The climate in Half Life is changed, that’s for sure. Most of the animal life is dead, so is our kind. The global work of the Combine (industry and so on) should have done quite an impact on Earth’s spheres. And if you want to go deeper, Ok. We can see that in Half Life, all people are wearing overalls of some sort and the weather isn’t that sunny but is not that cold, so this leads to a conclusion that an artificial (or lets say anthropogenic) global warming is at hand. Only the most resilient of plant life should’ve survived. And in that way of thinking, the consumers of O2 are almost gone - the few forests, having the most capable species of surviving, produce enough oxygen for them. So this is what we see in Half Life. And in Portal we hear birds singing, right? I don’t remember that in HL, except the crows of course, but they survive well in cities as well in forests…the cockroaches of the sky 
@Dr. Perky, Valve developed this source engine that puts the rl physics in a virtual game, they wasted much time and efford for this; the science part of the story has a logic too; time is also spent on working on the Xen creatures…creating new kind of life our of nothing; small details all over the place, most of it trully unnecessery but still in place…and you want to tell me that this is a “game” and they can just piss on all this and put 50000 years out of nothing in the story? I think the developers have in mind to put all this sci fi theme in the most realistic way possible. Otherwise, whats the point of having another fps…
