Strange how, just a knee-jerk you'll find some people defending the science, there are those that have the same knee-jerk reaction against any findings in this area. With all that uncontrollable knee-jerking on both sides - it seems that we have another great argument for universal health care, to get people's knees fixed again... But I digress...
Whether climate change is man-made or not - I don't think there is too much debate left on the matter. But, I'm no climate scientist, so for me personally it's a matter of "belief" that mankind is behind this. We may get some theories and models wrong on how fast global warming works - or why there may be a hiatus in it.
The question of whether we're behind this - take two past events and see how much influence we might have:
Remember the Icelandic volcano a few years back - in response to the volcanic ash, we grounded a lot of flights for a few days - and even in that time, we could measure how much the air changed - just by taking planes out of the picture for a few days.
Secondly, if you think mankind's influence isn't large enough given the size of the planet - look back at climate records around the time Krakatoa blew up - that one mountain exploding had a measurable impact on temperature and weather for 5 years; so, if a _single_ mountain on one day can create that kind of change -- are you sure, all of our industries around the world together over the course of years CAN'T?
What the planet is "too large for", is for us to do some quick and easy experiments to actually test our hypotheses quickly - so climate science does what it can mostly from observation and trying to identify as many factors as possible that DO have a measurable impact in order to MODEL what's going to happen and then wait and see how close these models correlate with what's happening.