Nature has a brief report on a PNAS paper, "Transient climate-carbon simulations of planetary geoengineering" by H. Damon Matthews and Ken Caldeira. BTW, before I get going, look at the sidebar on the right of PNAS - there is a "google scolar" search link built in. The abstract is:

Geoengineering (the intentional modification of Earth's climate) has been proposed as a means of reducing CO2-induced climate warming while greenhouse gas emissions continue. Most proposals involve managing incoming solar radiation such that future greenhouse gas forcing is counteracted by reduced solar forcing. In this study, we assess the transient climate response to geoengineering under a business-as-usual CO2 emissions scenario by using an intermediate-complexity global climate model that includes an interactive carbon cycle. We find that the climate system responds quickly to artificially reduced insolation; hence, there may be little cost to delaying the deployment of geoengineering strategies until such a time as "dangerous" climate change is imminent. Spatial temperature patterns in the geoengineered simulation are comparable with preindustrial temperatures, although this is not true for precipitation. Carbon sinks in the model increase in response to geoengineering. Because geoengineering acts to mask climate warming, there is a direct CO2-driven increase in carbon uptake without an offsetting temperature-driven suppression of carbon sinks. However, this strengthening of carbon sinks, combined with the potential for rapid climate adjustment to changes in solar forcing, leads to serious consequences should geoengineering fail or be stopped abruptly. Such a scenario could lead to very rapid climate change, with warming rates up to 20 times greater than present-day rates. This warming rebound would be larger and more sustained should climate sensitivity prove to be higher than expected. Thus, employing geoengineering schemes with continued carbon emissions could lead to severe risks for the global climate system.

Nature picked out the bit about precip: it gets drier. The commentary explains this: more CO2 but constant T means more efficient veg use so lower evap so lower (tropical?) ppn. But the commentary also points out that this may only be a model effect.

As for the rest, its fairly obvious: put in stratospheric aerosols and you can reduce T. This isn't the first paper to do this (though its the first one I've noticed); it claims to be the first to do it in a transient rather than 2*CO2 model; but its only an EMIC. And in fact they don't "put in aerosols"; they just reduce incoming solar to match CO2 forcing. The T response (fig 1) is then fairly small tropical cooling and polar warming. The other "exciting" but bleedin' obvious result is that if the geoengineering fails then it gets warmer very quickly.

I should probably add... one of the problems with these schemes is to achieve a global balance. For example, there was a scheme to increase the albedo of marine stratocumulus regions. Fine, but if done enough to keep *global* T constant it would have caused massive local decreases whilst allowing massive polar warming. This paper is "cheating" by allowing themselves to do it trivially globally.

Overall: interesting to see people doing this as a thought experiment. Needs a lot of refinement to be more realistic. Would be scary if done in reality.

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"And in fact they don't "put in aerosols"; they just reduce incoming solar to match CO2 forcing."

Would the extra aerosols be expected to have much effect in other* ways? Eg would they affect precipitation or cloud cover much?

*Obviously that would be from different research.

[I think it would be very hard to make the aerosols globally uniform, but I'm not sure where they would be expected to pile up. They would also be rather vertically different. And maybe they would be thick enough to have obvious optical effects. If they stay in the strat they they wouldn't affect weather much. But if they atart to fall out, as they would, then they probably would have an effect... who knows what it might be -W]

Putting global dimming aerosols into the upper atmosphere is a relatively inexpensive way to mitigate temperature rise associated with increased greenhouse gas levels.

On the other hand, it probably should be used only as a short term bandage, until human emissions fall below the ability of nature to remove the CO2 from the air. Besides, what about acidification of the oceans from excess CO2 in the air?

In my opinion, a much better solution is to remove the CO2 from the air after it has been emitted. Nature already removes about half each year, but that is expected to reduce 30% by 2030. Furthermore, a warming earth means that carbon sinks will become carbon emitters big-time.

I suggest improving nature's ability to remove CO2 from the air using genetic engineering-perhaps seeding a GMO into the oceans. Biosequestration is a low cost, highly scalable, and technically feasible solution to global warming. Besides, in a recent poll 40% of people would improve their childrens genes using genetic engineering.

By Brad Arnold (not verified) on 18 Jun 2007 #permalink

Although it is also an unproven technology, with a lot of problems yet and a few possible dangers, I personally feel that underground carbon sequestration may be one of the things we need to seriously be investigating as part of our arsenal of fixes. Note that I do NOT advocate storage as clathrates -- I think how unstable clathrates are or can be is way too downplayed.

There is just too much emission for us not to use sequestration, though; even if we cut down our carbon contributuion, it isn't going to be enough to mitigate the most certain effects, and from what I've seen in various journals we are also just on the edge of seeing some of the major sinks (peat bogs, for example) turn into sources. Once they really get going, we either need very good carbon capture/sequestration technology to reduce additions as much as possible, or realistically it WILL run away with us.

Underground sequestration ought to be safe enough if it is done well, carefully and in appropriate locations. The big problem there, of course, is the same problem we really have with nuclear energy as a solution: human nature. We cut costs by cutting corners, and go for "quick fixes" which are politically easier and cheaper for now, leaving it for the next administration to clean up the problems.

By Luna_the_cat (not verified) on 18 Jun 2007 #permalink

Brad Arnold wrote:

"Besides, what about acidification of the oceans from excess CO2 in the air?"

Yes. There are really two classes of geoengineering "solutions" to climate change.

One class just attempts to cool the planet, usually by modifying the planetary albedo, while leaving the underlying problem (excess CO2) intact. This is kind of like treating the symptom rather than the underlying disease. As Brad says, it fails to address CO2-related problems like ocean chemistry. As the original post says, it also puts us at risk of ultra-rapid climate change if the geoengineering fails or is abandoned at any point.

The other class of geoengineering schemes involves sequestering CO2 (underground, or via iron fertilization of phytoplankton, or whatever). Of course, there are questions about whether these would actually *work*, but at least they attempt to remove CO2 from the system rather than merely mask its symptoms.

Actually, though, I doubt that these kinds of things will ever be used on a massive scale. Once some agent (a company, or a country, or an international organization) begins consciously tampering with the climate in a large way, they will be held "responsible" in the public mind for all negative impacts of climate. Right now, if there's an untimely killing frost, or a flood, or whatever, it's still seen as an "act of God". With global warming, the blame is at least still diffusely ascribed to all of modern industrial society. But does one group or corporation really want to start assuming the liability for directly *managing* the climate? I don't think so!

Has anyone looked at the engineering cost of these sulphur injections to the stratosphere? Can a vortex be used or are rockets needed? What kind of tonnages are we talking about?

Please do your homework before this costs you your economies.

My point in all of this is that CO2 does NOT cause climate change; I am not arguing that a change in the climate might be occurring. The climate on earth changes all the time and that global change is caused by the Sun (a new NASA finding). All life on the planet is carbon based, CO2 is part of our food chain, and it is not a pollutant. The biggest "green house gas" is water vapor. If climate change is caused by human activity then we would need to start eliminating life on the planet, yes this is absurd, so is the assertion that humans are causing climate change. It just is NOT the truth.

It's psychobabel. Science fact 2+2=4 a scientific fact is a truth that never changes can be reproduced by anyone every time.

Additional information

[So far so wrong. Your top ref seems to be TGGWS, which is badly flawed: see for more. You're also wrong about the warming on Mars; RC has that too, or you may prefer… If you want to be taken serikously, you can't use Mars (well, or TGGWS, which is merely propaganda) -W]

Would they have a graph showing the temperature response curve for an instantaneous change in aerosol forcing?

[This is PNAS; you can read it too (can't you). They do say that if you go to the end of the century and then remove the "aerosol" you get some steep T rises: 4 oC in a decade or so -W]

I am sure you'll remember the discussion James Annan and me had on precisely this topic over on the global change group.…

And, wouldn't stratospheric aerosols be fairly well mixed? I remember an earlier study by Caldeira saying that (stratospheric) aerosols could indeed pretty well compensate for doubled CO2 even at the regional level (with regards to temperature deviations).

[The paper refs an earlier C study, but that too used change in solar as a proxy for aerosol. I don't think they have done the requisite studies, I don't see any evidence they have even done an imposed-strat-aerosol study, let alone a transport model to determine where the stuff would get to -W]

Thanks, I thought it was subscriber only. The 4C per decade is an instantaneous rate, ie it's 0.4C in the first year, after that the rate is much lower. It's nice to see that that's in good agreement with our earlier discussion over on global change.
On the stratospheric aerosol mixing, I don't quite remember the detail of what they did, but they certainly left me with the impression that it was reasonable to assume an even forcing across the globe from stratospheric aerosols. Maybe that assumption is not nearly as well supported by them than I thought it was.

Being a photochemist, I wonder what the UV is going to do to the aerosol mixtures. It could be Primal Soup II (the Sulphur Beast from the Sky)

"Would be scary if done in reality" - I agree. Such technocratic solutions are really scary movies...
There is *no feasible way* how to reduce CO2 output???

This reminds me of the brief cartoon by Matt Groening that Gore included in his film, where the people begin to put larger and larger ice cubes in the ocean every year, thus solving the problem now and forever.

On a more serious note, I recall hearing somewhere that injecting sulphate aerosols into the stratosphere would tend to turn the sky a more whiting hue. I'm curious if there is any truth to this, as a quick google search turns up little substantive.

[We had some good sunsets after Pinatubo -W]

By Zeke Hausfather (not verified) on 18 Jun 2007 #permalink

The Crutzen paper from last year mentions whitening and it seems a likely effect. Couldhave some reall impacts as well as sunsets (though I must say i remember enjoying teh sunsets). Bad for solar thermal systems using concentrators -- IIRC Mike MacCracken told me that one of the earlier solar concentrator plants lost as much as 20% of its efficiency after el chichon in the early 1980s. (PVs much less effected). There's also a plausible belief that more diffuse sunlight has a positive effect on photosynthesis. Whether this effect can be seen in the 1990s, driven by Pinatubo, is I believe a point of contention. Alan Robock says yes and has published on the matter, but I remember hearing that some fairly serious biosphere modelling types think you can save the phenomena of the 1990s without this effect.

By Oliver Morton (not verified) on 21 Jun 2007 #permalink

"If they stay in the strat they they wouldn't affect weather much. But if they atart to fall out, as they would, then they probably would have an effect... who knows what it might be"

I was reminded recently that there are (rare?) processes that can bring stratospheric air down to the surface. Normally this shows up as very dry air, but I guess in this case it would be very dry air, heavily laden with suplhate aerosols?

[I rather doubt that strat air gets to the sfc without heavy modification. It would be very very very dry if it did make it. Also very hot -W]