Appendix E. The Eye of the Beholder
E1. Climate Control Objectives
Geoengineering solutions to combat global warming generate heated debate. Polarized positions take two extremes: conservatives caution that tampering with climate could lead to disastrous unintended consequences; proponents point out that carbon dioxide and other greenhouse gasses will never be sufficiently reduced to avoid drastic climate change. Both are correct.
Geopolitical conflicts and cost considerations present almost insuperable obstacles to global climate control. However, the debate will change as climate extremes become acute. If world civilization maintains cultural continuity for the next one hundred years (or the next millennium), an engineered climate may be inevitable.
An inexpensive solution is to inject sulfur dioxide into the atmosphere (as happened naturally with the Mount Pinatubo eruption in 1991 and the Mount Tambora eruption in 1815), which produces a haze of sulfuric acid droplets that reflect sunlight and cool the planet. However, sulfuric acid is poisonous and increases atmospheric ozone destruction. Modifying the reflectivity of the atmosphere is likely to produce many unintended side effects. It is also blunt (not suited to regional weather control) and slow to reverse.
In view of the above, it's interesting to explore an optimal technological fix (bearing in mind that no single solution is sufficient to mitigate all effects of excessive carbon dioxide in the atmosphere, including ocean acidification). The proposition is that climate geoengineering must control the amount of sunlight that falls on Earth.
Ideally, sunlight should be regionally reduced (to tame heat waves or cool deserts) or increased (to heat selected regions, such as the Arctic). Since climate and weather are dynamic, sunlight control must be fast and flexible, especially to influence local weather. This means adjusting insolation (the amount of solar radiation energy reaching on the ground) in specific locations in hours or days, as opposed to months or years.
Quick control of sunlight has the potential to counteract weather extremes (taming hurricanes, mitigating drought or reducing floods). More importantly, sunlight control has the potential to improve local climate, for example providing rainfall for crops to reduce dependence on fossil groundwater for irrigation. Rapid response can also correct errors, reversing the worst unintended consequences.
Over the long term, climate control may save civilization from extreme climatic variations. As explained in the novel, sudden cooling can result from a volcanic super-eruption or meteor strike (or, hopefully never, a nuclear war). Other climate forcings, such as a Heinrich event (a sudden shift in the Gulf Stream), could cause continental glaciations or even a "Snowball Earth" scenario.
[N.B. Earth has experienced at least one (and maybe three) Snowball Earth episodes (a runaway ice age due to increased albedo from ice and snow). The most recent Snowball Earth was the Marinoan glaciation 650-635 million years ago, when the equator was as cold as modern-day Antarctica. Fortunately, volcanic outgassing of carbon dioxide ended the Snowball Earth episode, which in turn led to an "ultra-greenhouse" period. The Cambrian explosion of complex life occurred about 530 million years ago, after the Earth recovered from the Marinoan glaciation.]
With regard to long-term climate heating, the Earth resides at the inner edge of the habitable "goldilocks zone" around our sun. This means that the sun is almost too warm to support life on Earth (depending on the concentrations of various greenhouse gasses in the atmosphere). As the sun ages, its luminosity will increase and the habitable zone will move outward, beyond Earth's orbit, leaving our world to bake. Of course, this may take 500 million years. However, shorter-term mitigation of climate warming may be essential to maintain human civilization.
A flock of space mirrors is a small investment to mitigate or prevent these disasters — but space technology is nevertheless a complex and expensive solution.
E2. History of Space Mirrors
Geoengineering of global climate using mirrors in space is an old idea. I first discussed this with Freeman Dyson in 1989. At the time, he was contemplating terraforming Venus by collapsing the carbon dioxide atmosphere into liquid lakes. He wanted to use large reflective solar sails. [In the 1970s, Louis Friedman designed large solar sails at NASA's Jet Propulsion Laboratory. In 1989, James Early at the Lawrence Livermore National Laboratory envisioned a glass shield 2,000 kilometers in diameter to block sunlight.]
I proposed to Dyson an alternative idea that I'd been working on: platter-sized robotic mirrors at Lagrange One, steered by vectored sunlight. [L1 is a special solar orbit where the Earth and Sun's gravitational attractions cancel. L1 is about 1.5 million kilometers from the Earth.] This was a more flexible solution, since multiple mirror flyers can be deployed gradually, as needed. Dyson speculated that small robotic mirrors could be used to reduce the time needed to terraform Venus.
Subsequently, from 1992 to 1999, Russia experimented with various generations of Project Znamya, where space mirrors were used to illuminate Siberia in the winter. In 1992, the 20-meter diameter Znamya 2 deployed successfully; it produced a 5-kilometer-wide spot about as bright as the full moon; the spot traversed Europe from southern France to western Russia at eight kilometers per second. Znamya 2.5 had a 25-meter-diameter mirror, and was expected to produce a bright spot seven kilometers in diameter, with luminosity of between five and ten full moons. Unfortunately, in 1999 it snagged on a Mir antenna during deployment and was deorbited. The Russians then abandoned Znamya (much to the relief of astronomers who feared light pollution).
In 2006, astronomer Roger Angel, University of Arizona Regents' Professor, published a paper titled "Feasibility of cooling the Earth with a cloud of small spacecraft near L1" in the Proceedings of the National Academy of Sciences.
Angel's concept is to block 1.8 percent of the solar flux with space sunshades orbiting near the inner Lagrange point (L1), in-line between the Earth and sun. Objects at L1 circle the sun every 365 days, in synchrony with Earth, but require active station-keeping to stay in position. Fortuitously, objects of any size at L1 create a penumbra (a diffuse, partial shadow) that is slightly larger than the diameter of the Earth.
A major challenge with L1 is radiation pressure from the sun. Momentum from photons powers solar sails and will push any membranous satellite away from the sun and out of position at L1. Angel's answer to this problem is to make his flyers from transparent material used to deflect the sunlight, rather than to absorb or reflect it. The thin membranes will be refractive, blurring sunlight into a doughnut shape so that some of it misses the Earth. The 0.6-meter discs (weighing about a gram each) would also have three 0.1-meter-long protruding electronic "ears" with a solar power source so they could adjust their position, making them tiny, steerable spacecraft.
Angel's clever design will no doubt work to counteract global warming. However, since the discs are not mirrors, they cannot reflect additional sunlight onto the Earth's surface, where it might be needed for regional climate heating. Also, since the mirrors merely scatter light, they are not well suited to blocking sunlight from low Earth orbit to cool local hot spots.
Angel's design is not a general solution for global climate control. Rather, it is a single-purpose technology to counteract global warming. Of course, like all umbriferous technologies, it does not mitigate added carbon dioxide in the atmosphere (or the ocean).
E3. Robotic Mirror Design
The design for solar sailing mirrors presented in the novel is optically simple. Each mirror is a slowly spinning molecular membrane (to maintain flatness). The mirrors are designed to reflect all of the light impinging the surface, thus casting a penumbra (diffuse shadow) the diameter of the Earth.
To be useful, a solar sailing mirror must be able to station-keep at L1 or navigate to where it is needed, including LEO (low Earth orbit). To do this, there will be three arrays of movable micromirrors on the surface, much like those used for projection televisions. [A digital micromirror device is a microelectromechanical structure (MEMS) originally developed by Texas Instruments, designed for video projection. The tilt angle of a micromirror can be adjusted electronically, under computer control.] Instead of projecting a moving image, the micromirror array will vector sunlight to orient the solar sailing disc.
As the mirror disc rotates in space, the micromirrors must continually adjust the direction of vectored sunlight. Each disc will have an on-board CPU (a lithographed integrated circuit) to calculate navigational parameters. Radio receivers will monitor navigation beacons and accept navigation commands. The mirror discs will be powered by photovoltaic solar cells on the surface.
Therefore, each mirror disc will be semiautonomous robot. For station keeping at L1, the discs must decelerate (to drop closer to the sun) or accelerate (to raise the orbit). As discs fail, radiation pressure will blow them out of the Solar System, a neat form of automatic garbage disposal. Failed discs must be continuously replaced.
In addition to station keeping at L1, robotic mirrors can adjust the amount of sunlight falling on the Earth at any time. Tipping the mirror discs edge-on to the sun — like opening a Venetian blind — will allow full sunlight will shine though. Modulation of light intensity can happen in minutes, something that might be required, for example, to vary sunlight on the Pacific Ocean or the Eurasian Continent.
Another task for robotic mirrors is to strategically shade or illuminate specific areas of the Earth's surface, as Project Znamya was designed to do. For example, cooling the surface of the Gulf of Mexico may serve to reduce destructive hurricanes. Conversely, heating agricultural land in the north could extend the growing season. Mirrors in various elliptical orbits around Earth can provide regional cooling and heating.
Manufacturing of space mirrors could be done on Earth or in the microgravity of low earth orbit (or on the Moon). In any case, many tons of material must be launched into space. The geopolitics and technology needed for this giant engineering project are suitable subjects for speculative drama.