So if the earth has had a lot of climate changes in its life.. how do we know how much of it is us? If major warming and cooling has happened WAY before humans had any impact on the earth this too could be a natural cycle could it not? I agree we have a negative impact on the environment but that doesn't mean we are 100% the reason the climate changing.
It is hard to know our exact impact, however our actions induce the same influences that natural processes do.
It could, however there are no naturally occurring phenomena that can explain it. It is also possible (and apparently likely that man is playing a role in the warming).
In all likelihood we will never be able to know the exact amount of the warming is directly attributed to natural phenomena (which is really the sun's output, as it is about the only thing that man doesn't influence in the climate system), and what percentage man is directly and indirectly responsible for.
Neal