Detecting ETI via CO2


With the increasing success of detecting planets around other star systems,
the time is near that we will detect the first Earth-like planets (if they
are there) orbiting stars other than the sun.

If, and when we do so, I'm sure that scientists will go the next step, to
analyze these planets' atmospheric composition and attempt to find oxygen
and water.
With increased sensitivity and prolonged observation, they might even find
rather accurate readings for gases like CO2 and methane.

Now suppose that an ETI out in the Galaxy has found Earth and observed it
for hundreds of years. They know our atmosphere's composition pretty
accurately.
It might be possible that they have noticed a rapid increase of CO2 in our
atmosphere (from about 275ppm a few centuries ago, to almost 400ppm now).
Similar steep increase should be noticable for methane.

This should seem like a very uncommon, and artificial, increase in
atmospheric composition, since natural effects of changes in atmospheric
composition are much, much slower than that.
The ETI could thus conclude that there is probably an emerging technology on
our planet, burning massive amounts of fossil fuels....

Is this a possible scenario ?
Can CO2 levels in our atmosphere be observed by ETI's ?
If so, what kind of equipment would they need ?
And can a rapid increase in CO2 (30% - 50% increase in 100 years) be
explained as natural effects (such as massive change in vulcanic activity) ?
It surely did not change so fast in the past million years or so...

Could rapid changes in extra-solar planet atmospheric conditions be an
indication of ETI ?
And are there maybe other rapid changes (such as global warning) caused by
activity of emerging technology civilizations burning fossil fuels, that
could be observed from interstellar distances ?

Rob




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