Why our Earth is unlike Venus, and comparing two ways to keep it this way.

     The surface temperature of Venus melts lead, while Earth’s supports life, and water. While Earth is farther from the sun, this explains only part of the temperature difference. (1) The greenhouse effect and the different amounts of greenhouse gases in their atmospheres explains most of the surface temperature difference between Earth and Venus and explains why Venus is on average hotter than Mercury even though Mercury is closer to the sun.
     Earth avoided Venus’s fate, so far, despite starting with similar amounts of carbon (2) , by reacting carbon dioxide (CO2) with mafic rock. (3) Mafic rocks are rich in magnesium and iron, and underlie most of our ocean floors, and form Earth’s mantle, but are rarer atop continents. Most of Earth’s CO2 has reacted with such rocks already, yielding the carbonate rocks that store most carbon on Earth. (4)
     We increasingly have too much CO2 in our air and oceans now to keep the climate we rely on for our food. (5) The recent increase in air’s CO2 got here by our burning fuels for energy. (6) ‘Unburning’ these fuels would require energy we now don’t have, since we used it already. (7) But the reaction between these mafic rocks and CO2 is energetically favorable. (8) It occurs spontaneously whenever water, mafic rock and CO2 come together, requiring no further energy. So to react these rocks with this CO2, we need to bring them together.
      Should we bring the CO2 to the rock, or the rock to the CO2? Reasonable proposals differ on this. For example, let’s compare two carbon dioxide reduction (CDR) approaches: Olaf Schuiling’s Enhanced Weathering (EW) of olivine, a mafic rock, compared to a combination of David Keith and others’ Direct Air Capture (DAC), linked to Juerg Matter and others’ Injection(I) of CO2 into basalt rock, permanently forming carbonate rock. We’ll term the joint DAC and I process (DACI).
     Schuiling proposes bringing the rock to the CO2. Grinding the mafic olivine rock speeds the reaction between them. In part he suggests ground olivine substitute for limestone in agricultural use, because, while both will reduce soil acidity, olivine will also eventually bind with CO2 as carbonate rock at the ocean’s bottom, after CO2 in rainwater, as carbonic acid, dissolves that olivine.
     Bringing the CO2 to the rock, DAC’s CO2 would be injected in basalt. In DACI, first, Keith and others’ two-part process extracts CO2 from air. Then Matter and others’ injection (9) would combine this CO2 with Icelandic basalt permanently as carbonate rock.
     It is not fair to assume that this DACI combination would be a commercially viable process, as this combined process is neither a stated goal of DAC nor of basalt injection. But without attaching CO2 basalt injection to DAC, comparing it to EW would also be unfair, as EW sequesters CO2 permanently as rock, while DAC without injection merely isolates it in labile gaseous form, liable to leak out and renew troubles, and basalt injection assumes a stream of gaseous CO2, unlike EW’s starting point of CO2 dissolved in air. Thus we will compare DACI and EW processes with similar initial conditions and results: from CO2 in air to CO2 in rock.
     But how best compare these? Straight-forward economic analysis, while tractable, ignores non-monetized costs; like pollution’s effect on health, and reliance on non-renewable resources; and mis-monetized costs; like USA’s vast oil exploration subsidies. How should we evaluate and contrast these two distinct CDR approaches? We need a method to weigh different proposals’ costs and benefits to Earth, not to the individual human actors performing CDR.
     One cost measure is the amount of sunlight energy used up in every step needed and sufficient for each proposal, and a measure of benefit is the amount of sunlight energy acquired or saved by the proposal. Developed and used in the fields of environmental accounting and ecological engineering to rate multiple proposals, this approach compared differing methods, with varying environmental impacts, which addressed the same situation. (10)
     Let’s approach these two CDR proposals with this environmental accounting method to guide us.
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1. “ The CO2-rich atmosphere [of Venus] generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C; 864 °F) . [12 ] [58 ] This makes Venus's surface hotter than Mercury 's, which has a minimum surface temperature of 53 K (−220 °C; −364 °F) and maximum surface temperature of 693 K (420 °C; 788 °F) , [59 ] even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25 % of Mercury's solar irradianc e .” Wikipedia Venus entry, 2018-June 20th .
2. http://www.pnas.org/content/pnas/77/12/6973.full.pd f
3. Earth’s carbonate rock contains about 60 million gigatons carbon, while dissolved in the oceans as carbon dioxide is 38,400 gigatons carbon, with air containing 720 gigatons carbon. https://en.wikipedia.org/wiki/Carbon_cycle, citing doi:10.1126/science.290.5490.291
4. Same as footnote #3 .
5. https://en.wikipedia.org/wiki/Greenhouse_effect .
6. https://en.wikipedia.org/wiki/Greenhouse_effect .
7. No energy conversion is perfectly efficient, and we already used up the energy we got from burning the fuels. Thus reversal of fuel oxidation requires more energy than was first liberated .
8. https://www.sciencedaily.com/releases/2018/06/180605103437.htm
9. http://science.sciencemag.org/content/352/6291/1312
10. Table 12.1 Evaluation of Alternatives for Pulp Paper Wastes in North Florida, OdumHT 2007 Environment, Power and Society For the Twenty-First Century :359.