New Scientist reports astronomers suggest that pollution could be used in the future to detect the presence of intelligent life forms on exoplanets. Here's a snip from the article:
"CFCs are a very interesting idea to look for advanced civilisations," agrees Lisa Kaltenegger of Harvard University. But an exceptionally sensitive telescope would be needed to pick them up - more sensitive even than NASA's Terrestrial Planet Finder and the European Space Agency's Darwin mission, the most ambitious space telescopes now being planned. Kaltenegger says it may be feasible "in the far future with a flotilla of infrared telescopes in space".