Sitting in the driver's seat
"Free riders" are only half the problem. "Free drivers" may be as important. The allure of geoengineering derives from the simple fact that - given what little we know about it at the moment - it appears to be a comparatively cheap way to combat climate change. And it doesn't take a global agreement to act. It takes one actor - one country - in the driver's seat.
If, for example, the very existence of an island, nation, city, or agricultural region is threatened by global warming, the question among its leaders will no longer be whether geoengineering is an option, but what the effects, positive and negative, might be and how it could be carried out. That's also where the science stands today, and the economics points in the same direction.
One option that will inevitably come under consideration is the possibility of shooting reflecting particles into the upper atmosphere to create an artificial sun shield for the planet. Blocking some of the sun's rays from hitting the planet may sound like science fiction or hubris, or both. But geoengineers are already looking at which particles would work best, and how to deliver them: Planes, balloons or multiple mile-long hoses are all contenders.
All these options have one thing in common: They are cheap - at least from the narrow perspective of those doing the geoengineering. Hence the "free" in "free driver."
In fact, the price tag of these geoengineering strategies is likely to be negligible relative to the purported benefits: Columbia University's Scott Barrett, among others, has calculated that it would cost pennies to offset a ton of carbon dioxide from the atmosphere. By comparison, it costs dollars per ton to reduce carbon dioxide emissions in the first place.
The higher cost of attacking the problem head-on, by reducing carbon emissions, would still be a bargain compared to the financial, ecological and human costs of unchecked global warming. But "free riding" is so much easier, politically and financially.
That's what makes the "free driver" effect so powerful. Geoengineering is seductively cheap, and it doesn't take the collective will of billions of people - or policies guiding those billions - to have a major effect. Anyone capable of flying a fleet of planes at high altitudes could conceivably have a go at altering the planet's atmosphere, and do so at a fraction of the cost of decreasing carbon dioxide pollution. But here's the catch: Nobody knows the costs of potential unknown and sometimes unknowable side effects, and there could be grave political and legal repercussions when someone starts playing God with the climate.
Proof by volcano
What makes scientists believe geoengineering could work? It's been tried before - by nature, not by humanity.
When Mount Pinatubo erupted in June 1991, it forced the evacuation of 200,000 Filipinos and shot 20 million tons of sulfur dioxide into the stratosphere. The added sulfur counteracted the effect of 1,100 billion tons of carbon dioxide that had been accumulating in the atmosphere since the dawn of the industrial revolution. In 1992 and 1993, it decreased global temperatures by a bit less than 1 degree Fahrenheit by reducing the amount of sunlight that hit the earth's surface. That was about the same amount temperatures had risen at that point from carbon added to the atmosphere by human activity. In other words, Mount Pinatubo alone offset all temperature increases from the beginning of the Industrial Revolution.
The aftermath of Mount Pinatubo's eruption suggests the limitations of this kind of geoengineering. The excess carbon dioxide in the air isn't being removed - geoengineering would simply add millions of tons of sulfur dioxide (or some custom designed material) to the atmosphere. That might lower temperatures -- but it would not address other problems caused by global warming. For example, it wouldn't stop the ongoing acidification of the oceans, which may kill much of the life they hold.
And there will probably be a host of unknown, unexpected consequences. For example, some climatologists blame the Mount Pinatubo eruption for flooding along the Mississippi River in 1993 and for droughts in sub-Saharan African. That still pales in comparison to the 1815 eruption of Mount Tambora in present-day Indonesia, which caused the "year without a summer" linked to some 200,000 deaths across Europe in 1816. Incidentally, the eruption also had some unexpected cultural repercussions: All those overcast days also forced Mary Shelley and John William Polidori to spend much of their Swiss summer holidays indoors, jumpstarting the creation of both Frankenstein and The Vampyre (an inspiration for Dracula).