We have ample measurable impact of humanity on our atmosphere.

We know that even without such factors we do in fact have mini ice ages (History Channel had a nice doc called "the little ice age" and it took place during the middle ages/great plague time). Ironically ice ages are caused when a rise in temperature causes the ice pack to migrate south-we already have seen lots of ice bergs.

Now combine that with the green house gas/ozone depletion we know has been caused by our handywork and expertise at pollution. Mother nature mixing with industrial level pollution.

And most important for people to understand (and its hard for many) this kind of damage isnt immediate like a gun shot or a knife-it is slow and deadly like cancer. It wont happen with some kind of big flash and bang.....it will seep into our lives gradually in a thousand tiny ways until one day winter doesnt go away in the NE and summer is a desert maker in other places.