Denial ain't just a river in Egypt.
The "climate change is a myth" crowd is still at it, but their claims are growing simultaneously more shrill and less credible. The evidence is piling up at a rate which is accelerating as fast as the atmospheric GHG concentration: glaciers the world over are retreating, mountain snowpacks shrinking, New Mexico's piñon pine forests dying en masse, permafrost across the arctic melting, the whole of Antarctica losing ice mass while the West Antarctic Ice Sheet appears to be destabilizing, Greenland's glaciers accelerated to double their previous speed, the North Atlantic Conveyor slowing worringly, the Larsen B ice shelf broken up, arctic sea ice vanishing, heat waves killing tens of thousands even in the first world... the list goes on and on.
Some authors who appear to take the issue seriously nevertheless claim that we can wait half a century (p. 217) to act decisively about this. Once I would have believed that we had lead time and could get mitigation measures into place before the real crisis hit. I can't agree with this any more. We are already seeing some GW effects occurring decades ahead of naïve predictions, and other effects are likely to increase warming even further. Methane from thawing permafrost across Siberia and Canada and CO2 from fires in drought-stricken tropical rain forests are just two of the likely natural additions to human GHG emissions.
We don't have the luxury of either complacency or time; failure to act will allow the existing positive feedbacks to accelerate beyond anything we could to do arrest them. We need to take this seriously and do something to halt the progressive warming and its effects, NOW. We may wind up with an environment falling to pieces if we don't do something.
Some people react to the idea with a shrug, saying "What can I do about it?" If there's no remedy, there's no point in getting all worked up about it, is there? It's like growing old: something to be handled when it comes, planned for, but not worth worrying about at the cost of today. This seems to be the attitude pushed by those who have an interest in denying global warming but who have been forced to concede to the evidence. Some just go back to denial as the most comfortable response. But what if there was something we could do?
That something would have to meet several requirements:
If something could be done, quickly and cheaply... it would change everything. Some might oppose it as a license to continue burning oil and switch more demand to coal, but the psychological impact would be immense. Yes, we are having large effects upon our climate. Yes, we do have the capability - and thus the responsibility - to do something about it. It would be a step in the right direction.
The difficulty with trying to fix a problem on this scale is that the solutions must not have side effects anywhere near as large. Such things get into a nasty mess of fixes, counter-fixes, counter-counter fixes and so forth. Ideally the fix will work just like some natural mechanism that already does just what we need. It needs to be something that we can apply easily, and adjust or stop entirely if it has untoward side effects.
Fortunately, nature gave us an existence proof: aerosols.
The 1991 eruption of Mt. Pinatubo provides an example that we can follow. Up to 20 million tons of material was injected high into the atmosphere (my sources are not clear how much of that got to, and remained in, the stratosphere). The sulfur which got there oxidized to SO3 and then combined with water to form H2SO4; this material remained aloft for as much as 3 years.
The effects included:
Except for the ozone depletion, this is just what the doctor ordered. It is effective, it is reversible, and the world certainly did not end as a consequence. Far from it; some research concludes that soil carbon inventories increased as a consequence of the atmospheric effects. This not only helped offset the effect of warming, it directly affected the cause.
But can we do what Pinatubo did, and keep it up year after year while we deal with the root cause? Let's take a look at that.
Pinatubo injected on the order of 20 million tons of sulfuric acid into the mid-stratosphere, to 100,000 feet or so. Volcanic emissions contain a great deal of steam which condenses almost immediately, so a substantial amount of this probably fell out in a relatively short time; however, some of the clouds reached upwards of 120,000 feet and remained aloft for years. Could engineering duplicate this feat of nature?
I think so. And not just the same, but better:
Completely offsetting the anthropogenic greenhouse-induced warming requires reflecting roughly 1% of Earth's incident sunlight. This sounds like a big job, but coal-fired powerplants in the USA alone handle far more than 20 million tons of sulfur per year. This sulfur mostly winds up in scrubbers and on relatively nearby land (fallen out or rained out), but if it could be captured in a suitable form we could certainly put it elsewhere. If the lifespan of the sulfur in the stratosphere is on the order of years, it might take far less than 20 million tons per year to slam the brakes on global warming.
Atmospheric lifespan is something I know next to nothing about (aside from the sketchy information I have about the duration of Pinatubo's effects), but I was able to find something about the scattering efficiency of sulfate aerosol particles. According to figure 1 of this abstract , the effective surface area of sulfate particles even under the least effective (driest) conditions has a broad peak around 10 square meters per gram for optimal particle sizes. The Earth has a total surface area of approximately 5.12*1014 m2, so intercepting 1% of the sunlight needs a reflector of 5.12*1012 m2. At an effective surface area of 10 m2/g, this could be accomplished with as little as 512,000 metric tons of sulfate in the atmosphere at any time. Unfortunately, most scattering occurs at relatively small angles so the total amount of material required to reflect that much light away from Earth is several times as much.
The abstract also gives a graph of radiative forcing vs. dry particle size and relative humidity. According to Figure 2, the peak forcing for 40% RH comes to about 250 watts per gram. A 1% reduction of insolation is a forcing of roughly 1.74*1015 watts; at 250 W/g, it would require around 7 million metric tons of aerosol particles. Greater levels of relative humidity could cut the required amount by almost a factor of eight.
On the scale of human effort, this is nothing. This is only a few times the tonnage of airliners flying at many times of day, and they can only remain aloft for hours; sulfate aerosols can last years. Ecological impact would probably be minimal. If the sulfur was drawn from industrial sources which are currently going into the troposphere, both the total amount emitted and its chemical effect on the landscape could be slashed drastically. Compared to the status quo, it looks like a great improvement.
The rate of fallout would average the same as the rate of injection. This rate is broadly equivalent to the required mass of particles divided by the lifespan. The lifespan would be partly determined by the exact location of injection, the chemical form and the weather conditions in the stratosphere. I have no information on these so I will not address them further.
The USA is currently burning approximately 1 billion tons of coal per year, 90% of which goes into electric powerplants. If this coal averages 3% sulfur, the annual sulfur flow is 30 million tons (30 million tons of sulfur can make over 90 million tons of sulfate ion, SO42-). If the current level of GHG's requires 7 million tons of sulfate aerosols to offset them and the aerosols have a lifespan of 2 years, the replacement rate is 3.5 million tons of sulfate per year. If we assume 100% conversion, this could be added as 3.6 million tons of H2SO4, 2.9 million tons of SO3, 2.3 million tons of SO2, or 1.24 million tons of H2S. This is on the order of 4% of the sulfur in the coal mined in the US each year: clearly not a big difficulty to obtain.
Where to get it seems obvious, but the question becomes how to get it in a usable form? So long as we intend to burn coal to make electricity for the next decade or two, the most viable option seems to be to convert powdered-coal combustion plants to one or another type of IGCC. Oxygen-blown IGCC yields sulfur as H2S when the fuel gas is scrubbed, and air-blown IGCC may produce it as ZnS which is further oxydized to ZnO and SO2; design may permit capture of pure SO2 in the scrubber effluent. Either gas could be liquefied under pressure and stored until it could be dispersed.
The use of IGCC with cold-gas cleanup is desirable for other reasons as well; it offers almost complete elimination of fly ash and other particulates, sulfur can be scrubbed to levels well below EPA requirements, and both mercury and other toxic metals can be scrubbed to 99% removal with activated-carbon filtration of the cold fuel gas. On top of all that, the thermal efficiency can be improved from 33% to 40% or greater. These improvements turn a coal-burning plant into a far better neighbor.... almost a good one.
Supposing that we've got a million tons or two of liquefied sulfur gases: how do we get them to where they do the most good? This leaves the realm of chemistry and takes us to aviation. One possibility is to mix some hydrogen into the gas to make it sufficiently lighter than air, and then release balloons filled with it; if the balloons burst (or were set alight) reliably at a known altitude the dispersion could be controlled rather precisely. This classic method would be immediately understood by the Mongolfier brothers.
If aerostats are too fragile, too unreliable or too toxic (who wants the results of a burst balloon near ground level?), aerodynes are the obvious step up. Modern jumbo-jet engines are powerful, efficient, and relatively quiet; an ultra-short haul aircraft might be designed around one to fly freight from the ground to 24 miles up, one way. Let's take the GP7000 engine as our baseline. The GP7277 variant is 6033 kg of machinery with a static thrust of 342.5 kilo-Newtons; it can (theoretically, under standard conditions) lift almost 5.8 times its weight. If a drone was built around a GP7277 core and its airframe, tankage, and other systems weighed as much as the engine, it could lift as much as 22.8 metric tons of cargo on a vertical takeoff. Bypass-burning could increase this figure considerably, at the cost of new enclosures of a heat-resistant material like Inconel or titanium.
If such a drone could take 20 metric tons of gas to altitude on each flight, the system would require roughly 62,000 flights per year carrying H2S or 117,000 flights per year carrying SO2. A fleet of 100 drones could handle this at less than 2 flights per day carrying H2S, and less than 4 flights per day carrying SO2. Many commuter aircraft fly more legs each day over longer distances, albeit not to such extreme heights.
If aircraft won't do the job, we could always bring in the big guns... literally. Firing tanks of gas to the desired altitude and dumping them might be the most efficient and cheapest method of all. Tanks could be recovered by parachute. I'd worry about noise and misfires, though.
Could we capture a million tons or so of sulfur each year? Absolutely; at about 8.5 tons per thousand megawatt-hours, the conversion of only 17 GW of coal-fired generators could supply everything we need. Could we build a fleet of 100 drone aircraft around jumbo-jet engines? If Scaled Composites can make a flying model of an orbital spacecraft and power it with 4 RL-10 motors, I'm sure we could. Could we have something next year? Hand a signed check to the right people and then stay out of their way, and I bet it would happen.
We appear to have the way, and then some; all we need now is the will.
Visits since 2006/05/11: