Find alien civilizations by their pollution?

There’s been speculation that advanced telescopes may be able to find hallmarks of alien life by looking for oxygen in the spectrum of light reflected off of exoplanets, but this article suggests using the James Web Space Telescope to look for pollution: Pollution on other worlds may show advanced alien life – space – 27 June 2014 – New Scientist.

Life is messy. So to find aliens, why not look for their pollution?

As part of its mission, NASA’s upcoming James Webb Space Telescope (JWST) will be able to look at starlight filtered through the atmospheres of Earth-sized planets and search for signs of life. Most proposed plans involve hunting for highly reactive gases such as oxygen that usually need a living source to replenish them. But these methods might only hint at relatively simple life such as plants and microbes.

Henry Lin at Harvard University thinks we could find more advanced civilisations if we look instead for industrial pollution. His team calculates that JWST should be able to spot two kinds of chlorofluorocarbons (CFCs), complex carbon-based gases used in solvents and aerosols.

“Their production requires a network of chemical reactions that are only known to be produced artificially on Earth,” says team member Avi Loeb at the Harvard-Smithsonian Center for Astrophysics.

My initial reaction to this is that we don’t know how long the polluting phase really lasts for a civilization.  It might only last a century or two before a typical civilization switches completely to renewable or non-emitting energy sources.  And an advanced civilization may be eons past their polluting phase.  Then I read this part.

JWST would only be able to filter out signs of CFCs from highly polluted atmospheres, the team found, but still within levels that humans could tolerate. In addition, the telescope could in principle detect the remnants of civilisations that annihilated themselves, since some CFC molecules survive for up to 100,000 years and could outlast their sources, says Loeb.

100,000 years is still a pretty brief period in astronomical and geological scales, but if the great filter is that most civilizations destroy themselves, then we might be lucky enough to spot a remnant or two this way.  I can’t say the prospect of finding something like this would be enticing, since it would indicate that our long term outlook is bleak.  Still, if it happens to be reality, I would want to know it.