Other SG Concepts

Solar geoengineering (SG)  (also called Radiation Management, RM, by some authors, Climate Engineering, CE, by others, and simply geoengineering by others) is not a new concept.  There have been hundreds of studies and dozens of major conferences over the past three decades discussing the possibility, and there is widespread consensus of the imminent need for some form of substantial solar dimming solution.  The problem is, thus far, only five potential solutions have been widely discussed, and all have a slate of costs, concerns and unknowns that have served to stall the next step towards implementation.

There is little doubt that a combination of the previously studied SG methods could slow or stop global warming at least for some period of time.  After all, we only need to reflect 1% of the sunlight that the Earth receives in order to counteract all of warming caused by the current increase in atmospheric CO₂ levels over pre-industrial levels. 

SG doesn’t address some problems (particularly, ocean acidification) associated with CO₂ increases, and SG alone would only result in an ever-increasing global demand for further SG, as the greenhouse gas levels continuously build while effort is expended to shade the Earth and keep warming at bay.  Efforts toward increased usage of low-carbon energy must continue at the fastest rates possible, but even the most massive and rapid scale-up of green and nuclear energy – beyond anything imaginable – will not be sufficient to prevent 100 million people in coastal cities from being forced to flee their homes over coming decades. Clearly, something else must be done.

Briefly, we’ll review the potential, the concerns, and the “known unknowns” of competing solar geoengineering concepts.  But before we begin, it’s critical to note that we strongly support additional research and study into some of these concepts (just as we strongly support the continued investment in reducing GHG emissions as quickly as possible).  We believe that all but one of these concepts may play an important though limited role in mitigating the ice shelf collapse and rapid sea-level rise that is becoming a more and more certain near-term threat.  We also believe that the threat of ice sheet collapse is significant enough that more than one response may be required, and all reasonable ideas should be fully explored and utilized in ways that are most efficient for stalling and reversing the threatened ice sheet collapse.
(Please note: while we look here at the effects, side effects and cost of other SG proposals, we also took a serious look at these issues in our proposed Tropical Cloud Generation plan. These assessments can be found on the Home page, in TCG Side Effects, and in the Sea Mixer page.)

Surface Albedo Enhancement (SAE)

Starting (literally) from the ground up, the first proposed method for solar geoengineering is the most obvious: Surface Albedo Enhancement (SAE).  Albedo is a measure of the reflectivity of a surface, or the measure of the percentage of solar energy that is reflected back into space rather than absorbed.  Obviously, the more energy that is harmlessly reflected back, the less warming we will encounter.  Most SAE involves land-based measures that will not significantly impact the sea level rise, but may result in considerable reduction of heat in urban “heat islands”… simple measures ranging from painting roofs white, considering whiter building materials, changing “blacktop” asphalt into lighter shade coloring by painting it white, etc… 

Land-based SAE is interesting in terms of city management, but not interesting to this discussion as its potential for mitigation of global warming or collapse of major ice sheets is negligible.  What IS interesting is the reflectivity of the Arctic Ice Pack, the Greenland Ice Sheet, and the Antarctic and West Antarctic Ice Sheets and Ice Shelf.  White ice covered with snow has an albedo of over 0.9 (meaning that more than 90% of the solar energy that irradiates it is reflected, rather than absorbed).  Compacted ice without snow has an albedo of between 0.7-0.8, while smooth ice (ice that has melted and then re-frozen smooth) is ~0.6, and smooth ice covered by melt ponds (regions of standing liquid meltwater on the surface of the larger-deeper ice sheet or ice pack) has an albedo of ~0.4.  Meanwhile, sea water has an albedo as low as ~0.06.  So the less ice that remains over the Arctic and Antarctic seas during the summer and autumn, and the more early spring melts that result in smooth ice or melt-ponding, the greater the amount of energy transferred during the summer and autumn months into the warming of the polar seas and melting the great ice sheets. 

Much of the discussion of solar geoengineering (SG) revolves around the ocean tropics: that region receives the greatest share of insolation, and it is readily absorbed into the seas.  But in order to stop the collapse of the ice sheets and forestall the projected near-term rapid sea level rise, we need to focus on limiting the rise of Arctic and Antarctic temperatures

Marine Cloud Brightening (MCB)

            The second SG method that has been proposed is marine cloud brightening (MCB).  The goal is to make the low-level cumulus clouds over the ocean tropics reflect more of the incoming (shortwave) radiation before it reaches the ocean – which again has an albedo of only 0.06, so virtually all of the radiation that reaches the ocean is absorbed by the ocean.

The method proposed with MCB is to spray fine sea water mist upward into low-hanging stratocumulus clouds… the micro-salt particles would then serve as condensation nuclei for micro water droplets within the clouds. At constant water content, the reduced droplet size means more droplets and hence more reflectivity. One proposed method for doing so would use specialized high-pressure (up to 800 bar) jet nozzles to spray fine streams of sea-water and air together with water velocities approaching the speed of sound.  The sea water streams then pulverize themselves into a very fine saline mist (sub-micron droplets).  The mist hangs over the ocean (which temporarily increases solar reflectivity) until the micro-droplets of water evaporate away.  Some of the remaining micro-salt particles are then lofted up into the cloud layer by atmospheric turbulence, helped by convection after the mist warms back to the temperature of the ocean from its lower-density (because of its increased humidity).

 The primary goal here is to decrease the average droplet size in the clouds (from 14 to 11 microns) by introducing millions of trillions of very small particles that serve as condensation nuclei.  With a greater number of nuclei, the same amount of liquid water would be distributed into smaller droplets – which would result in higher albedo for the cloud.  We think this could work, in part because of experience with aerosol emissions over-land.  In regions with high aerosol emissions, clouds have higher albedo (they are “brighter”).   However, over the ocean, there are much lower aerosol emissions (except in the paths of ships), so there stands to be a benefit.

There are overlaps between the advantages of MCB and those of Tropical Cloud Brightening TCG:  Both target reducing local heating of warm sea surfaces by increasing cloud reflectivity or cloud fraction over a localized region, and they both do not rely on introducing a foreign compound into the local ecosystem (as we will see in SAI – which is next). 

There are remaining concerns regarding MCB:

A recent small scale experiment by the Harrison group demonstrated feasibility of generating a fine mist from a small ferry (see above picture, near Townsville Australia), and the mist was lofted successfully. But because of that small scale, it remains uncertain whether this was due to specific weather conditions. The mist was produced by a rather large modified turbine with 320 nozzles (or maybe 100, different numbers have been reported) supplied with very high pressure sea water.  We have not been able to find useful engineering data on this experiment, but they estimated it would take 1000 mist generators, each 10 times larger than this one, to reduce incoming radiation over the Great Barrier Reef by 6.5%.

First, there is the question of how to generate the needed fine sea mist.  Some researchers have argued for the past decade that it should be possible to generate the needed sub-micron mist very efficiently by using arrays of 100 million (or more) sub-micron tapered orifices in Si3N4-coated silicon wafers, together with ultrasonic vibration or high-voltage acceleration.  (A proposed thickness where the nozzles are etched was just 2.5 microns!)  We have not been able to find data on any remotely relevant demonstration, of even an array of a few dozen suitable sub-micron nozzles, though perhaps some will be forthcoming from the Wood group in a few years.

Secondly, there remains uncertainty in the fraction of salt particles that would be lofted into the clouds rather than settle into the ocean, as well as questions about some important cloud physics.  Recent simulations (9/2021) suggest a narrow range of successful particle sizes, which would differ for different conditions. Other recent research shows it is possible that the salt micro particles will increase precipitation, causing cloud thinning, leading to increased incoming radiation instead of decreased.  It will take a demonstration at least 100 times larger than the above to begin to answer these questions, which will help determine the ultimate complexity and cost of the nozzle array and the method.

Thirdly, the proposed propulsion assisting and power generation method for the ships: wind. Flettner rotor sails have generally resulted in only marginal savings in fuel for ship propulsion, and it adds substantial costs. More substantial still are the restrictions required by seeking to generate the electrical power needed for the proposed nozzle arrays from wind.  The nozzle arrays required to produce particles of appropriate size by the only method demonstrated thus far (high velocity water jets) would require many megawatts per ship.  (We note others have estimated power requirements as low as 300 kW per ship, but they assumed mist generating methods that have not been shown to be practical, even in simulations.)  In order to provide even 5 MW of generated power, the hull sizes of the ships in question would have to be very large to avoid capsizing in winds capable of generating the power that they need – which only would serve to further increase costs and propulsion requirements.  The ships used would almost certainly require either diesel or nuclear power in order to have the speed and flexibility to approach optimal cloud banks and brighten them before they either dissipate or rain out.

Fourthly, there remains major uncertainty in the number of ships that will be required. MCB advocates place that number at 800-1500 – though that assumes that a number of the remaining uncertainties are proven to be very favorable for the concept. A peer-reviewed study estimated it would require injecting 200-500 Mt/yr of sub-micron sea salt particles below tropical marine clouds to counteract the effects of CO₂ doubling, and it would require 16,000 ships, which seems roughly in line with recent estimates from the Harrison group.   The recent NASEM study notes that they had not found a useful cost study.  Neither have we..  However, we can do some simple calculations. If it requires generating 300 Mt/yr of sub-micron salt particles, and the method of doing so results in 30% of the pumped sea water producing salt particles of useful size being lofted, and doing so requires accelerating the water to 300 m/s in micro nozzles, then the kinetic energy density in the accelerated water is 45 kJ/kg.  

For a reality check, in snow-making machines almost all of the water droplets created are in the 500-1500 micron size range (a few percent are in the 50-micron size range, to create nucleating sites), and their energy usage (from the 90 psi (620 kPa) air and the 1000 psi (68 bar) water is ~15 kJ/kg in the larger systems.  If we assume that somehow it will be possible to produce mostly sub-micron particles of sea water with only 45 kJ/kg, then the minimum continuous power required by the needed fleet will be ~40 GW. (If the water is heated from 25C to 50C before going into the nozzles to ensure smaller droplets are formed and the plume rises quickly, the required power per mass flow is tripled, but maybe it would then take only a third the mass flow rate, so the same power.)

The groups advocating billions of sub-micron nozzles in ceramics believe that by making only 100-nm salt particles, the needed cloud brightening can be accomplished with an order of magnitude less sea water, accelerated to velocities an order of magnitude lower.  If so, the fleet power requirement might be only 40 MW instead of 40 GW.  (We at DotyNMR actually have decades of experience making tens of thousands of micro-nozzles in Si3N4 and other hard materials, by various methods.)  We suspect the nozzle sheet will need to be 20 times thicker than the 2.5 microns some have suggested and of a material 10 times tougher than silicon for sufficient robustness.  The only method we are aware of that could then make the desired sub-micron nozzles is an extreme-UV laser.  We suspect the cost for a robust sprayer with 200 million nozzles (along with all the other required apparatus) would exceed 100 million dollars.  Perhaps some other group will have an experimental data point (with at least 10,000 micro nozzles) in a few years from which a better cost estimate can be made. In the meantime, maybe someone should look at what can be done to reduce droplet size from snow-making machines by three orders of magnitude. There are more than a hundred nuclear submarines that have the power needed to drive them!

Even if we assume it only takes 1500 ships (each with ~30 MW of high-pressure pumping capacity) this would still present a challenging initial cost, as well as a challenging staffing problem. 

But far more uncertainties need to be clarified before any real estimates could be made for the cost of this proposal, or for costs for limited efforts using MCB to protect critical ecosystems in more shallow waters – like coral reefs – where Tropical Cloud Generation (TCG) could not be applied.  However, it’s worth noting that even assuming optimistic projections, the costs for MCB will likely be more than an order-of-magnitude higher than TCG in regions where TCG will be effective.

Stratospheric Aerosol Injection (SAI)

Of the SG methods that have received much attention, Stratospheric Aerosol Injection (SAI) sometimes referred to as the “Pinatubo Option”, has generated the greatest interest and for some time was thought to hold the greatest potential (but may also hold the greatest potential harm). When Mt Pinatubo erupted in 1991 and injected 18 million tons of SO₂ into the lower stratosphere, it cooled the earth by 0.5°C for a little more than a year before the temperature began to rise again. So when talks began of reducing incoming solar radiation (insolation), serious people immediately began discussing SAI.  SAI is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere (at altitudes in the 16-25 km range) in order to increase the reflection of incoming sunlight. Cost considerations have resulted in sulfur dioxide – SO₂ – being the primary “aerosol” considered.  (SO₂ isn’t an aerosol, but in the stratosphere it oxidizes to SO₃, then bonds with water vapor to form H₂SO₄, and then condenses to sub-micron-size particles of solid sulfuric acid, that grow over time.) 

Due to emissions regulations, we have seen a massive reduction in SO₂ emissions from global fossil fuel power plants over the past several decades, and this is likely to continue for many decades to come, resulting in the reduction of a hundred million tons of annual SO₂ emissions.  The goal of SAI is to inject the SO₂ into the stratosphere, where most of the H₂SO₄ microparticles might remain aloft for about a year (assuming mean particle size of ~0.5 micron, at 20 km) before descending and gradually being rained out, as compared to land-source emissions which rain out very quickly.  So, much smaller quantities of SO₂ would be needed as compared to land-based emissions.  In 1980, global land-based emissions of SO₂ were ~150 million tons/year (Mt/y).  By 2019, that had fallen to under 50 Mt/yr, and is likely to continue to decrease.  Linear extrapolation from the Mt Pinatubo data point suggests it would take 50-90 million tons of SO₂ sustained within the stratosphere in order to offset the 3.4°C global temperature increase expected from a doubling of CO₂ over pre-industrial levels (this may be an over-estimate due to the interactions between SO2 and volcanic ash).  There is considerable uncertainty over the rate of transport downward into the troposphere (or – if you will – the half-life of the lofted aerosols in the stratosphere) as it depends strongly on how quickly the particles coagulate from their initial most effective size of ~0.1 micron to several microns.  Some simulations indicate that this happens much more quickly than has generally been assumed.  In the sorst case, however, the total resulting acid rain would be only a fraction of the concentrations seen in the 1980’s, and it would be dispersed over the entire globe, rather than concentrated in small regions around point source emissions.  Acid rain is not a concern here.

Causes for hesitation:

            Before continuing we must stress that more research and development on this front MUST be allowed to continue apace.  We are facing a global existential threat that will cause tremendous hardship and burden every global economy in the world to the tune of tens of trillions of dollars, tens of millions of displaced refugees, and a rapidly increasing rate of privation, disease and death that is unthinkable:  We must be able do research into viable paths towards mitigating or forestalling those threats.  The recent actions of “environmentalists” that shut down Harvard’s recent – literal – trial balloon experiments are unconscionable, and absolutely not environmentally responsible.  We need to do research, and know what the potential gains and costs of various actions might be.  Any other response is as irresponsible as the global warming denialists camps.           

There are, however, two major concerns, several uncertainties, and one minor concern:  The first major concern is cost, and the second (more significant) concern is the potential impact on the ozone layer.  A minor concern is the additional fossil fuel consumption and jet exhaust of the mission to loft the aerosols.

Costs:

            The cost here should not be overlooked, as it is significant.  The cost projections here are all over the map, but many of these projections are coming from a completely Pollyannish perspective. 

Often the discussions around lofting these aerosols center on using blimps or balloons to lift the payload, and sometimes even involve concepts like “tethers” which would pump the payload into the stratosphere where it would be ejected by the blimp or balloon. (Note the image that we used at the top of this section).  But these concepts cannot be taken seriously.  The goal is the stratosphere, and presumably not the absolute lowermost edge. There is no possibility of a “tether” that stretches ~15-20 km vertically and can sustain the force of its own vertical weight.  That’s cartoonish.  Furthermore, the densities in this region of the atmosphere are ~10% of the density of air on the ground!  The lifting potential of a balloon or blimp is the buoyant force of the gas minus the mass of the airship.  By the time you ascend into the stratosphere, the buoyant force of helium (per volume) would decrease to ~10% of its buoyant force at sea level, or ~1 N/m³ when in the lower stratosphere.   For balloons to ascend into the stratosphere, they have to be designed to expand by a factor of more than 10 as they ascend.  The largest airship in the world today is the “Airlander 10” – which has a tremendous volume of ~38000 m³, and weighs ~20 tons. However, it isn’t designed to expand much in volume as it ascends. Its maximum design ceiling is only 6 km.  In order to reach an altitude of 16 km, there would have to be roughly 8 times as much balloon material for helium expansion as it continues to rise… which would increase the mass of the airship considerably, leaving far less available lift for a possible payload.  One data point to consider: World View Enterprises – a company based in Tucson Arizona – is offering tourists a chance to ride a high-altitude balloon into the stratosphere.  It costs $50,000 per person for a very short time hovering in the stratosphere before beginning descent.
There is simply nothing feasible about the concept of using gas volume buoyancy to loft millions of tons of payload into the stratosphere.

This will take specially modified high-altitude planes.  A lot of them. 

We don’t currently have planes that are designed for carrying heavy payloads into the high stratosphere to dump these payloads.  They don’t exist. 

Boeing’s largest next-generation plane, the 777-X, will have a payload of 74 tonnes, with 160 t fuel capacity and a price tag of $440M. With its two GE9X jet engines (and two smaller engines) its maximum certified ceiling is 13.1 km (43,000 ft).  With extensive modifications, no passengers (only crew, for safety reasons) and four specially designed engines, a similar plane could probably get to the 60,000 ft (18.3 km) altitude of the Concorde or the new Boom supersonic airliner. That will cost a great deal of money in new planes, infrastructure (airports are already overtaxed), maintenance, staff (including hundreds of new pilots), and of course, fuel.  It would take hundreds of flights per day of such planes to loft 6 Mt/yr of aerosols into the stratosphere.  If we assume a price per aircraft of ~500 million, and assume only ~200 of these planes would be required, that’s still a $100-billion price tag before the airport infrastructure expansions begin to be taken into account. The cost of the jet fuel alone for 200 flights/day would then be ~$20B/yr. This is assuming the low-end projections will prove to be valid once the uncertainties are clarified.  The final costs could be well higher. The proposal cost here is very small compared to the cost of inaction, but it will not be inexpensive. The costs are a concern.

Ozone depletion

The greater concern – by far – is the potential impact on the ozone layer from SAI at the scale that is needed.  The ozone “layer” – the region in the lower stratosphere where the ozone concentration is greatest – is precisely where SAI would inject SO2.  Populating the stratosphere with aerosol particles of H₂SO₄ will result in a significant reduction of the ozone layer.  Following the eruption of Pinatubo, the ozone layer depleted at record rates. As the H₂SO₄ particles from the eruption gradually fell, the rate of ozone destruction reduced and gradually stabilized at a record nadir roughly 19 months later).  

Ozone is depleted because the aerosol H₂SO₄ particles react with NO_x gasses to form nitric acid aerosols (HNO₃).  NO_x gasses are critically important in maintaining the ozone layer, because they bond with chlorine (Cl^–) radicals and chlorine monoxide (ClO) to form chlorine nitrates. Chlorine is an extremely effective “catalyst” for breaking down ozone, as both of these reactions can be repeated indefinitely:
Cl + O₃ -> ClO + O₂
ClO + O₃ -> Cl + 2O₂
A single chlorine ion could destroy over 100,000 ozone molecules before bonding to NO_x and becoming an inert chlorine nitrate molecule or gradually transporting through the tropopause into the troposphere where it could rain out.  So, the prospect of seeding the stratosphere with millions of tons of sulfuric acid molecules that are quite effective at eliminating NO_x molecules doesn’t sound good.  Recent modeling has suggested 2°C of cooling (-2.2 W/m² of radiative forcing) could be achieved with 32 Mt/yr SO₂ injection into the mid stratosphere and that it would increase UV exposure by 14% in the northern hemisphere but twice that amount in the southern hemisphere.

While SAI could legitimately, at a high but justifiable price, reduce the global temperatures, the true cost would come in the sustained reduction of the ozone layer, exposing life on earth to significantly more UVb – which could cause more rapid biodiversity loss and crop damage than the warming that SAI is intended to offset. 

Stratospheric heating

            Another significant unknown that could become a major issue is the potential of stratospheric heating caused by the continual flights of a global fleet of stratosphere-flying cargo planes which would dump much more CO₂ and H₂O into the stratosphere than SO2. The impacts on stratospheric circulation of this uncertain degree of heating are still quite unknown.

Increased climate forcing from additional long-distance high-altitude flights.

            This is a minor concern, but as the world desperately seeks to reduce emissions from burning fossil fuels, SAI will call for a very large ramp-up in long-distance high-altitude flights.  Beyond merely the consumption of fuel – which is substantial – the contrail exhaust from high-altitude flights is credited with causing some additional climate warming effects, for the same reason that high-altitude cirrus clouds are credited with increased climate forcing: Cirrus clouds are thin enough that they do very little to block incoming solar shortwave radiation (low albedo), but they absorb significant amount of the outgoing infrared radiation from Earth and re-radiate much of that (at yet longer wavelength) back to earth.  So unlike the low-level clouds which have very high albedo (because they’re much thicker), the high cirrus clouds have a net warming effect.  The contrails from high-altitude flights work in similar fashion – the plumes of high temperature CO₂ and H₂O from the jet engines are immediately expanded and cooled, with the water freezing into aerosolized ice microparticles (the contrail ice crystals are much smaller than those found in naturally occurring cirrus clouds).  They quickly dissipate, and in a relatively short time aren’t packed in sufficient density to reflect incoming insolation, but they are still effective at absorbing and re-radiating the outgoing infrared radiation back to Earth. 

The aviation industry at large is credited with ~2% of total anthropogenic greenhouse gas emissions, but over 3.5% of anthropogenic climate forcing.  Depending on many unknowns, the total contribution of the SAI fleet may increase that number to as much as 5%. 

Cirrus Cloud Thinning (CCT)

The  most recently proposed SG method is cirrus cloud thinning (CCT), which proposes to seed cirrus clouds with a non-toxic, inexpensive, and highly effective aerosol such as bismuth tri-iodide to reduce the number of ice particles in them and thereby make them trap less heat.

The science supporting CCT has recently become stronger, and it looks like it could be implemented in a very small way at moderate cost. Still, the potential benefit is unclear, partly because the warming/cooling forcing ratios of various types and thicknesses of cirrus clouds are not yet well clarified. One key issue is the thickness of the (ice-crystal) cloud formation, as that dictates how much aerosol needs to be dispensed to achieve the desired effect.  The picture above seems to depict wisps of feathery clouds, but in reality the “wisps” are the thicker cloud banks, gradually feathering into thinner and thinner clouds (of ice crystals), which interconnect the thicker clouds.  Much of the “blue” sky in the image also contains cirrus clouds, we just can’t see the small amount of light reflecting off of them amidst the large amount of blue light shining through them.  When the cirrus clouds are only ~100m thick, they may reflect only ~9% of the incoming solar energy (visible light and UV_a), while still absorbing more than 50% of the outgoing long-wave infra-red radiation from Earth.  The energy that they absorb is then re-radiated, half of which is re-radiated back towards Earth.  To make matters worse (from a climate-forcing standpoint) cirrus cloud cover has a nocturnal-diurnal cycle. These are high-altitude ice crystals: at night, when there is less total energy, more water freezes into more cirrus cloud cover, which then sublimates under the morning sun and we see far less cirrus cloud cover during the day, so we get more blanketing absorption and re-radiation of our outgoing energy during the night, and less cover reflecting incoming solar energy during the day. 

There is only so much long-wave radiation that is currently absorbed by cirrus clouds, and half of that energy would be re-radiated back into space from the cloud that had absorbed it. Also, the amount of visible light and UV_a radiation that is reflected by the clouds ramps up relatively quickly with greater thickness, so thinning some clouds to reduce their long wave absorption will also thin other clouds and reduce their albedo.   

Recent simulations have determined that seeding with somewhere between 1 and 15 particles per liter would likely be optimum (negative forcing of up to 1.4 W/m2), and seeding with more than 40/L would likely cause a positive (warming) forcing. If we assume the wake from a large airliner could disperse an aerosol of 7-micron bismuth tri-iodide particles (probably a good size, to keep the mass down, though most simulations have used particles 20-50 microns in diameter) throughout a trail 500 m wide and 200 m deep at a particle density of 3/L in a night flight from New York to Paris, that 1.6 tonnes of aerosol (~2% of its payload) could thin cirrus clouds over 0.0005% of the earth’s surface.  If all night-time flights over oceans around the world were doing that every night, perhaps the total mean forcing could be as much as negative 0.0035 W/m2. If the mean cost of these flights is ~$100K each, there are 500 similar flights around the world every night, and 2% of their payload is for CCT, then the transport cost of achieving this negative 0.002 W/m2 would be $1M/night. Of course, adding the needed equipment to a few thousand aircraft could cost more than $1B, and the overhead costs could easily make the total daily operational costs more than $3M. That would put operational costs at ~$1B/yr for achieving ~0.1% of what is needed to offset the global warming from a doubling of CO2 over preindustrial levels.

Still, there is much that we have not yet clarified, and further modeling may find carefully targeted benefits that provide some regional cost-effective improvements.  More research is needed here to know more, but it is unlikely that CCT could be a significant factor in the critical next two decades.

Space Junk

While we embrace the need for solar geoengineering and the potential for many of the most commonly discussed platforms (though we believe our TCG method will prove to be the most cost effective with the fewest deleterious side effects and the most beneficial side effects) we should note that there are some ideas that are clearly junk.  Chief among those would be the concept of launching reflective foil into a low-Earth orbit to reflect 1% of incoming solar energy.  Even with the extremely laudable advances in rocket technology seen over the past 10 years, the launch costs using a SpaceX “Heavy Falcon” is still ~$1400/kg.  In order to have enough ultra-thin foil to reflect 1% of solar insolation, you would need ~2.5 million km² of 2um aluminized mylar, which would weigh ~6.5 billion kg, for a total launch cost of ~$9.1 trillion dollars.  This is before weighing the threat of closing off all access to space by means of literally junking the region up with foil.  Mylar might be low density, but if it were to intersect a satellite at 10,000 m/s, it would slice the satellite clean through.  The foil would continuously be struck by small meteorites which would deflect pieces of it into unplanned orbital patterns, making satellite deployment impossible.  So not only is the price tag in the multi-trillion dollar range, it would effectively seal the sky from us.  The idea of using space junk for solar engineering is nonsense, and is the only mentioned concept for which no support should be considered.