We propose that certain atmospheric technosignatures do not suffer from this longevity problem.

In contrast to industrial pollutants, artificial greenhouse (“terraforming”) gases would represent an intentional effort to modify a planet’s climate, and could persist for the entire remaining history of a civilization or beyond.

Terraforming, by definition, requires sufficient modification of the atmosphere to adjust a planet’s global energy balance, which correspondingly implies a large spectral signature in the thermal infrared portion of the planet’s spectrum, incidentally supporting remote detectability.

Maintaining a terraformed atmosphere may also require the consistent and intentional replenishment of the contributing gases.

Fortuitously, many such gases tend to have long atmospheric residence times of thousands to tens of thousands of years, which would help make the cost of doing so nonprohibitive.

Civilizations may be motivated to introduce highly efficient greenhouse gases to forestall a global ice age on their own home world caused by analogs to Earth’s Milankovich cycles.

Alternatively, they may use terraforming gases to make another planet in their home system (or beyond) more suitable for life, as humans have proposed for Mars . For the case of Mars, the idea of mobilizing available CO2 and other volatile inventories as a terraforming strategy appears to be largely infeasible, so the use of additional artificial gases would be needed for an effective terraforming strategy.