air capture via nanotechnology

[mitchell porter]

Hello all. I've just discovered this group and its companion
(climateintervention) and have spent a happy hour digging through the
archives. I'd like to first present a perspective on overall climate-
change strategy, and then move directly to the specific issue which is
my focus.

First, I'd simply like to put a name to a climate policy position that
the world could have adopted. I call it the Triple-A strategy, of
Aerosols, Air Capture, and Adaptation. After the subprime experience,
dubbing something "AAA" may not seem the best marketing, but they do
all start with "A".

The reason why I think this strategy might have been optimal is that I
expect cheap air capture to be possible long before any program of
emissions reductions could make a difference to the world's overall
welfare. (The historical imminence of cheap air capture technology is
a theme I'll take up below.) And aerosol geoengineering seems to be
the only thing we could do right away that would make a change to
global temperature in the short term. As for adaptation, I assume I
don't need to make a general case for planning ahead!

So in short, the Triple-A strategy is one where air capture is the
long-term solution, aerosols are used in the interim if a bit of
cooling right now is deemed necessary, and all other efforts are
focused on adaptation, e.g. anticipation of near-future impacts. I
imagine that a few people in the geoengineering community have a view
somewhat like this.

Instead, the world is mostly embarked on the enterprise of
coordinating national mitigation strategies through the Copenhagen
negotiations, establishing a carbon price, and so forth. I'm not going
to say we should dump Copenhagen and take up Triple-A, but I am
interested in exploring the continuum of possible policies between the
two approaches, opportunities for air capture to enter the emerging
global system (e.g. as a recognized form of offset, as an adjunct to
capture-at-source research like CCS, as an adjunct to
biosequestration), and so forth.

However, since this is an engineering list, and since the political
viability of air capture as climate policy is mostly going to depend
on its technical and economic viability, let me move on to the more
technological discussion.

I made a claim above about the historical imminence of cheap air
capture technology. By imminent, I meant within a few decades. The
real inspiration for this view comes from the "mechanosynthetic"
school of nanotechnology associated with Eric Drexler. The idea is to
combine the atomically precise positioning of submolecular reactants
which can be achieved with atomic-force microscopy, with the
atomically precise assembly of molecular components achieved by
biological systems. (This description is not quite true to the
historical origins of the idea, but I believe it conveys something of
how mechanosynthesis is supposed to work.)

One may find simulations of nanomechanical components, typically made
of diamond and/or carbon nanotubes, which establish something of what
their properties would be, without demonstrating that such structures
can actually be synthesized. There is also a very small research
community trying to find actual pathways to the fabrication of such
structures. In years past there was some controversy over the very
idea (see, for example, debates between Drexler and Nobel laureate
Richard Smalley), but the debate seems to have died down. The existing
situation is that "nanotechnology" refers to a very broad spectrum of
applied chemistry, and the minority who want to pursue
mechanosynthesis, specifically, are free to do so. (I should say that
it's not an either/or situation, and that interest in both
mechanosynthesis and self-assembly can coexist in the one
researcher.)

The thing which really caused problems for Drexler's early
collaborators was their forecast that nanotechnology was going to
revolutionize everything - medicine, space travel, industry, warfare.
Also, this conception of nanotechnology has been popularized in ways
that are problematic. I imagine that most people on this list have
some familiarity with the pop-culture notion of a nanorobot, that can
turn anything into anything else because everything is made of the
same thing, atoms. If we consider a biological cell as a real-life
nanobot, it can certainly perform remarkable material metamorphoses,
but it is still constrained by thermodynamics, dependency on a
particular chemical feedstock, and so forth. There has therefore
developed a pop-culture critique of the pop-culture vision of
nanotechnology, namely that it overlooks these and other limitations,
which is a sort of shadow of the more rarefied discussions exemplified
by Drexler vs Smalley.

I'm saying all this just to set the scene for what follows, and to
describe my own thinking. Basically, while cognisant of the ways in
which the real world of atoms differs from a set of Lego blocks, I do
think that ultimately something like a self-reproducing nanodevice,
whose diamond-and-nanotube components are mechanosynthetically
assembled in an evacuated interior space, ought to be possible. It is
going to be a very complex and advanced technology, relative to where
we are right now, but I do not see a law of nature that makes such an
entity simply impossible.

And yet this is the nanodevice-as-doomsday-machine scenario which
Smalley in particular was keen to reject as impossible. I think the
most straightforward discussion of how such a device constitutes a
doomsday machine may be found under the name of "aerovores", devices
which "eat the air" - carbon dioxide, specifically. (Aha, says the
reader, the connection to air capture technology comes into view.) If
you suppose, hypothetically, a nanodevice which can reproduce itself
using only solar power for energy and atmospheric molecules for
feedstock, then biological rates of replication will have it
extracting *all* the carbon dioxide from the atmosphere very quickly,
while also smothering the cooling earth in a layer of indigestible
diamond dust.

Since I do in fact think this is a technological possibility, I am
very concerned (to put it mildly) about the prospect of human survival
in a world of advanced nanotechnology. But there is one upside to this
belief, and that is that air capture on scales capable of returning
the atmosphere to its preindustrial condition no longer looks like it
costs trillions of dollars. :-) Just one aerovore can do the trick;
but you need to program it to stop reproducing after the appropriate
number of generations. And good luck with that!

I don't propose to hijack this list in order to discuss what to do
about nanotechnology in general; there are other forums for that. But
what interests me is whether better, cheaper, faster forms of air
capture than those we know about can somewhere be found in the
interior of the design space bounded by photosynthetic carbon
fixation, mineral carbonate formation, air-scrubbing technology, and
the vision of a mechanosynthetic replicator. Ultimately it's all
chemistry. Is there something in there which has the ferocious carbon
appetite of the hypothesized artificial replicator, but which is both
less dangerous and more immediately attainable? That's the question
occupying my mind.

Thanks for your attention,
Mitchell Porter
Brisbane, Queensland, Australia