It's sad, really, how little effort these people go to when thinking about deep questions; and, similarly, how little effort they think is needed to overturn a consensus that -- remember! -- was only arrived at after decades of debate within the community, when multiple alternatives were explored. Both those that are advanced in this thread, and countless others besides that are beyond their ability even to come up with.
We are, in effect, as sure that human activity is the principal driver behind the current period of climate change as we are about how gravity works.