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I think that man definitely has had an impact on the environment. But, and it’s a big but, we just have no way of knowing if man has had an impact on long term climate trends.

And, to the extent that man has impact on climate trends, we don’t know how much is man made.

I agree with all of your points except that we do have a way of knowing the extent of man's influence.

While it is true that we don't fully understand Nature's role on climate, do not misunderstand this to mean we cannot assess the role of human activity. If we restrict our definition of the impact on global warming by human activity as through the mechanism of green house gases (CO2 emission) then we can say with certainty if this is significant or not. We are justified in restricting our definition of human activity to CO2 emissions because that is precisely the feature of human activity that the global warming policy issues are based on.

Let's take a moment to remind ourselves of the scientific process: When a hypothesis is proposed, it must be able to predict some non-trivial observables in order to test the hypothesis. By non-trivial, I mean that the reasoning should not be circular. For example, evidence of warming is not evidence of anthropogenic warming. As an illustration, one could propose a hypothesis that aliens are projecting an energy beam on the Earth causing the Earth to warm. Hence, evidence of warming would be evidence that our 'Alien' theory is correct. -Not! The point is, evidence of warming is not evidence of any particular cause of warming and is, hence, invalid in terms of answering the question of human influence on warming.

However, greenhouse gas warming theory does indeed predict certain non-trivial observables and we can in fact seek to observe them and test the theory. One of the predicted observables is that the temperature trend in the tropical troposphere round 10,000 ft should be increasing at a rate of three to four times the surface rate. We have over 60 years of balloon satellite date measurement to check this and we observe that the trend in the tropical troposphere is about half to three-quarters the surface rate. The exact opposite of what greenhouse gas theory predicts. From that observation alone we can conclude that temperature is not driven by greenhouse warming.

There is yet another prediction made by greenhouse gas theory which predicts that as the surface temperature increases, the amount of energy radiated into space will decrease by a certain amount. This prediction is based on the idea that increased temperature will cause more CO2 and water vapor to be produced by the oceans, resulting in a greater amount of radiation trapping. But again, we have decades of satellite data to test this observation and we find that the amount of radiation into space increases by an amount about three times what greenhouse gas theory predicts it should decrease. So again, greenhouse gas theory predicts the opposite of what we observe.

From these tests, even if we don't fully understand what is actually driving temperature, we can absolutely conclude that the warming (if any) is not driven by greenhouse gases. Independent of whatever else we don't know, this fact is incontrovertible.