While it is a given that mitigating global warming isn't going to come cheap, this is by far the most ambitious and expensive plan in the series. Professor Greg Benford of the University of California estimates that the cost of this project would be around $2 trillion.
Other critics have reservations about the science. The lenses would be placed at what's called the "Lagrangian Point" — a position in space where the effect of the sun and Earth's gravity cancel each other out. However, scientists like Tom Wigley think that the lens will slide off this "saddle."
Professor John Shepherd, of the University of Southampton in the U.K., doubts that the control system to "shepherd" the lenses into position will work, even if they can be launched successfully. He also worries that if the lenses don't work properly, everyone on the Earth could be fried.
Angel counters by planning to hold the lenses in the correct position by what's known as MEMS — micro-electromechanical systems. In other words, they are automatic, robot-like navigational devices on each lens, realigning them if they do drift off course.
Aside from the cost, another objection is Angel's method of getting the lenses into space. He envisages some form of electrical propulsion — also known as a coil gun — rocket fuel being too expensive and not carbon neutral. But to do this would involve building a gun with a barrel more than a mile long, mounted at the top of a mountain. Although the plan sounds like science fiction, the technology is well-known and does exist.
Professor Shepherd thinks the whole plan misses the point — it won't do anything for ocean acidification by CO2, which he describes as "a very serious deficiency." Professor Angel would counter this by claiming it is one of several ideas we need to consider, not the only one; there is no magic bullet.
It's harder to bat away another of Shepherd's objections — that the idea requires an unprecedented level of global cooperation. As he says, who will control the system? And as Professor Marty Apple says, what if "crazies capture the whole thing?"
