Science News

Engineering a cooler planet is both possible and affordable

Sun, 9th Sep 2012

Alan Boyd

What do aeroplanes, airships, rockets, guns and gas pipes have in common?  They could all help to cool the Earth by delivering millions of tonnes of global warming-busting aerosols into the Earth’s stratosphere.

AeroplaneA group of researchers at Aurora Flight Sciences, Harvard University and Carnegie Mellon University conducted a study to predict the cost of using 6 possible systems to put 1-5 million tonnes of material into the atmosphere, at altitudes of 18-25 km in the Tropics.  The prediction of the costs involved was based on previous statistical models developed for use in the aeronautics industry.   The systems included retrofitting existing aircraft, designing new types of aircraft or airship, or shooting material into the sky using rockets or modified artillery guns.  The group even considered permanently suspending a 20km long pipe from a helium-filled floating platform in the stratosphere.

These systems would deliver a slurry of material into the atmosphere that would increase the reflectivity (or albedo) of the Earth by reflecting the Sun’s rays in the same way as a mirror reflects light, resulting in a reduction in energy reaching the surface and heating up the planet.

The study, published in IOP Publishing's journal Environmental Research Letters, found that the design and deployment of new high-altitude aircraft was by far the cheapest possible option, with costs of around $1-2 billion dollars.  The most expensive options were the rocket and gun-based deployment systems, costing 10 or 100 times more than the other options due mainly to their lack of reusability.  This may sound extremely expensive, but the cost of reducing carbon dioxide emissions is currently estimated to reach between 0.2 and 2.5 per cent of U.S. Gross Domestic Product (GDP), which is the total value of the goods and services produced by a country in a year by the year 2030, or roughly $200 to $2000 billion.

The more original ideas, using an airship or suspended pipe system led to a high degree of uncertainty, due to the difficulty in predicting their cost of development from a basic idea to a fully functional system.  However, the predictions show that the suspended pipe would, if successfully implemented, give the cheapest recurring cost-per-kilogram of particles delivered of all the systems considered.

The authors of the paper stress that reducing incident sunlight using aerosols does not reduce concentrations of greenhouse gas in the atmosphere, nor does it combat the resulting rise in the acid content of the oceans, again due to the increase in greenhouse gases.  Rather, as co-author Professor Jay Apt of Carnegie Mellon University, comments, “If for some reason, we are unable to stop ourselves from putting greenhouse gasses in the atmosphere...then it’s useful to have in our hip-pocket an understanding of what the emergency measures are that might quickly cool the planet.”

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It just seems to be counter-productive to fly airplanes and use extra fuel specifically to make high clouds.  And, I would think there would have to be a lot of flights, for a fairly short-term effect.

Is there any way to work this in with commercial jet traffic?  What are the effects of northerly intercontinental flights (summer vs winter?)

The development and deployment cost of some kind of a space mirror would be very high, but once up there, it could last for essentially as long as one wanted it.  And, if designed right, one might be able to heat one part of the planet and cool another part of the planet.  Or, perhaps capture energy for some other use. CliffordK, Mon, 5th Nov 2012

Heat one part of the planet and cool another, CliffordK? I doubt that that would "capture energy". Much more likely that it would provide the driving force for super storms on a scale way beyond anything we have yet seen!

Yes this strategy would provide energy, but energy that would surely be expressed in rogue, untameable ways!

If we are planning to produce a lot more stratospheric aerosol, we are also planning a lot more ozone depletion. The chemistry that produces the Antarctic Ozone Hole is largely catalyzed by the surfaces of particles in stratospheric aerosols. At mid latitudes and tropical latitudes, the chemistry would be a little different, but it would still amount to significant ozone depletion.

Lethal UV radiation can be blocked out by the effectiveness of scattering of short wavelength radiation by aerosol, but even so we would need to think very carefully about lowering our ozone shield! damocles, Sun, 18th Nov 2012

Much of the recent ozone depletion seems to be related to polar winter weather.  I.E.  When the sun doesn't shine, in a place where few humans are living. 

Much of the ozone hole fills back in every summer when one gets the solar exposure.

Yet, I agree that any geoengineering could have unintended consequences. 

Targeted artificial warming the poles in the winter may in fact reduce the temperature differences, and reduce storms.  However, it would depend if one chose to just warm the land masses, or warm both the land and water. CliffordK, Sun, 18th Nov 2012



The ozone story is rather more subtle than that CliffordK. The chemistry that leads to the Antarctic phenomenon, and a similar less developed phenomenon in the North, involves aerosol, and visible sunlight. It occurs not in the winter, but in the spring, with returning sunshine after the polar night. And aerosol plays a very important part.

The stratosphere is a very dry place. Only in the Antarctic winter does it get cold enough to saturate with water vapour, and form ice aerosol. Nitric acid aerosol also forms at those temperatures. September and October are the peak months for ozone depletion. Not before September because there is no sunlight, and not after mid to late November because all of the clouds evaporate in the warmer sunshine.

Aerosol that is thrown into the stratosphere by certain types of volcano does not evaporate -- it consists largely of non-volatile sulfates. It also provides surfaces for the critical ozone-depleting reactions to occur, although the detail of the chemistry is different because of the absence of a long winter darkness to pre-process the chemicals, and the different chemical nature of the sulfate particles.

We can tell when there is volcanic aerosol in the stratosphere: if you look in the region of the sky near the sun on a clear day (and assuming no local smog or raised dust), it will have quite a pink tinge, that fades into the normal light blue as one's eye wanders to the sky further from the sun. The last time that I remember this happening in the Southern hemisphere was about 20 years ago when there were significant major eruptions of Pinatubo in the Philippines and of a major volcano in Chile within a few months of each other.

There has been a lot of work done on the chemistry of stratospheric ozone in the presence of sulfate aerosol -- I am not fully in touch with the latest modelling and evidence. however it is almost certainly the case that (1) there is some ozone depletion involved, and (2) that scattering of solar UV light at high altitude by a stratospheric aerosol more than compensates in reducing the amount of UV that penetrates to the Earth's surface. damocles, Sat, 24th Nov 2012

Sure we can, a major atomic war should do it, and decrease population world wide too. It won't be a pretty sight though, and the aftermath will be hard to live with. yor_on, Tue, 30th Apr 2013

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