The human impact on the Earth's ecosystem has expanded gradually (but at an increasingly accelerated rate) ever since we got those opposable thumb thingies. Or ever since Eve bit the apple. Whatever.
Global Warming is largely accepted by the scientific community. Geologists and climatologists and some other ologists have scientific data showing the Earth goes through regular periods of warming and cooling. Whether or not the current warming cycle is being caused by or getting too much help from humans is not something that can be concretely determined.
The doom sayers tell us global warming is the trigger that will herald the end for humanity. Ice caps melting, sea levels rising, ozone layers dissapearing, bottom-of-the-food-chain types being wiped out and starting cascading die-offs all along the way up the food chain, etc.
Why focus on global warming though? Humans are not the type to just try one thing. With all the billions of people in the world all trying to get ahead of the game, we've come up with plenty of other ways to ruin the planet.
Invasive species are taking over whole regions of the globe and causing die-offs of native flora and fauna, changing whole ecosystems. Radioactive waste, oil spills, strip-mining, deforestation .. i'd say our play book is pretty full.
If all industrialised nations made giant leaps toward curbing emissions and engaged in proactive restoration of the environment we'd still have all that other stuff.
Yeah, i'm pretty sure global warming is real and a major threat, but it's just the latest one we've recognized.
By the way, you all have way too much time on your hands. I wish I could post more but I have to work at least a little bit while I'm on the clock.