Small changes in delicate systems have big effects, Loosehead.
Using tree rings which capture carbon isotopes in their growing cycles, and cross checking with ice cores and corals and sponges, we can check to see what type of carbon are present in the atmosphere at any given point. C13 naturally occurs at a rate of about 1% of total carbon. In fossil fuels, the ratio of C13:C13 is much lower.
From 1850, the start of the industrial revolution, those lower ratios suddenly appear. This also marks the point at which the levels of CO2 in the atmosphere significantly rise.
Current levels of the C13:C12 ratio are the lowest they've ever been, directly correlating with our use of fossil fuels.
The changes in the ratio of different isotopes of CO2 (C12 and C13 respectively) going from full glacial-to-interglacial change over a period of several thousand years is 0.03%.
The changes in same isotopes in C02 levels as a direct consequence of antropogenic C02 levels since 1850 is 0.15%.
We know that CO2 levels are significantly higher now than at any point in history. We know that our activities are directly responsible for that CO2. We are producing CO2 at a level that cannot be completely taken up by the oceans, resulting in an increased (and increasing) level of atmospheric CO2.
CO2 causes climate change. QED.
http://www.realclimate.org/index.php?p=87