I don't know why it's so hard for some to understand. There is a balance in nature. Volcanoes, plants, and animals do emit gases and other substances that are harmful, but nature has counter-measures for those things. Car emissions, industrial emissions, other human caused emissions, and human re-working of vast areas of terrain are not part of the natural order. The natural systems were not designed/evolved to handle the excessive human emmissions.
I'm not advocating Luditism, but if we are seriously considering whether we have an impact or not, the clear answer is yes! How can we not be? Should we just ignore that effect? If we can minimize that impact, should we not? Or should we just continue on until the effects become so bad that it makes economic sense to do something?
It's best to make changes NOW, rather than wait until it's too late, while it is affordable, and can be done with minimal societal unrest. If we wait for the "proof" before acting, it will probably be too late, as the momentum will only carry the effect to a much worse situation. Add to this the social turmoil when governments force it's citizenry to become Ludites or die and you'll see the end of Democracy, Capitalism, Freedom, and Civilization as we know it. This is truly the biggest threat to our existance, simply because once the balance goes out of kilter, there is nothing we can do to stop it's effects. Except to wait it out.
So what's it gonna take? How many thousands of Americans will have to die before you believe?