Free Astronomy Magazine November-December 2020

26 NOVEMBER-DECEMBER 2020 ATMOSPHERIC POLLUTION used (which, especially in the presence of low abundances, may not allow for the cor- rect distinction of gases), but also condi- tioned by our current models on the formation and evolution of planets and their atmospheres. Despite this, there is no doubt that, in certain scenarios, specific molecules either cannot be produced by natural processes or cannot persist in plan- etary atmospheres without the support of biological activities. There are no solid ar- guments to assert that the life present on other planets might have a totally different impact on their atmospheres compared to the only case we know. Therefore, if an- other civilization has evolved by polluting its own atmosphere, our methods of in- vestigation should be able to discover it (within certain limits, of course). Obviously, this opportunity is not one-way. Pollution of the Earth’s atmosphere is also de- tectable from other planetary systems and it would be interesting to know how far away that detection can occur, as that is the limit beyond which an alien observer can only detect the generic presence of life on our planet but not the presence of an evolved civilization. This is equivalent to saying that, beyond such limit, it does not make much sense looking for possible mes- sages intentionally sent to us. Understand- ing how far the light reflected from our polluted atmosphere has gone can ulti- mately improve the targeting of some SETI programs. To determine that distance, we must take a trip through the Anthropocene. This term (introduced in the 1980s by the US bi- I n this graph of global carbon dioxide emissions from the time of the first Industrial Revolution, it can be seen that that initial, intense period of atmos- pheric pollution is negligible com- pared to the emissions of the last century. [CDIAC]