Are we alone in the Universe?

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humans have been modifying our atmosphere for several thousand years...

Several thousand years

Yes. Ice cores show changes in CO2 that appear to correlate with human caused deforestation going back at least 2000 years, maybe more. I don't know how someone with an advanced understanding of geological and climatological processes would view that, but I certainly don't exclude the possibility that they would interpret it as an indicator of technological life.

• Frankly, I think that you must mean atmospheric LEAD:

<<Lead, a product of the ancient silver smelting process, was produced in the Roman Empire with an estimated peak production of 80,000 metric tons per year – a truly industrial scale. The metal was used along with other materials in the vast water supply network of the Romans for the manufacture of water pipes, particularly for urban plumbing.>>

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Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity

Joseph R. McConnell, Andrew I. Wilson, Andreas Stohl, Monica M. Arienzo, Nathan J. Chellman, Sabine Eckhardt, Elisabeth M. Thompson, A. Mark Pollard, and Jørgen Peder Steffensen

PNAS May 29, 2018 115 (22) 5726-5731; first published May 14, 2018

<<An 1100 BCE to 800 CE record of estimated lead emissions based on continuous, subannually resolved, and precisely dated measurements of lead pollution in deep Greenland ice and atmospheric modeling shows that European emissions closely varied with historical events, including imperial expansion, wars, and major plagues. Emissions rose coeval with Phoenician expansion and accelerated during expanded Carthaginian and Roman lead–silver mining primarily in the Iberian Peninsula. Emissions fluctuated synchronously with wars and political instability, particularly during the Roman Republic, reaching a sustained maximum during the Roman Empire before plunging in the second century coincident with the Antonine plague, and remaining low for >500 years. Bullion in silver coinage declined in parallel, reflecting the importance of lead–silver mining in ancient economies.>>

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[THX1138]

The problem with contact is that the life span of life life is short compared to a planets overall life and intelligent life is even shorter! So: contact would have to be made in this short period of time! JMHO! Orin

I'm betting on eventual contact between our robot overlords & their robot overlords.

I for one will welcome our robot masters and Oil them regularly