My wife mentioned this to me yesterday, hip as she is, several hours before the interconnected sci-fi community I'm part of caught onto it. I had a while to think about it and I've gotta say I disagree with the general concensus that he's right.
Maybe, I thought, just maybe Hawkings isn't being imaginative enough. There's not much water or oxygen in the universe, but there's tons of carbon and nitrogen and solar rays from a billion stars. Why isn't it just as likely that we'll meet aliens who are photosynthetic or whose metabolism requires a constant influx of nitrogen? They could drink our sewage and lick the sweat off of our bodies! Ok, ok, I'm being gross now, but think of it: they're basically living plant organisms because that's pretty much all that can exist on a place that's not-Earth, and they come from a planet full of useless, choking oxygen and they're starting to look for places that gaseous carbon dioxide is present in abundance. Whammo! Suddenly no more global warming on Earth. They can just slowly siphon off the excess carbon dioxide from our planet and replace it with all that waste oxygen they produce. They could move here and mingle with us in our cities, their photosynthetic respiration would slash the smog and air pollution numbers down to their pre-industrial levels. It would be like a rain forest within a city, cleaning and gradually scrubbing the air. Who cares if they get sick from the sulphur and mercury and lead and other toxins we dump into the air? If they could metabolize carbon monoxide somehow, we could pipe the exhausts of our cars into pressurized tanks and sell the waste-exhaust to them for a few bucks! Or maybe not sell it to them, maybe trade it to them in exchange for the cures to cancer and the common cold and for the schematics to cold-fusion, faster-than-light travel and female Viagra. I'm telling you, Hawkings, you've got it all wrong man; it's just as likely that these aliens could be our friends!
What if (stay with me here), what if because their world is so polluted with oxygen that they've stopped singing and they don't allow open flames on their home planet? They sit around all day in a silent, uninteresting stupor where all their food is raw and they can barely get enough energy built up to have a conversation. Their last-ditch effort to find a planet capable of sustaining their species finds Earth.
Holy smokes is right.
They'd forget all about their ray guns and interstellar warships the first time they set foot on our CO2-rich planet where their life-giving smog pours out of every smokestack and industrial cooling tower. What's junk food to us would be like vitamins for them. They'd flock to McDonalds for their charred, carbon-rich, trans-fatty hydrogenated deliciousness. Imagine such an alien getting their first earful of rock and roll music and their first bong hit. Carbon dioxide and cannibanol, mana from heaven. A typical college rock concert would be like going to church for them, and they'd worship the band Phish as living gods. They'd reveal that an advanced scouting team made up of Jimi Hendrix, KISS and Marilyn Manson had been sent to Earth decades ago to determine that we wouldn't immediately try to blast them out of the sky. They'd usher in a new era of music and share their starship technology with us so we could put a Taco Bell and a Starbucks and one of those grimy bodega-style gas stations where the attendant is behind a sheet of bullet-proof plexiglass on the planets circling Alpha Centauri.
They could teach the plants on Earth how to talk and after the roars of "STOP CLEAR CUTTING US, YOU ASSHOLES!" settled down, we'd have a whole population of sentient beings on earth that we could converse with and learn from and tax and charge college tuition to and marginalize and exploit.
You need to lighten up, Hawkings. How could our civilization encountering aliens possibly go wrong?