> I don't believe there is any scientific basis for these sort of
> opinions. All of the "commitment" stuff is based on maintaining constant
> GHG levels into the future, and _not_ especially on the principle that
> we might actually flip over a hysteresis threshold. On glacial time
> scales, as soon as we reduce the CO2 enough, the temperature comes down
> and the melting stops. There is quite possibly a hysteresis if Greenland
> does melt, but that "flip" is expected to take at least several hundred
> years.
If emissions go down enough, then even without sequestration
concentrations will fall, and I think over a 1000 year time frame
starting at 450 ppm they'd fall to around 300 ppm (?).
>From this it would follow that as long as there isn't a hysteresis as
you call it in between, stopping emissions would be enough to
eventually reverse most of the melting.
> Actually I'm not aware of anyone having done precisely this modelling
> experiment (rapid reduction of CO2) but that is probably because it is
> (a) bloody obvious and (b) politically unattractive. Anyone who takes
> Branson's air capture prize seriously might plausibly consider that this
> CO2 reduction can be left to 6 generations hence, if they choose to go
> back to a colder world.
What is your opinion on what is possible in terms of taking CO2 out of
the air?
There are well tested methods of doing it, by far the most obvious
being production of biomass and conversion of said biomass to a
product consisting mostly of carbon, which could then be dumped, with
or without energy recovery (eg dump vegetable oil in the ground, or
burn biomass with sequestration of the resulting CO2).
CaO could also be turned into CaCO3 by exposure to air, and the CaCO3
could then be exposed to nuclear or solar heat to recover the then
concentrated CO2. That would be energy intensive, but not nearly as
bad as you might think. Back of the envelope I'd guess, if we go from
present energy consumption of 200 million barrels of oil equivalent to
around 500 million by 2100 and energy consumption saturates then and
is entirely met by nuclear/renewables, taking 1000 billion metric
tonnes of Carbon out that way might require something like 200 million
barrels of oil equivalent in additional clean energy for 100 or 200
years.
I am not sure what Branson's prize is about exactly. Presumably it
would be for technology that could do the CO2 removal at very low
energy and overall cost? What I've read about it sounds awfully vague.
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