We've all heard of the ravaged rain forests and the plight of the polar bear. But as far as reasons for saving the planet go, the one offered by scientists Thursday is truly out of this world.

A team of American researchers have produced a range of scenarios in which aliens could attack the earth, and curiously, one revolves around climate change.

They speculate that extraterrestrial environmentalists could be so appalled by our planet-polluting ways that they view us as a threat to the intergalactic ecosystem and decide to destroy us.

The thought-provoking scenario is one of many envisaged in a joint study by Penn State and the NASA Planetary Science Division, entitled "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis."

It divides projected close encounters into "neutral," those that cause mankind "unintentional harm" and, more worryingly, those in which aliens do us "intentional harm."

Extraterrestrial intelligence (ETI) "could attack and kill us, enslave us, or potentially even eat us. ETI could attack us out of selfishness or out of a more altruistic desire to protect the galaxy from us. We might be a threat to the galaxy just as we are a threat to our home planet," it warns.

One such scenario is the stuff of many a Hollywood blockbuster, a "standard fight-to-win conflict: a war of the worlds." But another might resonate more with fans of Al Gore's documentary film "An Inconvenient truth."

It speculates that aliens, worried we might inflict the damage done to our own planet on others, might "seek to preemptively destroy our civilization in order to protect other civilizations from us."

"Humanity may just now be entering the period in which its rapid civilizational expansion could be detected by an ETI because our expansion is changing the composition of Earth's atmosphere (e.g. via greenhouse gas emissions), which therefore changes the spectral signature of Earth," the study says.

"While it is difficult to estimate the likelihood of this scenario, it should at a minimum give us pause as we evaluate our expansive tendencies."

But before we brace ourselves for alien annihilation, the report suggests things could turn in humanity's favor.

"As we continue the search for extraterrestrials into the future, perhaps our thinking about the different modes of contact will help human civilization to avoid collapse and achieve long-term survival," it suggests.