Well I'm glad that now some consensus has been arrived at as to who to point the finger at.......of course mankind is a contributory factor in Global weather changes.
Not all the effects are caused purely by Man.
If you took the Human race out of the picture, as some would like to do, you would still have climate variations, not as you have now, but variations pro rata the conditions that affect the weather with the conditions that would occur if just one link in the chain is missing.
Enter the Human Race, and the effects of it's being will cause a shifting of factors that we now go into panic mode for...........but the fact is, the weather is just adjusting to the influence as it now is, and if an asteroid made an entry to the scene, no doubt the effect would for a time be variable until the weather balanced to the new conditions and you'd end once more with........a change in the climate.
By that time the asteroid, if it were of significantly large enough volume to fundamentally alter the whole World environment as the last big one did, would not be part of any equation as Mankind won't be around to attempt a balancing act to the new conditions.....Mother Nature would just get on with it and invent a new species that would adapt to the new conditions......just as it always has been.
The Human race will be the ones to cop the full impact of an environmental disaster, like an asteroid impact, and being regarded as the lesser of all the species due to the fact that they are slow if not impossible to adapt to any change in their habitat, they will, like last year's snow fall, vanish never to be heard of or be repeated again.
I think the only thing Mankind learned from it's existence was that if you are a species that lives up in a tree you don't mess with fire, so the evolutionary process, per se, meant they all scrambled down the tree when it caught fire, never to go back again.....that's probably the biggest adaptation Mankind was able to achieve......but that was long after the previous asteroid made it's presence known.
From this observation the fact that the lesser primates who still live in trees, realising that their environment was at risk, decided to forgo the making of fire as a dangerous experiment that did no one any good.
From this observation too, it can be stated that the moment Mankind learned how to make fire, that was the moment when the climate balancing act shifted and caused the changes we now think of as unnatural.
It doesn't take a wizard in a pointed hat to know that if you introduce another factor to an experiment the outcome will shift to another dimension, and calling it AGW or climate change, for convenience, is just overstating the obvious.
Ian.