The climate changes of so long a go were absolutely huge. But even the far lesser one of today will have very big consequences for mankind. Our species is extremely susceptible to the slightest variation and even if we're not responsible, we've got to understand it and certainly change some habits.
Adapting the economy and changing our habits are not the same as putting the blame somewhere.
But it's true that blaming human activity makes the job much easier as it would mainly depend on us to change our way of life.
What other convincing variables are there? Sun cycles? Cosmic dust in the solar system? Volcanism? Are non-Earth events measurable in the past? Is what scientists have in their database enough for them to get a picture of the years to come?
But obviously, something must be done and only humans can do something about it.

Raoul