Doug Wilson wrote an essay responding to this article. Here's a portion of the Guardian's article on the potential for alien invasion that caught my eye:
"Green" aliens might object to the environmental damage humans have caused on Earth and wipe us out to save the planet. "These scenarios give us reason to limit our growth and reduce our impact on global ecosystems. It would be particularly important for us to limit our emissions of greenhouse gases, since atmospheric composition can be observed from other planets," the authors write. [Emphasis Mine]
Leaving aside all the other issues of the silliness of this paper and the fact that someone actually thinks aliens might be "green", we should note that because light travels at a certain speed, even if the aliens were to look at it today from their planet, they'd only see what the atmospheric composition was like decades and, more likely, centuries ago. So we can all breath a sign of relief, if man-caused global warming is true, and if aliens take notice from their planet--it will be centuries before they may see anything that concerns them. By that time, we might not have any air left to breadth or we'll be advanced enough to erect planetary shields and to shoot them down with our phasers.