Andrew and list
1. I have been meaning to comment on this article by Dr. Andrew
MacDougall, that you cited in October. I was reminded by it being referenced
in the last Science issue (Dec. 6, p 1149) I received. The article is NOT
behind a paywall. It is at:
http://onlinelibrary.wiley.com/store/10.1002/2013GL057467/asset/grl51021.pdf?v=1&t=hp8v0lqy&s=27b2334b807e010482b6e4fa3f41a8703797473f
2. I took the main result to be quite negative for CDR. But there still
some positives to gain from an analysis I believe can be improved. The faults
I find are only in the papers four scenarios, which are:
a) that only as much carbon can be removed from the atmosphere as was
inserted (no explanation on why this limit), and
b) is to be taken out as a “mirror” of the way it was inserted (and I am
not understanding details of the mirror, but this seems highly unrealistic), and
c) requires going back to 280 ppm (not the usual 350 ppm)
d) looking at very high out year peaks, that are inconsistent with the
concept of CDR.
3. I am of course unhappy that there is nothing about biochar in this
article (for out-of-date reasons), but the specifics of the CDR approach are
not critical and are not I believe in the model. There are a number of useful
references to somewhat similar papers. There is no sense of “Irreversibility”.
So I am still hoping to see modeling as detailed as in this paper, that
includes the potential for more soil carbon. There are some out year freebies
with biochar that have not been modeled by any group, to my knowledge. To the
best of my knowledge none of the removed CO2 ends up in soil.
The model is better than many by including Arctic methane release. The
long time periods to reach the year-1850 conditions are because of the four
faults I am claiming above in the four assumed scenarios. These should be
readily fixable.
4. The positives that I think we can take from the article relate to what
can happen with more realistic scenarios, which should have these four
features, that replace the above #2 assumptions:
a) No limitations on the amounts that can eventually be removed from the
atmosphere.
(The required removal amounts are a very small percentage of CO2
in the oceans, or allowed deep underground - and I know of no reason that soil
carbon couldn’t be doubled or tripled without harm.)
b) Use annual removals that emphasize speed and are reasonably least cost
- not a “mirror” of anything.
(For biochar options, the limiting speed at first will be the
manufacture of pyrolysis hardware - which presumably could have a doubling time
at first measured in months. For sure, the output of such factories will not
be time-independent, so the growth rate should not be linear. When a
production plateau will be reached is a function of costs and benefits, which
can be controlled by policy decisions ($10 or $100/tonne CO2, etc). This can
be handled parametrically, with numbers like 1, 10 or more Gt C/yr being
assumed and costed. The important point is that for decades I think we can
assume an annual increase in removal, not the annual decrease that (I think) is
in the MacDougall model. I would hope for a model that talks of getting to 350
ppm well before 2100.
c) First investigate the 350 ppm end point; continue to 280 ppm as a
second stage effort.
(350 ppm just because there has been so much emphasis on that
number. 280 ppm should certainly be modeled as well.)
d) I would restrict attention at first only to the two lowest of the new
IPCC AR5 peak scenarios. I believe his present analyses already adequately
argue against the two higher.
5. So, I hope that Dr. MacDougall or others will extend his analyses in
these directions. The cause of CDR can be enhanced by trying to achieve what
some climate scientists are saying we need to do - or showing why it cannot be.
Lower ultimate costs will result by striving for speed and avoiding terms like
“the year 3000” (in the abstract).
To repeat, I am not intending to be critical of this paper. I believe the
results are reasonably correct for the assumed scenario. I am suggesting we
need also to look at scenarios that are much less restrictive.
Ron
On Oct 8, 2013, at 3:18 AM, Andrew Lockley <[email protected]> wrote:
> http://onlinelibrary.wiley.com/doi/10.1002/2013GL057467/abstract
>
> Reversing climate warming by artificial atmospheric carbon-dioxide removal:
> can a Holocene-like climate be restored?
>
> Andrew. H. MacDougall
>
> DOI: 10.1002/2013GL057467
>
> Geophysical Research Letters
>
> Keywords:
>
> Holocene;Climate warming;Reversibility
>
> Abstract
>
> Most climate modelling studies of future climate have focused on the affects
> of carbon emissions in the present century or the long-term fate of
> anthropogenically emitted carbon. However, after carbon emissions cease there
> may be a desire to return to a “safe" CO2 concentration within this
> millennium. Realistically this implies artificially removing CO 2 from the
> atmosphere. In this study experiments are conducted using the University of
> Victoria Earth system climate model forced with novelfuture scenarios to
> explore the reversibility of climate warming as a response to a gradual
> return to pre-industrial radiative forcing. Due to hysteresis in the
> permafrost carbon pool the quantity of carbon that must be removed from the
> atmosphere is larger than the quantity that was originally emitted (115–180%
> of original emissions). In all the reversibility simulations with a moderate
> climate sensitivity a climate resembling that of the Holocene can be restored
> by 3000 CE.
>
>
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