[See https://www.climatetechnologyprimer.com/ for new version]

This is the third of a series of three blog posts intended as a primer on how technology can help to address climate change. In the previous posts, I covered a review of climate and energy basics, and CO2 removal technologies.

This post — meant more as a reference collection and literature review than any kind of position statement — is a summary of what I learned at a technical level about the controversial topic of potential engineering interventions, beyond reducing emissions and introducing negative emissions, which would set out to mitigate some of the large-scale effects of climate change… i.e., “geo-engineering”. These include solar radiation management, and direct interventions to slow the loss of sea ice.

(Overall, these posts are focused disproportionately on CO2 removal and geo-engineering, as opposed to traditional mitigation or adaptation, though I did cover some cool mitigation technologies in the first post — the total set of opportunities for technology in climate is much broader than I can hope to cover here. I recommend this paper for a review that is more focused on applications of machine learning, but covers a much wider set of problem/solution areas.)

Note: you can annotate this page in Hypothes.is here:
https://via.hypothes.is/https://longitudinal.blog/co2-series-part-3-other-interventions/

Disclaimers:

1. I am not a climate scientist or an energy professional. I am just a scientifically literate lay-person on the internet, reading up in my free time. If you are looking for some climate scientists online, I recommend those that Michael Nielsen follows, and this list from Prof. Katharine Hayhoe. Here are some who apparently advise Greta. You should definitely read the IPCC reports, as well, and take courses by real climate scientists, like this one. The National Academies and various other agencies also often have reports that can claim a much higher degree of expertise and vetting than this one.
2. Any views expressed here are mine only and do not reflect any current or former employer. Any errors are also my own. This has not been peer-reviewed in any meaningful sense. If you’re an expert on one of these areas I’ll certainly appreciate and try to incorporate your suggestions or corrections.
3. There is not really anything new here — I try to closely follow and review the published literature and the scientific consensus, so at most my breadcrumb trail may serve to make you more aware of what scientists have already told the world in the form of publications.
4. I’m focused here on trying to understand the narrowly technical aspects, not on the political aspects, despite those being crucial. This is meant to be a review of the technical literature, not a political statement. I worried that writing a blog purely on the topic of technological intervention in the climate, without attempting or claiming to do justice to the social issues raised, would implicitly suggest that I advocate a narrowly technocratic or unilateral approach, which is not my intention. By focusing on technology, I don’t mean to detract from the importance of the social and policy aspects. I do mention the importance of carbon taxes several times, as possibly necessary to drive the development and adoption of technology. I don’t mean to imply through my emphasis that all solutions are technologically advanced — for example, crucial work is happening on conservation of land and biodiversity. That said, I do view advanced technology as a key lever to allow solutions to scale worldwide, at the hundreds-of-gigatonnes-of-CO2 level of impact, in a cost-effective, environmentally and societally benign way. Indeed, the rights kinds of improvements to our energy system are likely one of the best ways to spur economic growth.
5. Talking about emerging and future technologies doesn’t mean we shouldn’t be deploying existing decarbonization technologies now. There is a finite cumulative carbon budget to avoid unacceptable levels of warming. A perfect technology that arrives in 2050 doesn’t solve the primary problem.
6. For some of the specific technologies discussed, I will give further caveats and arguments in favor of caution in considering deployment.

Acknowledgements: I got a bunch of good suggestions from friends, many of whom are more expert in these fields, including Lowell Wood, Michael Nielsen, Sam Rodriques, James Ough, Evan Miyazono and Eric Drexler. 

Outline:

• Introduction
• Why even talk about this
• Albedo modification
  • Stratospheric aerosols
  • Marine stratus brightening
  • Space mirrors
• Other kinds of interventions
  • Countering sea level rise directly
  • Preserving the tundra
  • Ocean pipes
  • Terraforming the desert
• Some take-aways

Why even talk about large-scale climate engineering interventions, beyond a) emissions reduction and b) carbon dioxide removal?

First, some important caveats on this controversial subject. Because these caveats are important, many thoughtful people who have looked at this tend to repeat them, and I too will both link to an expert opinion from the University of Oxford, and paraphrase my own set of caveats with some additions (and if that is not enough, here is a position statement from the Union of Concerned Scientists):

1. Geo-engineering should not be seen as an alternative to rapid deployment of renewable energy and other decarbonization technologies. In the case of solar radiation management at least, doing so, as pointed out by many key quotes in this excellent Scientific American article, could be dangerous. Specifically, imagine a very-high-carbon scenario coupled with a solar shading mechanism that was suddenly turned off for some reason; that could create a much more unstable situation with rapid warming. Even if such a shading mechanism worked well and was not turned off, it would be highly problematic if it was used as a “bandaid” while action to decarbonize the economy and prevent other effects like ocean acidification was delayed.
2. To reiterate, it is very important to limit CO2 and other greenhouse gas concentrations in the atmosphere, and possibly ultimately bring them back close to pre-industrial levels through sequestration, for a variety of environmental reasons other than the impact on the average global temperature (“global warming”), e.g., ocean acidification and potentially changes in rainfall distribution. A paper by Ken Caldeira’s group notes, for example, that a significant fraction of existing coral is already in trouble due directly to ocean acidification (some are trying to help). Also, don’t forget that particulate air pollution from, e.g., coal power plants, is also a big health problem, not addressed by solar geo-engineering.
3. No single conceived solar geo-engineering technology would effectively stabilize all forms of dangerous climate instability, e.g., solar radiation management may not be able to stabilize undersea melting dynamics of the Antarctic ice
4. It would probably not be easy to build a consensus around the use of such technologies, at least in the present geo-political environment. They are not without real risks. There are also concerns that they would impact different regions differently, exacerbating potential ethical and political issues. (In general, these technologies would not perfectly restore a lower-CO2 climate: they would lower average temperature, but each individual locale would not necessarily return to the exact temperature or precipitation features that it had at lower CO2.)
5. These technologies are still basically just ideas. It would take years or decades to work them out just at the engineering level to the point where they could be reliably and practically applied.
6. This post does not cover the policy and human dimensions, which are clearly in many ways more challenging than the technical dimensions, and are extremely important. As a result, this post is admittedly woefully incomplete. In particular, even if the technology was basically perfect, the moral hazard issue is a real one. Should we be developing solar radiation management faster, because of the potential for emergencies, its likely cost effectiveness and speed for certain purposes (but not others), and the fact that any technology takes time to prove out and refine? Or should we be developing it slower because vested fossil fuel interests would use any progress on these lines as ammunition in a massive ongoing lobbying campaign to slow their inevitable decline while maximizing their profits (or, you could say, lobbying for their ability to continue providing for market demand)? This is a separate question from whether the technology is going to be effective, safe, equitable, controllable, localized, and so on. I don’t have an answer for it, but I will point out the following. The lack of a viable solar radiation management approach should ideally not need to be a necessary weapon against excuses from fossil fuel emitters to keep emitting — we should have a whole arsenal deployed, like carbon taxes, fines, and crucially lower cost carbon-free alternatives. Would we need to postpone development of solar radiation management technologies if we had a strong carbon tax or cap in place already? What if renewables + storage + a bit of small modular nuclear was at 1/10 the price of coal for grid-scale electricity (and we had cost effective carbon-free versions of steel and cement manufacturing and so forth)? Whether we in practice need to hold back this R&D for moral hazard reasons, at the moment, is a real question, but we should be able to shut down fossil fuels regardless — without needing to hold back research on emergency adaptation approaches — just as we have strong laws against arson, and fire-safe buildings, but still keep fire extinguishers around just in case. (Alas, we don’t have a trillion dollar entrenched industrial base trying to start fires… in that case, we’d sadly have to ask about the net benefit of bringing about the potential arrival of fire extinguishers sooner versus later.)
7. My purpose here is simply to review what the existing technical literature says about what physics and technology might allow. It is intended as a set of pointers to already-published technical material. I’m not prescribing any particular action or inaction beyond what has been published.

With that said, I’m not sure I can do better in advocacy of analyzing these potential technologies than by a) linking to a US presidential candidate already discussing them (and not incoherently either) and b) emphasizing that we still don’t perfectly understand potential tipping points in climate dynamics (or for that matter global human social dynamics) that could lead to emergency scenarios where a diverse portfolio of technological options might be needed.

Background materials: For an introduction to geoengineering, a few years ago Scientific American had a nice article covering certain approaches and their risks. The National Academies report on the subject is worth reading, as is a European equivalent. A group at Leeds in the UK has a great site on integrated (technological, social, economic) assessment of geo-engineering proposals. I would also recommend checking out the slides from this graduate course on the subject. A recent technical evaluation of geo-engineering approaches was recently published by Lawrence et al, which I cite heavily below. A first principles calculation paper from 1992 is also worth looking at.

Albedo modification

Referring to my first post on climate (or to any introductory course on climate science), one possibility is to try to increase the A term in the radiation balance, i.e., the albedo, i.e., to prevent some incoming solar radiation from making it to the ground. 

How much would one theoretically want to change the albedo, to achieve a given temperature reduction? The standard number, used in many geo-engineering papers, corresponds to lowering the incoming solar radiation by about 1.8%, with a cooling effect of about 2K relative to a doubled-from-preindustrial-CO2 planet configuration (which we must of course fight to avoid facing in the future by other urgent means). We can readily calculate this using the simple formulas covered in my first climate post. In particular, the impact of a 1.8% decrease in solar flux in that scenario would be an average temperature of about 285K instead of 286.3K, or a 1.6K cooling.

One could consider modifying the albedo on areas of the land surface, e.g., with mirrors in the desert or reflective paint on roads and roofs and so forth, but only a tiny fraction of the overall albedo comes from the land surface, so increasing that term by a few percent gives a small contribution overall. Thus the field has focused on modifying the atmospheric albedo rather than the surface albedo. Dyson and Marland pointed out exactly this in the 1970s:

(Sam Rodriques points out that this does depend on how much one could actually increase the surface Albedo. If it were only by, say, 5%, then the analysis above holds, as 0.05*0.03 = 0.0015, a fraction of a percent, whereas if we can change the cloud albedo by the same percentage, we get a 6x larger effect. But a 50% increase in the surface reflectivity would be a big effect. If 4.5% rather than 3% of the incoming radiation was reflected by the surface, say, then we’d have a fractional change in incoming solar radiation of 1 – (1-0.295)/(1-0.28) = 2%, similar to what is being proposed for the stratospheric scattering particle approach. Because the surface is so non-reflective now, increasing its net reflectivity by 50% could be done with a relatively small area (so to speak) of mirrors. Now, mirrors are presumably much cheaper to build out over large areas than solar panels, so we can do a fanciful comparison. 18 TerraWatts global energy use / (optimistic 30 Watt per square meter solar panels) works out to an area of solar cells about the size of France. Given a typical surface reflectivity of say 0.1, if we crudely estimate the effect of covering that same area with 95% reflective mirrors instead, we get only a 3.5% change in surface albedo, not nearly enough. If we cover a 20x larger area, that’s a 70% increase in surface Albedo, but covers the whole of China or of the Sahara desert. From a cost perspective, mirrors are so much cheaper than solar cells that they could in a fanciful sense be “cost effective” despite the larger total area needed for mirrors, but doing this would not to be feasible in practice for reasons like local weather effects not to mention issues of land ownership. Mirrors over arctic sea ice, however, have been seriously suggested, as has local modification of the surface albedo via, for instance, more reflective crops, which might also have other benefits. Another fascinating potential land based idea is radiative cooling to space!)

The seminal early paper in this area involves one of my favorite people, the polymathic and benign Lowell Wood, and offers a taxonomy of ways to do this:

1. Particles in the upper atmosphere which would back-scatter some of the incoming sunlight
2. Particles in near-Earth orbit which would back-scatter some of the incoming sunlight
3. A structure in space between the Earth and sun which would deflect a fraction of the incoming sunlight through a small angle such that it would miss the Earth

Aerosols in the stratosphere

The authors, and the field subsequently, seem to have focused heavily on #1, for reasons that they outline:

“Of the three deployments, the stratospheric location is by far the least expensive on a pound-for-pound basis; positioning mass in the stratosphere currently is at least 10^4 times less costly than putting it into low Earth orbit. Moreover, the mid-stratospheric residence time of sub-microscopic scattering particles of anthropogenic and natural origins is comparable to the half decade residence time of its molecular components, so that appropriately fine-scale particulate loadings of the middle stratosphere will persist for five-year intervals. However, the stratosphere is a chemically uncongenial location due to the high flux of ultraviolet radiation from the Sun and the presence of oxygen, particularly in the more reactive form of ozone. Ideally, we would prefer to deploy scattering systems – or their principal components – that would remain in place and retain their performance-pertinent properties for a century, which is of the order of the interval required for a CO2 emission pulse to be effectively sunk into the deep ocean. However, we consider the half-decade mid-stratosphere residence time to be sufficiently long for practical deployments. We may re-constitute the deployment of a scattering system twice per decade (or 20% per year), and we even consider such a short duration to constitute a relatively rapid, naturally-operating means of disposing of possible unwanted side-effects of insolation modulation.”

In other words, the particles in the stratosphere approach is cheap (arguably none of the other approaches discussed here are within 2-orders-of-magnitude of the economic cost of the aerosol method), and it naturally needs to be replenished on a timescale of less than a decade, so it naturally disposes of itself if you want to remove it on that timescale.

There is also a precedent for this approach, namely the eruption of Mount Pinatubo in the Philippines (1991), which reduced the average temperature of the Earth by about half a degree Kelvin for a couple of years:

The idea is to recreate the atmospheric effect of such an event a few times a decade.

Unfortunately, as the authors note, the chemistry of the upper atmosphere is complex, and in addition to potentially damaging the functionality of the injected light-scattering particles, a worry is that, depending on the material composition of the particles, they could cause some damage to the ozone layer (although some have proposed putting the particles in the high troposphere where they would, assuming they stay there, not have as much opportunity to do so).

David Keith has an excellent talk that covers other aspects, including potential local effects on precipitation and temperature across the world, early stage ideas about how to properly engineer the particles to maintain the right sizes, lifetimes and distributions, the need for microphysics experiments to constrain the particle design, and many important policy related aspects. 

Recently, Keith at Harvard and other researchers have been studying some of the chemically most benign possibilities, namely using tiny particles of calcium carbonate, better known as “chalk” or calcite. This was highlighted by Danny Hillis in his TED talk, where he emphasized that only 10 teragrams a year of chalk (about twice the mass of the great Pyramid of Giza) would need to be injected into the upper atmosphere to undo the warming effects of the CO2 we already have put in. Hillis points out that this is “a handful of chalk in every olympic swimming pool of rain”, and that the volumetric throughput required to deliver it to the atmosphere would be quite tractable, equivalent to continuously running just one or a few firehoses.

The paper Hillis is referring to with the chalk appears to be this one from PNAS. From their abstract, “A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol.” Note that the positive radiative forcing from the CO2 we have now is calculated to be roughly around (5.35 Watt per meter squared) * ln(408/278) = 2 Watt/meter^2, while the reference radiative forcing in the Lawrence paper was RFGref ≈ 0.6 Watt/meter^2.

The PNAS paper uses Mie scattering theory (a method of calculating the amount and direction of light scattering by particles of a size scale on the order of the wavelength of light or a bit larger) to calculate the change in albedo due to the injection of various sizes of aerosol particle into the upper atmosphere, which causes back-scattering of the incoming light. Anyway, as Hillis points out, a few teragrams per year of this stuff is quite achievable. He illustrates that that corresponds to a big fire-hose operating at full blast. The cost could be very low, on the order of a few billion dollars a year.

The Lawrence paper concludes: 

“calcite particles91 are non-toxic, would not cause significant stratospheric heating, and may counteract stratospheric ozone loss, but their microphysics and chemistry under stratospheric conditions are poorly understood”

In general, even if the particles perform their function well, and don’t cause ozone layer damage, there are some remaining big issues:

• Does not solve ocean acidification, which is a very big issue. (Though, see Ocean Liming in the last post for something that might be done about it, beyond of course limiting the amount of additional CO2 we release.)
• Causes changes in precipitation, in addition to temperature. Pinatubo had precipitation effect including lowering of river flow in Ganges and Amazon. A key concern is that it monsoon seasons could be disrupted by this. Keith points out, however, that directly ballparking this based on the effects of the Pinatubo eruption is misleading since transient albedo perturbations like a volcano eruption are expected to have larger impacts on precipitation than sustained perturbations like ongoing geoengineering.
• Impacts different areas differently — although a recent study of an idealized model found that “while concerns about the inequality of solar geoengineering impacts are appropriate, the quantitative extent of inequality may be overstated”.
• Could cause heating of the stratosphere — although the calcite rather than sulfate approach could be significantly better on this: “…the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol”
• Changes the light scattering by the atmosphere in way that may impact ecosystems, in particular making the sunlight slightly more diffuse rather than direct. This will have an impact on the “canopy structure” of forests and other ecosystems. Caldeira seemed to indicate in a talk that he thought experiments to study these effects might be worthwhile at some point in the future. Keith indicated in a talk that overall productivity could increase due to this scattering, as the sunlight is now coming in from a (very slightly) broader range of angles and thus is less shaded in its direct path. A paper from Caldeira’s group finds a potential for crop yields to on average actually improve in one particular scenario where this is used. Caldeira pointed out in a talk that there should be a roughly linear decrease in photosynthesis, e.g., ~2% with ~2% decreasing solar radiation, but that, compensating for this, you have high CO2 fertilization and decreased heat stress (due to the intervention) in a (otherwise dangerous!) high carbon scenario where this might be used as an emergency measure. Another recent study suggests that yield reduction is similar in both cases.
• Depending on how and when it would be done, it may not fully mitigate sea ice loss. IAGP has a nice integrated simulation, in which they simulate effects such as the difficulty of hypothetical future climate engineers in measuring whether the solar radiation management is working and to what extent and thus in managing the project; in the end the simulated world manages to control the loss of sea ice. But in the case of the Antarctic, things appear complicated. While the ice on Greenland is mostly atop land, in Antarctica melting is heavily driven by processes occurring deep under water. As this detailed paper explains, “Solar geoengineering may be more effective at reducing surface melt than a reduction in greenhouse forcing that produces the same global-average temperature response. Studies of natural analogues and model simulations support this conclusion. However, changes below the surfaces of the ocean and ice sheets may strongly limit the potential of solar geoengineering to reduce the retreat of marine glaciers… It may be that significant losses from some West Antarctic glaciers are unavoidable by simply returning climate and oceanic driving conditions to the preindustrial conditions and perhaps that even doing so would not be sufficient to arrest the retreat.” We will discuss below some more specific localized engineering approaches that might be able to help where solar geoengineering might not cut it.

(IAGP also has an integrated simulation of solar radiation management combined with carbon capture and emissions reductions. Interestingly, solar radiation management itself causes some decrease in atmospheric CO2 in that situation through effects mediated by the biosphere.)

The Teller/Wood/Hyde paper is both the place to find all the back of the envelope calculations for this approach, and a range of particle designs and schemes that tile a design space much wider than most of the literature in this area apparently considers these days, e.g., the use of resonant scattering and other effects to enhance these systems. In the paper, they give a back of the envelope calculation for the number of dielectric scattering particles needed, which holds in the small particle (Rayleigh scattering) regime but breaks down as particles get larger such that much of their scattering becomes forward scattering: we would need a 60 kg/second average particle injection rate into the stratosphere.

Meanwhile, the definition of a “firehose” I found online lists up to 200kg/sec. This calculation is similar to the one Hillis shows on a literal back of the envelope in his TED talk, in which he explains this firehose analogy. 

According to this paper, “we settle upon an aircraft-based delivery system… developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive” and “average costs of ~$2.25 billion yr−1 over the first 15 years of deployment. We further calculate the number of flights at ~4000 in year one, linearly increasing by ~4000 yr−1”. That’s super cheap.

Cloud whitening/brightening

A distinct way to potentially increase atmospheric albedo would be to “spray seawater into low-lying marine cloud formations thereby whitening clouds and increasing their albedo (reflectivity)”. This could probably be done with a fleet of on the order of hundreds to a few thousand specialized wind (sailing) or solar-powered boats. The Scientific American article concludes that this method might be comparatively benign, locally controllable, inexpensive and easy to shut off (but with consequences for localized weather effects that may be hard to predict)! And it is just spraying seawater, not any exotic chemical.

This idea was originally proposed by Latham in 1990 in a commentary in the journal Nature, and refined somewhat through interaction with Salter and others. The basic idea is to alter the droplet size inside clouds:

One project in this area is based at the University of Washington:

• onlinelibrary.wiley.com/doi/10.1002/2017EF000601/full
• atmos.washington.edu/~robwood/Geoengineering/PJR_UW_Jan2011_v2.pdf
• washington.edu/news/2017/07/25/could-spraying-particles-into-marine-clouds-help-cool-the-planet/

From their paper, “Early studies suggest that it might be possible to offset a doubling of CO2 globally by brightening 10%–30% of marine clouds.”

The scheme is also very fast to switch on and off, since any given cloud system only lasts a matter of days.

Australia is apparently considering some local testing of the idea. 

The authors emphasize that preliminary studies of such “Marine Stratus Brightening” (MSB) could also serve to help pin down key parameters involved in aerosol-cloud interactions, which are a key uncertainty in climate models generally. Thus research in this area is valuable for the fundamentals of climate science even without reference to geoengineering per se. 

The Lawrence paper concludes that this science is needed: “the limited knowledge about key microphysical and dynamical processes involved results in a large uncertainty in the maximum cooling that could be achieved via MSB, with estimates111,113,117,119,120,121,122 ranging from 0.8 to 5.4 W/m^2, i.e., likely well above RFGref”.

To illustrate just how much our knowledge of aerosol-cloud interactions remains in flux, consider this article from Science in Feb 2019, which states: 

“The development of novel methodologies to retrieve cloud droplet concentrations and vertical winds from satellites represents a breakthrough that made this quantification possible…. Aerosols explain three-fourths of the variability in the cooling effects of low-level marine clouds for a given geometrical thickness. Doubling the cloud droplet concentration nearly doubles the cooling. This reveals a much greater sensitivity to aerosols than previously reported, meaning too much cooling if incorporated into present climate models. This argument has been used to dismiss such large sensitivities. To avoid that, the aerosol effects in some of the models were tuned down. Accepting the large sensitivity revealed in this study implies that aerosols have another large positive forcing, possibly through the deep clouds, which is not accounted for in current models. This reveals additional uncertainty that must be accounted for and requires a major revision in calculating Earth’s energy budget and climate predictions. Paradoxically, this advancement in our knowledge increases the uncertainty in aerosol cloud–mediated radiative forcing. But it paves the way to eventual substantial reduction of this uncertainty.”

The Leeds IAGP project has done some more detailed simulations of at least one version of the Latham-Salter cloud whitening idea, and uncovered some potential problems, concluding:

“Our simulations found three issues that reduced the efficacy of the spraying mechanism: only certain clouds were susceptible to spraying at certain times of day; many of the sea-salt particles coagulated and rained out before they reached the cloud; and the particle plume generated by the moving ship had a tendency to sink rather than rise to cloud level (due to the evaporation of water from the generation of sea-salt). No doubt many of these obstacles would be surmountable, but development and testing take time.”

There is also the possibility of thinning the Cirrus clouds. Per the Lawrence paper, “A maximum net cooling in the range of 2–3.5 W/m^2, considerably exceeding RFGref, has been computed based on model simulations131,132,135,136,137, though the high end of this range is accomplished by modifications in the models which are far removed from what could likely be achieved in reality (e.g., increasing the cirrus particle fall speeds 8–10-fold everywhere).”

The 2015 National Academies report on solar radiation management concluded the following in terms research needs and potential problems, but overall seemed to judge it feasible that it could make a sufficiently large impact on the planet’s energy balance if this research is resolved:

“Research beyond the use of computational models is needed to address some of the key open questions on the potential for marine cloud brightening to be useful for albedo modification purposes. The reason is that the uncertainties of cloud susceptibility, scale-up, and feedbacks are not sufficiently understood to be included with confidence in models. These issues produce the largest uncertainty in quantifying marine cloud brightening feasibility and, hence, assessment of cost and risks… there are many potential climate impacts from MCB that are essentially unexplored, and more attention is merited with both models and possibly field experiments if they can be done at smaller scales. The committee is specifically aware of a lack of knowledge about (a) impacts on ocean circulations, (b) consequences to ecosystems due to significant reductions in sunlight reaching the surface where MCB is operating, (c) interactions of MCB with dominant modes of interannual variability like ENSO and the Pacific Decadal Oscillation (PDO), and (d) the nature of the remote impacts to precipitation like that found in the U.K. Met Office model discussed previously (Jones et al., 2013). These processes are all likely to operate at longer timescales and be sensitive to forcing on larger space scales and should also be explored.”

The Lawrence paper notes that cost estimates for both stratospheric aerosols and Latham-Salter style cloud whitening could be <$100B. In other words within some people’s personal net worth. This clearly presents both positive and negative aspects.

Update 2021: Australia is doing work on marine cloud brightening to forestall damage to the Great Barrier Reef.

Space based sun-shades

Somewhat less attention has been paid to option #3 above, a space-based sun-shade. The most popular version of the idea (although to be clear, no version of this idea is particularly popular) appears to be a version by Roger Angel proposing to deliver sun shades to the L1 Lagrange Point, a point which remains at a constant relative position in between the Earth and Sun as they orbit. Angel’s paper proposes three things:

1. “First is an optical design for a very thin refractive screen with low reflectivity, leading to a total sunshade mass of 20 million tons.” 
2. “Second is a concept aimed at reducing transportation cost to $50/kg by using electromagnetic acceleration to escape Earth’s gravity, followed by ion propulsion.” (Hmm…)
3. “Third is an implementation of the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure.”

There are some obvious and enormous practicality issues:

• Why “low reflectivity” and why would it need to be “stabilized”? Apparently in part because solar radiation exerts a radiation pressure on the spacecraft, which tends to push them away from the sun, and in part because orbits around L1 are inherently radially unstable.

• “Very thin” means < 1 micron thick, significantly complicating fabrication and deployment as far as I can tell, e.g., can films much thinner than saran wrap really survive the environment of space? It turns out that this range of thickness is widely considered in the solar sail literature. Not that this makes it easy.

• In total, the sunshade would need to be a couple of thousand kilometers on a side. That would take a long time to assemble.

How do the costs then work out? SpaceX’s proposed/forecast launch costs to low-Earth orbit using the BFR rocket are $100/kg. So if the refractive screen weighs 20 million tons as proposed, the launch cost would be 2e7 tons * 1e2 $/kg  = 2 trillion dollars, which is hefty. Angel’s novel electromagnetic launcher concept helps but the cost is still at best above $100 billion.

Interestingly, the earlier paper with Lowell Wood (“Global Warming and Ice Ages: I. Prospects for Physics-Based Modulation of Global Change“) proposes a potentially lower-cost variant:

Now, 1e14 grams = 1e8 tons, which is similar to the 2e7 tons for the Roger Angel proposal. But the Teller/Wood/Hyde proposal suggests 1e5 smaller mass requirement than that: see footnotes 23, 24 and 25 in that paper. So that seems more economically feasible, though I am wondering about the challenges in actually building and maintaining such a system in space.

The other idea mentioned in the above footnote is using lunar regolith as a construction material, which has a much shallower gravity well to escape versus stuff launched from Earth; granted, this approach requires moon mining and manufacturing machines, probably with regolith-to-fuel processing powered by photovoltaics, and ferry rockets!

Clearly, we’d need to massively up our game in space technology to make something like this work. Reminds me of Jeff Bezos saying “We Have to Go to Space to Save Earth“, although this is not the context in which he meant it. A long shot at best, to be sure.

The National Academies report on solar radiation management concluded:
“…these ideas require the ability to manufacture in space, making them impractical at the current time. Overall, the committee has chosen to not consider these technologies because of the substantial time (>20 years), cost (trillions of dollars), and technology challenges associated with these issues (GAO, 2011; The Royal Society, 2009).”

But perhaps this hasn’t been studied to its absolute logical limit — for instance, there are other cool ideas for reducing mass. Did you know that we have a spacecraft at L1 already?

Other kinds of interventions

Outside of targeting CO2 removal or solar radiation management per se, one could in theory make more local, or more effect-specific, modifications to major climate processes.

Countering sea level rise directly

Freeman Dyson proposed, as a zany conceptual sketch illustrating a space of possibility, kites or balloons to modify the air flow over the coast of East Antarctica — to dump more snow on comparatively-stable East Antarctica, potentially sucking out from the ocean sea water released by melting of Greenland and/or West Antarctica, and thus lowering sea levels:

“Snow-dumping in East Antarctica would be a good way to stop sea levels from rising. Sea levels have been rising since the end of the most recent ice age 12,000 years ago. Most of the rise had nothing to do with human activities, but a further catastrophic rise by fifteen meters is a possible worst-case consequence of human activities in the next two centuries. A fifteen-meter rise would be the result of a complete meltdown of the ice in Greenland and West Antarctica caused by global warming. Such a meltdown is unlikely but not impossible. Fortunately, East Antarctica is much colder and larger than Greenland and West Antarctica, and the ice cap on East Antarctica is not in danger of melting. A permanent high-pressure anticyclone over East Antarctica keeps the air over the continent dry and the snowfall meager. The same anticyclone keeps a strong westerly flow of moist air circling around the southern ocean… To dump snow onto East Antarctica, we must move the center of the anticyclone from the center to the edge of the continent. This could be done by deploying a giant array of tethered kites or balloons so as to block the westerly flow on one side only. The blockage would cause a local rise of atmospheric pressure. The center of the anticyclone would move toward the blockage, and a fraction of the circulating westerly winds on the opposite side of Antarctica would move from the ocean onto the continent. The kites or balloons might also be used to generate massive quantities of electric power for use in other projects of planetary engineering. With or without electric generators, the onshore flow of moist air at a rate of a few kilometers per hour would produce an average snowfall equivalent to a few meters of ice per year over East Antarctica. All the ice added to the continent would be subtracted from the ocean. This would be enough snowfall to counteract the sea-level rise produced by a complete meltdown of Greenland and West Antarctica in two hundred years. Year by year, we could raise or lower the kites and adjust the flow of moist air across the continent so as to hold sea levels accurately constant.”

nybooks.com/articles/2008/09/25/the-question-of-global-warming-an-exchange/

There is a sketch of a related idea for the Arctic, where the situation by some assessments doesn’t look good — here the idea is to reduce the ice melt itself, whereas in Dyson’s idea, you would be lowering sea levels even if the ice had already melted, and regardless of the cause of that melt (as long as East Antarctica is still available as a snow dumping site). Methods for increasing ice reflectivity, e.g., with coatings of glass microspheres, have also been suggested, as has localized albedo change to the Arctic ocean.

Different major Arctic and Antarctic bodies of sea ice are melting via different processes and due to different proximal causes, and would theoretically require different types of interventions to mitigate their ice loss. An interesting aspect for the Arctic is the ice albedo, and the effect of pollution on that, as opposed to global warming per se

although the effects of pollution are multifarious and depend on the pollutant. Another fascinating paper suggests a different primary driver of melting of Greenland, however:

“In an analysis of recent changes over Greenland, Hofer et al. (2017) found that the substantial reduction in cloud cover over Greenland in the past 2 decades is the likeliest cause for the accelerated mass loss from the ice sheet over this period. To arrive at this result they simply calculated how much melt would result from the change in downward surface shortwave energy received over the melt season as a result of the change in cloud cover and compared this against the other contributions to melt and accumulation. They find that the ∼ 10 % reduction in summer cloud cover over Greenland in the past 2 decades led to a ∼ 4000 Gt loss of mass making it the dominant driver of surface mass balance change in this period.“

This article covers three potential ways to reduce glacial melting, including a) blocking warm water, b) supporting ice shelves, and c) drying subglacial streams. As the authors state:

“Even if greenhouse-gas emissions are slashed, which looks unlikely, it would take decades for the climate to stabilize… Geoengineering of glaciers will not mitigate global warming from greenhouse gases. The fate of the ice sheets will depend on how quickly we can reduce emissions. If emissions peak soon, it should be possible to preserve the ice sheets until they are again viable. If they keep rising, the aim will be to manage the collapse of the ice sheets to smooth the rate of sea-level rise and ease adaptation.”

Here is a peer-reviewed paper on the idea of supporting the West Antarctic ice, and a diagram from it:

Because major sea level rise is a key risk area, attempting to define a backup of the backup of the backup options like this seems important. The engineering required would be fairly heroic but the stakes are very high — collapse of the Antarctic ice could lead to many tens of meters of sea level rise.

Preserving the tundra

Another interesting Church lab concept is to restore large mammals to the arctic tundra, where their activity on the land helps to prevent thawing:

Eli Dourado’s blog post has an interesting analysis of this.

Ocean pipes

This one is apparently a bit of a dud. Since the deep ocean water is cold, one idea is to install pipes to facilitate heat exchange between the surface and deep waters, to effectively cool the surface. Unfortunately, based on a modeling paper from Caldeira’s group, it seems that this idea doesn’t work even in theory, and in fact creates warming due to its effect on clouds. 

Terraforming the Sahara desert

Researchers have even studied theoretically the notion of locally terraforming the Sahara desert by albedo increase, e.g., putting down large light-absorbing sheets to create heat that would drive more rainfall, or other schemes. I’d want to see on a technical perspective on the detailed meteorology of that. (This is not to mention the economics and politics obviously.) There are also other desert greening schemes applied in various locations around the world.

The UAE is already doing cloud seeding for rainfall, and China started a project in the area. The science of cloud seeding is also improving. The history, politics and perception of cloud seeding is checkered, however.

Slightly relatedly, Y Combinator lists “Desert Flooding” as one of their Frontier areas for carbon capture. Rather than “terraforming”, they propose basically shipping water inland to create reservoirs. That seems vastly more energy-intensive, but also more easily controllable. I think it this intended as basically a “straw man” to stimulate brainstorming.

Some take-aways

Again, simply reviewing the technical literature on a topic is not an endorsement of deployment, or even of serious engineering development.

Nonetheless, in early-stage solar radiation management research, I was pleased to see more chemically benign particulate formulations (e.g., chalk) being researched, which might not damage the ozone.

It was also encouraging to note some of the theoretical advantages of marine stratus brightening: it is fast to turn on and off, can be locally controlled, only involves spraying sea water, and the cost and number of boats theoretically needed seems manageable. But there are a lot of scientific unknowns as to how well it could work, still, and how best to build it and run it.

In addition, in the literature, there is ongoing modeling and analysis of potential effects of solar geo-engineering on key variables like precipitation patterns and crop yields, regional inequalities in effects, and so on, for the stratospheric aerosol approach. So those key issues are not being ignored by researchers studying this.

Solar radiation management still seems like a fraught area overall — in terms of severe governance challenges, potential moral hazard issues, and the need to deal separately with ocean acidification and other non-temperature effects. That doesn’t make it unimportant to flesh out at a scientific and technical level, as an emergency adaptation strategy. But those issues are serious.

The idea of studying aerosol-cloud interactions in this technology context seems intriguing, and experimental knowledge in this area is also much needed for climate modeling in general. 

Because even solar geo-engineering might not be able to reverse catastrophic collapse of the Antarctic ice sheets, additional schemes to preserve the ice, or to actively lower sea level, need to be studied as a backup of the backup emergency option.

Update Feb 2021: Two interesting data points suggesting discussing this may be becoming somewhat more mainstream: Article by Deutch and Zuber, with Zuber being one of the chairs of the President’s advisory council on science and technology, and Ezra Klein featuring Elizabeth Kolbert’s new book called Under a White Sky: The Nature of the Future.

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In any case, I leave you with one of the loveliest talks on the internet.