Humans are altering the environment

You can't argue with this (although some people ridiculously try). Human activity is altering our environment in a huge way, from the massive deforestation of the planet to the release of gases into the atmosphere. We've poisoned the rivers, destroyed natural habitat, polluted the oceans (Gulf of Mexico, anyone?) and altered the chemical composition of the atmosphere. These are undeniable scientific truths. No sane person can reasonably argue that human beings have not radically altered the environment of our planet over the last 200 years.

If you visited North America 200 years ago, for example, you wouldn't even have recognized it as the same continent dominated by human beings today. A few hundred years ago, North America was teeming with life, with huge old-growth forests, pristine rivers and abundant plains. Today it is relatively dead, having been over-developed, over-paved and over-population to a point so extreme that our ancestors would largely consider it "dead".