>> That's not complex.
Humans only contribute around 3% of total CO2 emissions... That's almost certainly less than normal year-on-year variability of CO2 emissions from purely natural sources. The level of volcanic activity (which, on its own contributes more CO2 than humans) is not the same every year. It's difficult to see how human impact on climate change could be significant.
Another thing is that CO2 is good for plants and trees; so increased CO2 should contribute to global greening; and there is evidence of this occurring. Since plants consume carbon dioxide, one would think that more plants and trees on earth would mean that more carbon dioxide would be taken out of the atmosphere. It seems like mother nature has already figured out how to achieve CO2 equilibrium.
Do the climate change models account for things like global greening? Do they account for the fact that plants can evolve to adapt to a changing environment (with more CO2) through the process of natural selection? A good environmental model would have to account for all the different plant populations around the world and their DNA changes over time... We would have to understand how plant genes affect CO2 consumption; and the simulation would involve trillions of variables and probably require millions or billions of times all the world's current computational capacity to compute accurately. That's barely scratching the surface of the complexity. It's arrogant and disingenuous to suggest that it's simple.
All scientists have been able to establish to far is correlation. There is no evidence of causation. Establishing causation is likely impossible due to the huge number of variables involved. Not only do we not fully understand how all the variables interact, we don't even know what all the variables are because the earth is so complex and massive.