Geoengineering refers to massive engineering projects that may indirectly mitigate some of the detrimental effects of increased greenhouse gases (GHGs), though geoengineering would do nothing to solve the energy challenge. At best, some geoengineering projects might be shown to be worthwhile in reducing the worst effects from global warming decades from now– though at great cost. However, on Oct 30, 2010, geoengineering was banned by the UN Convention on Biological Diversity. 192 nations (not including the U.S. and a few other nations) were party to the agreement. (See references below.)

(We note that there has been an attempt recently to obfuscate geoengineering by co-opting this word and using it to mean geotechnical, geological, or geophysical engineering. Websites have been devoted to that purpose. Perhaps their goal is to try to make ejecting massive amounts of sulfate aerosols into the atmosphere appear as innocuous and beneficial as building a large dam.)

By far, the most discussed and researched possibility is aforestaion - increased planting of trees. However, recent studies suggest this is worthwhile only in warm and reasonably fertile areas. In the upper latitudes (above the 50th parallel), the increased solar absorption in the slower growing forests more than offsets the beneficial effects of their increased carbon sequestration. In arid areas, the cost and soil consequences of irrigation would far outweigh the benefit. (We have already seen the destructive consequences of massive, poorly planned irrigation efforts projects in several places around the world.)

The second-most mentioned geoengineering possibility is iron fertilization of the oceans. Some very crude calculations in the late 1980’s suggested it would be easy to get phytoplankton to sequester enormous amounts of carbon in the ocean depths by spraying a small amount of iron sulfate over the surfaces of the oceans. After two decades and more than a dozen small-scale experiments, the current consensus is that at best this might be able to help a little; but it would be quite costly, and there are numerous unknowns that may lead to increased risks for currently threatened fisheries and ecosystems. At least another decade of studies will be required before there could be any decisions to proceed (in carefully selected areas) at a scale that might make any significant contribution to CO₂ sequestration. A recent article in Nature (9/2009) says the concept should be abandoned.

Engineered Cooling. The debate on the reality of global warming amongst reputable scientists ended in 2007 – partly because of data such as that shown here.

Unfortunately, the debate whether or not anthropogenic CO₂ is the primary cause of global warming seems likely to continue for at least several more years; and as long as this is debated, projects aimed at reducing warming without dealing with its cause will continue to be proposed. Some suggestions for lowering the earth’s temperature include: high-altitude reflective balloons; orbiting mirrors; and continuously dispensed sulfate or sulfide aerosols at high altitudes (see Sci., 30 May, 2008). Of course, such projects would do nothing to address the CO₂-induced acidification of the oceans, which appears to be a major culprit in the decimation of coral reefs around the world.

To our knowledge, none of the above “global-cooling” proposals has been subjected to much serious engineering analysis. We do not expect any to be either cost effective or environmentally acceptable at the scale needed to address the atmospheric CO₂ challenge. For example, one recent small study (see Sci., 30 May, 2008) concluded the proposal for stratospheric sulfate aerosol injection could destroy the ozone layer and severely reduce precipitation in important agricultural regions in Asia and Africa.

CO₂ Mineralization. One idea has received far more attention than it deserves is crushing rocks and spreading them over hundreds of thousands of square miles of the deserts. Many naturally occurring rocks (especially igneous magnesium-calcium silicates, such as olivines and serpentines) are in a higher energy state than their carbonates and thus they will naturally (exothermically, but extremely slowly) react with CO₂ to form the solid carbonates. The problem, however, is in the kinetics.

Olivine carbonation rates calculated as a function of CO₂ temperature and pressure show a factor of 2E6 increase as pressure and temperature increase from 300 K, 4E-4 bar CO₂ (normal atmosphere) to 100 bar, 470 K. About 50% carbonation was achieved for 90µm, high-grade olivine (Mg₂SiO₄, such as forsterite) in 100-bar CO₂, at 470 K, in 1 day. Diffusion times are usually quadratic with grain dimension. Thus, we estimate it would require about 700,000 years for 50% of an olivine sand of 1 mm grains dusted over a desert to convert to carbonate-silica sand. The rates for serpentines are lower yet. Naturally, there could also be adverse environmental effects, both at the enormous rock quarries and in the deserts, as many serpentines and olives poison soils from their high chromium and nickel contents.

As a cost reference point, fine limestone gravel costs about $40/ton when oil is $100/bbl. Pulverizing and dusting over the desert would probably add a similar amount to the cost. Hence, the up-front cost of absorbing CO₂ in an olivine sand would probably be ~$200/ton of CO₂ absorbed, but it would take several hundred thousand years to realize much of the benefit.

A variation on the above theme that avoids poisoning the deserts and reduces the time from hundreds of millennia to days is pulverizing the sand to an extremely fine powder (micron sized) and reacting it in carbonic acid at over 150⁰C and pressures above 80 bar. Separation processes involving substantial solids handling achieve a lower fraction of theoretical efficiency limits than those involving limited to liquids and gases. It is hard to imagine how any mineralization process could begin to be competitive with CO₂ injection into saline aquifers.

It has been suggested that mineralization rates are sufficiently rapid at high temperatures and pressures to enable in-situ conversion of CO₂ in uplifted portions of peridotite mantle, which has high concentrations of olivines and pyroxenes (Mg₂Si₂O₆). One idea is to fracture hundred of cubic miles of warm mantle and pump CO₂ (possibly separated from the atmosphere) into the mantle at high pressure. However, it has been shown that the biggest energy penalty (and hence, cost) associated with conventional CO₂ sequestration options is associated with CO₂ separation from the point sources; but it is still more cost effective to first separate the CO₂ from point sources, rather than simply compress the raw effluent (that is three orders of magnitude more concentrated than the atmosphere) and pump it into geological formations. Moreover, the energy required for CO₂ separation from the atmosphere would be at least seven times greater than for separation from point sources by any method. Hence, CO₂ injection into fractured peridotite mantle would certainly be much more expensive than conventional options.

Hot water also reacts exothermically (but again extremely slowly) with magnesium silicates, so it has also been suggested that one could simply pump seawater through uplifted portions of the mantle, achieve sufficient heating from the hydration, and preferentially suck the CO₂ out of the water. The CO₂-depleted seawater would then be returned to the sea. However, the oceans are only about 0.009% CO₂. Hence, this idea seems three orders of magnitude away from being realistic.

Another idea that has been heavily promoted is to look for a better method of getting dolime (an admixture of CaO and MgO) from novel sources and using that to make a commercial-grade cement with properties similar to Portland cement. However, notwithstanding VC money and well publicized claims to the contrary, no method has yet been published that would lead to significantly reduced total CO₂ emissions in the production and use of a hydraulic cement compared to current processes – calcining carbonates. In fact, the opposite appears to be true of the process described in the pending patents by Constantz.

We considered writing a detailed scientific analysis on the Constantz-Calera-Khosla cement scam, but that is probably no longer necessary. Ken Caldiera has begun to expose it, so plenty of others should now be willing to speak up:
http://cleantech.com/news/4327/you-say-caldera-i-say-caldiera
http://www.greentechmedia.com/articles/read/more-clues-in-calera-cement-controversy/

Hurricane killing. There have been a number of physically plausible approaches to stopping the destruction from hurricanes for at least the past three decades. These do not really fall within the category of Geoengineering, but rather weather control. However, we will mention one idea here that has recently received considerable attention because it was advocated (and patented) by Bill Gates and Ken Caldiera – pumping cool water from ocean depths ahead of an advancing hurricane to the surface to cool waters and thus kill its primary energy source – the low atmospheric pressure caused by the water vapor from warm water. Of course, to kill a hurricane by such a scheme would require an enormous investment of capital and energy. (A more cost effective concept was proposed two decades ago.) The two biggest problems with their deep-water-pipe method are (1) there is no sure economic justification (one never knows there would otherwise be major economic damage from the hurricane), and (2) the enormous release of CO2 from the deep water (saturated with CO2 at high pressure) would probably cause more harm to the climate than the economic benefit from killing a hurricane. There are also other huge potentially disastrous effects on sea life associated with the abrupt change in local salinity, oxygen, and nitrogen contents, to mention but a few.

Energy Needed. Recent global climate simulations (published in Nature, April, 2008), indicate it is likely that cyclic variations in ocean currents have played a role in accentuating the observed global warming over the past decade, and their now reversing effects may reduce warming over the next five years. Unfortunately, the urgency of the energy challenge will only grow with each passing year if major progress is not seen toward real energy solutions.

References:

UN Ban on Geoengineering:
http://pubs.acs.org/cen/news/88/i45/8845news1.html

http://en.wikipedia.org/wiki/Geoengineering

http://www.bbc.co.uk/news/science-environment-11076786

“Ocean Fertilization: Time to Move on”, Nature, 9/2009,
http://www.nature.com/nature/journal/v461/n7262/full/461347a.html

P Goldberg, ZY Chen, W O’Cnonor, R Walters, and H Ziock, “CO₂ Mineral Sequestration Studies in US”, NETL, 2000.

PB Kelemen and J Matter, “In situ carbonation of peridotite for CO₂ storage”, PNAS, 105, 45, pp 17295-17300, 2008.

Alan Robock, “Whither Geoengineering”, Science, 320, pp 1166-1167, 30 May, 2008.

see C&EN, Mar. 31, 2008

David MacKay, “Sustainable Energy – without the hot air”. It’s available for free download here:
www.withouthotair.com