Much more speculative are some ambitious plans for high-tech parasols to block sunlight before it reaches this planet. At the NASA Institute for Advanced Concepts (NIAC) meeting last fall, Roger Angel, an astronomer and optics expert at the University of Arizona, produced a highly detailed - and highly futuristic -- proposal for a sunshade huge enough to cut incoming sunlight by 1.8 percent. That, he says, should counteract the warming expected from a doubling of atmospheric carbon dioxide.

Angel's plan builds on an early design by James Early of Lawrence Livermore National Laboratory, but it slims the mass down from 100 million tons to 20 million tons - something that, he says, conceivably could be launched from Earth.

Angel does not consider solar sunblock the optimum choice in the struggle against global warming, but rather a fallback position "if things get seriously bad." If, for example, ice starts sliding faster from Greenland than expected, sun shields "may be a useful idea" to prevent vast coastal flooding.

The Early concept calls for a giant sun-shield near the L1 Lagrange point, about 1.8 million kilometers (about 1.2 million miles) above Earth. Here, the gravity of Earth and sun balance, enabling a shield to remain stationary for years.

Angel suggests that the shield, covering an area of 4.7 million square kilometers (slightly smaller than the area of the continental United States west of the Mississippi River), would be best made as a cloud of 16 trillion free-flying circular refractors, each 0.6 meter (2 feet) in diameter. . Each refractor would be about 5 microns thick and weigh 1.2 grams. The refractors would be launched in stacks and then deployed upon reaching the target zone.

At every stage, Angel has proposed high-technology solutions to staggering challenges. He would launch the refractors to escape velocity with an electromagnetic coil gun, which propels a missile based on electromagnetic repulsion, then propel them to L1 with ion thrusters using argon as fuel. Once in place, each disk would sense its position using hyper-miniature cameras that detect sun and Earth. Adjustable trim tabs (tiny mirrors) catch solar radiation pressure as needed to maintain the disk's correct orientation and position in space.

If the disks had reflective mirror surfaces, they would quickly be pushed toward Earth by solar radiation pressure, so they will be designed to refract (bend) sunlight, not reflect it. Since they would make only a small deflection, the disks would evade most of the radiation pressure, Angel says. He estimates the disks could remain in orbit for at least 50 years, until their solar cells degraded and they could no longer position themselves.

With money from NIAC, Angel says, "we have built a prototype optical element, a micron-thick [refractive] hologram on glass.. When you hold it up and look directly at the moon, the moonlight disappears off the axis and is spread away into radial spectra."

Despite every effort to control weight, the total launch weight for the project would be about 20 million tons: 20 million launches at 1 ton apiece. Angel proposes launching stacks of 800,000 disks from a 2-kilometer coil gun. The mouth of the gun would be 18,000 feet above sea level, enabling the projectiles to start their journey above half of the atmosphere, where air friction is reduced.

Because coil guns are so efficient, the electricity necessary for 20 million launches, even if generated by burning fossil fuel, would cause only a minimal increase in global warming, Angel estimates. However, giant electricity-storage facilities would still be necessary, as would enormous factories to produce 16 trillion disks.

Angel suggests the total system could cost $5 trillion. That's a heap of money, but if the shield lasted 50 years, the average annual cost would come to $100 billion. That is just 0.2 percent of current world gross domestic product, and a lot less than some estimates of the cost of global warming.

Given the size and futuristic nature of the technology, the program would need plenty of advance planning - as much as 30 years for feasibility studies, manufacturing, and launching, Angel says.

The pies in the sky do have some advantages over competing plans, Angel says. Unlike suggestions for placing reflective particles in the high atmosphere, his plan would not need constant renewal.

"It probably has a minimal number of side effects, it just turns down the knob on the sun, does not put anything in the atmosphere," says Angel. "On the other hard, it's probably the most expensive route" to blocking the sun.

Robert Kennedy, an engineer from Oak Ridge, Tennessee, wonders about funding. "How are you going to fund it? With a magic waving of hands, you spend several hundred billion, or several trillion dollars. The human race generally does not do that, it generally suffers the pain rather than spending the money."

In 2000, Kennedy and colleagues proposed a plan to place giant mirrors at L1. The mirrors would contain photovoltaic arrays, and so could both block sunlight and beam power back to Earth. "In addition to solving the primary problem," he says, his proposal "actually makes money" through electricity sales.

Although the refractors proposal could be an ace in the hole if global warming gets out of hand, Angel hopes humans will be smart enough to devise less-radical ways to confront warming. But if, as some suspect, the climate system is near a "tipping point," the refractor plan might start looking attractive, he adds. Right now, he says, "You want to understand your options, have some idea of what the thing might cost, how long it would take, and what the side effects might be."

For a few hundred million dollars, he says, it should be possible to test whether disks at the Lagrange point indeed can remain stationary enough to deflect sunlight over decades. In addition, Angel says, serious research is needed into such nitty-gritty questions as this: "What is the most practical way to make a million square miles of [ultra-thin] glass?"

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NASA Institute for Advanced Concepts
University of Arizona