For years science fiction writers and astronomers have speculated about the feasibility of terraforming other planets. 

One dream is to make Mars habitatable for humans by warming the planet and therefore building up a wetter and thicker atmosphere.

The irony today is that scientists are now talking about “retro-terraforming” Earth of all places!  The goal is to try and offset the effects of mankind’s contribution to global warming.  For example the production of electricity by burning oil and coal accounts for 40 percent of emissions that dump the greenhouse gas carbon dioxide into the atmosphere.

In a bold move toward cleaner energy, last week President Obama announced $8.3 billion in loan guarantees to build the first U.S. nuclear power plant in nearly three decades. (In hindsight I find it very ironic that our timidity about nuclear power since the 1970s has helped contribute to climate change.)  With the shift toward minimally-polluting energy sources we could “de-carbonize” electricity production within 25 years, say some experts. 

But even if we instantly stopped industrial carbon emissions world temperature will remain high for the next 1,000 years predicts David Keith of the University of Calgary. He calculates that only after 10,000 years global carbon dioxide levels would drop to about half of what they are today. (By contrast after 10,000 years nuclear fission waste is 1/3000th as radioactive as the day it left the power plant.)

This has inspired a spirited discussion among scientists about a planet fix-it strategy called geoengineering. Major technological steps would be taken to alter the atmosphere to cool off Earth faster.

One approach is to inject a sulfur aerosol into the atmosphere to make it reflect away more sunlight. Alternatively, spraying seawater into the atmosphere would create more white clouds that would reflect sunlight.

Another approach would be finding ways to absorb excess carbon dioxide in the atmosphere. One idea is to spread iron filings on the surface of the Pacific Ocean to boost phytoplankton growth, which in turn would absorb more carbon dioxide.

 “Our current understanding of climate systems suggests that intelligently executed climate engineering would reduce risks to the environment,” says Keith. 

But critics of geoengineering say that it isn’t nice to screw around with Mother Nature any more than has already been done. And, if the global cooling experiments fail we have no “Plan B.” There is no other planet to evacuate to! Also, if the nations just stopped funding a large-scale geoengineering program it could accelerate global warming say some experts.

“Geoengineering is in fact untested and dangerous,” says James Fleming of Colby College in Maine.  We don’t understand it, we can’t test it on smaller than planetary scales, and we don’t have the political capital, wisdom, or will to govern it.”

Geoengineering proponents emphasize that we need to do experimentation first to see if the idea is even feasible. They feel it is important to take steps now rather than simply sitting back and doing nothing.  “We need to break out of the foolish ‘all or nothing’ dichotomy and ramp up with learning,” says Keith.

The idea of controlling the atmosphere isn’t new. During the Cold War weather modification was considered feasible and a potential military weapon. In 1963 the U.S. Weather Bureau’s chief scientist warned that an enemy could explode a “chlorine bomb” high in the atmosphere to deplete the ozone. In the 1950s General Electric Co. scientists and the U.S. military unsuccessful tried to steer a hurricane by seeding clouds with dry ice and silver iodine to generate rain. Cloud seeding was also secretly used during the Vietnam War.

On balance the geoengineering risks seem to far outweigh the rewards. “Though geoengineering might cool Earth on average, for some regions of the world climate change plus geoengineering might leave you worse off than climate change alone,” says Martin Bunzl of Rutgers University.

Some climate models from geoengineering projections come up with scenarios that could condemn hundreds of millions of people to drought.  Other predictions are that the oceans could become more acidic, ozone would be depleted, and crops would be  damaged by reduced sunlight for photosynthesis.

A natural example of unexpected consequences can be found in the 1991 eruption of Mt Pinatubo on the island of Luzon in the Philippines.  It ejected 20 megatons of sulfur dioxide into the atmosphere that caused global cooling for two years. But it also reduced rain, soil moisture, and river flows in many regions. 

This makes me wonder if we could ever see evidence for astroengineering on inhabited extrasolar planets. Imagine a constellation of orbiting star shades built to cool a planet by blocking starlight. This might appear in telescopic photometric data as an unusual series of pulses in the planet’s reflected light. Once astronomers ruled out any natural phenomenon causing this, they might hypothesize that it is evidence for astroengineeering. 

But I hope that a technological civilization evolved past the fossil fuel era might have a better history of stewardship over their home planet.