I'm fully aware of the PNAS paper (and others like it which have been published). Let me point out a few other times in history where scientists, without clear evidence for their beliefs, were wrong:

When Galileo championed the idea of heliocentrism, the vast majority of all other astronomers during his time opposed his views. http://en.wikipedia.org/wiki/Galileo_Galilei

The idea of phlogiston was quite prevalent in the chemistry community for about a century before it was definitively proved to be a pile of combustible turds. http://en.wikipedia.org/wiki/Phlogiston_theory

Even recently, in the early 20th century, physicists believed that: a) The smallest particle of matter (the atom) had been quantified, measured, and fully explored. b) There was no more to discover in the field of physics. Clearly, they were wrong.

Let me rephrase my objection more clearly. I believe that the Earth's climate (across all biomes) has been rapidly shifting over recent time. I know that humans can impact local ecosystems and one of the results is a shift in local climate. I have yet to see evidence for (cum hoc ergo propter hoc is a danger when looking at studies) humans causing climate change on a global scale.

This does not mean that we should stop our efforts to prevent deforestation. Nor does it mean we should ignore local environmental disasters, but linking local impact to global impact has not been done definitively yet, so as a scientist, I question that belief. It is not out of arrogance that I question, but out of the desire to be a better scientist. I am trained to base my beliefs in: Hypothesis -> Evidence -> Examination -> Belief. What we do have currently is a very good "guesstimate" hypothesis (we observe that humans can cause local ecosystem shifts). What we do not have is evidence. And thus, we do not have the opportunity for discussion or the transition to belief.