From Paul Thornton, the Los Angeles Times letters' editor:

As for letters on climate change, we do get plenty from those who deny global warming. And to say they "deny" it might be an understatement: Many say climate change is a hoax, a scheme by liberals to curtail personal freedom. [...]

[W]hen deciding which letters should run among hundreds on such weighty matters as climate change, I must rely on the experts -- in other words, those scientists with advanced degrees who undertake tedious research and rigorous peer review. ... And those scientists have provided ample evidence that human activity is indeed linked to climate change. Just last month, the Intergovernmental Panel on Climate Change -- a body made up of the world's top climate scientists -- said it was 95% certain that we fossil-fuel-burning humans are driving global warming. The debate right now isn't whether this evidence exists (clearly, it does) but what this evidence means for us.

Simply put, I do my best to keep errors of fact off the letters page; when one does run, a correction is published. Saying "there's no sign humans have caused climate change" is not stating an opinion, it's asserting a factual inaccuracy.