What do you mean by "Natural" climate change?
You really need to qualify what you mean there.
The science isn't political. The solutions may have to be political, but I am not betting on that working out well. Better hope for a technological innovation that changes everything.
Essentially, we measure things like solar output on a constant basis, so we know how the Earth's position and the solar output affect temperatures, and it's not enough to keep the whole planet from freezing completely over absent greenhouse gasses.
Greenhouse gasses, then, make it possible for us to live on the planet. Over the last two decades, a lot has been learned about the convection qualities of different atmospheric compositions and the role of different gasses in raising or lowering surface temperatures.
Now, a certain amount of CO2 and methane is given off absent human activity, and a certain amount of sulfoxides that lower temperatures are given off as well, and while these vary, absent cataclysmic events, they vary gradually.
Knowing that these gasses raise and lower surface temperatures (among other things), all that remained was quantifying concentrations and their relationship to global temperatures. That is now a reasonably mature science, and we can then plug in the billions of tonnes human activity is adding into the varying concentrations and establish a trend projection, and these are easily confirmed by measuring the lagging mean temperature changes.
It's not complicated.
As an aside, CO2 emissions fell for the first time in a long while last year because of the economic crisis. Also, you might want to go beyond this basic question and look into secondary effects like ocean acidification.
Man has, by shear numbers, become a geological force.
It's not the first time in the planet's history that a species has become a geological force, but hopefully it turns out better for our species than for others.