If the prospect of catastrophic climate change doesn't give you enough to worry about, Shawn Domagal-Goldman of Nasa's Planetary Science Division and his colleagues posit this: The fact that we're artificially altering our atmosphere may bring us to the attention of extraterrestrial intelligence (ETI), who "may seek to preemptively destroy our civilization in order to protect other civilzations from us."
A preemptive strike would be particularly likely in the early phases of our expansion because a civilization may become increasingly difficult to destroy as it continues to expand. 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.
Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis (Acta Astronautica, 2011, 68(11-12): 2114-2129) is as entertaining and provocative a scientific paper as you are ever likely to find--and one that sheds a stark light on our current environmental predicament. The authors conduct what they call a "scenario analysis," gaming out the the various possible forms of human/ETI contact in terms of whether the consequences would be beneficial, neutral, or harmful to humanity. One of the biggest unknown variables they explore is the ethical nature of the extraterrestrials: Are they "selfish," seeking to maximize their alien self-interest like certain hominids we know? If so, then watch out. Here the authors quote Jared Diamond:
[E]xtraterrestrials might behave the way we intelligent beings have behaved whenever we have discovered other previously unknown intelligent beings on earth, like unfamiliar humans or chimpanzees and gorillas. Just as we did to those beings, the extraterrestrials might proceed to kill, infect, dissect, conquer, displace or enslave us, stuff us as specimens for their museums or pickle our skulls and use us for medical research.
However, it could well be that selfish beings such as ourselves are poor models for Conquerors of the Universe:
Perhaps rapid expansion is unsustainable at the galactic scale, just as rapid expansion is often unsustainable here on Earth. . . . [R]apidly expanding civilizations may face ecological collapse after colonizing the galaxy, analogous to the fate of Easter Island.
So if the ETIs are not selfish beings bent solely on their own inexorable advancement throughout the cosmos, they might be "universalists" who place equal intrinsic value on all life forms, even us. In the best case, "an advance ETI may be capable of solving a great many of humanity's problems, such as world hunger, poverty, or disease." Alternatively, however, universalist extraterrestrials might see humanity as a threat--either to themselves, to other civilizations that they seek to preserve, or even to the biota of our own Earth:
Human civilization affects ecosystems so strongly that some ecologists now often refer to this epoch of Earth’s history as the anthropocene. If one’s goal is to maximize ecosystem
flourishing, then perhaps it would be better if humanity did not exist, or at least if it existed in significantly reduced form. Indeed, there are some humans who have advanced precisely this argument. If it is possible for at least some humans to advocate harm to their own civilization by drawing upon universalist ethical principles, then it is at a minimum plausible that ETI could advocate harm to humanity following similar principles.