Because of biology, geology and meteorology... and, honestly a whole host of other words ending in "ology". We understand that the earth's climate changes over time, fluctuating from hotter to colder. We similarly understand that human beings do impact that change.
What we don't understand is how much humans impact the change. There are a wide array of data and studies out there that prove both the "not that much" and the "oh mah gawd, humans are killing the planet" side of the argument. Either way, it's likely within humanity's best interest to be good stewards of this planet... so the argument is largely moot. Until we start doing drastic crap that hobbles world-wide energy production, wrecks the world economy and plunges ascendant third-world nations back into starvation and poverty.
Want a middle ground? Nuclear energy. There... I just solved most of the world's energy consumption issues. Now it's up to the enviro-freaks to get behind the concept instead of standing in its way. A few might actually be sincere enough to help figure out how to make nuclear facilities even safer than they already are?! But maybe I'm asking too much.