burned to produce the 100 ppmv increase in the 1 m^2 column of air.
Divide 12240000 J by 1.5 W to get 8160000 s. Or 94 days. CO2 persists for centuries, so the crossover point comes very early in the process. It’ll be a bit longer for natural gas, but not by an order of magnitude.
Be careful about making ‘simple’ calculations like this. The concentration of carbon dioxide by altitude can vary significantly with altitude. At high altitude, where carbon dioxide concentrations are meaningful in terms of creating a greenhouse effect, CO2 is well mixed, but near ground level where a lot of historical measurements are taken there can be significant variation. Also, for non-pulverized coal combustion, a significant portion of the carbon output in is particulates and ‘soot’ (solid products that do not contribute to atmospheric CO2). Particulates have their own very significant environmental hazards but they don’t produce greenhouse gases.
As for greenhouse gases, although CO2 Is the greatest by overall volume, it is far less significant per unit mass compared to methane (CH4), which has a global warming potential (GWP) of 72; that is, the near term effect it has on increasing radiative forcing. Fortunately, methane breaks down after about a decade, but it breaks down into CO2, so it still contributes to greenhouse effects. Similarly, nitrous oxide (N2O), and various fluorinated gases (HFCS, PFCS, SF6) are not large by proportion but have both long duration and more significant effects than CO2. So there is a lot more to excessive radiative forcing than just burning hydrocarbon ‘fossil’ fuels, although obviously introducing so much captured carbon into the atmosphere over such a short period of time can have dramatic effects. We need to be looking at overall industrialized footprint rather than just carbon emissions even if the latter is the largest component by volume.
Stranger
I only claim that this is probably within an order of magnitude of the right answer, for some “reasonable” definition of the problem statement (which isn’t all that well-defined). Even if I undershot by 10x, the energy of combustion is still negligible compared to the greenhouse effects. And if I overshot by 10x, it’s truly infinitesimal.
My link above estimates CO2 as contributing ~60% of the total radiative forcing. The other gases are by no means insignificant, though CO2 still dominates. CH4 is at 20% despite being present in far smaller quantities.
Thank you very much, that is the kind of rough estimation/calculation I was hoping to get. Three months! And again, and again… Wow.
That is also a way to approach the problem, stated like that it is evident. Thanks to you too!