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I think it would be more accurate to say that human activity has resulted in the emissions of CO2. Increases in CO2 concentrations depend on the performance of sources and sinks, many of which have substantial error bars.

A very good observation. I don't know what the evidence is, if any, that measured CO2 increases are primarily due to human activity. Information about recent history is probably very good (we know how much fossil and other fuels are burnt and we know what atmospheric CO2 concentrations are) so we can reliably say today what the natural sinks and sources are doing in the aggregate, even if we can't identify and understand them in detail. But extrapolating that aggregate behavior into the future seems very chancy. As evidence, it's my understanding that we have inferred significant historical swings in CO2 concentrations which cannot have been due to human action.

Supposing those inferences are reliable, and supposing they cover siginficant historical periods, I'd trust that extrapolation into the future. You could add that variability as another uncertainty into the models.

The reason that CO2 increases lag temperature increases is that they are a result, not a cause of global warming. This is demonstrated by the same lag being repeated in multiple deglaciation cycles in the paleoclimate data. This lag and its irresistible logical implications are very inconvenient for the AGW argument.

A discussion of the mechanism is presented here. http://www.rocketscientistsjournal.com/2006/10/co2_acquittal.html. The basic idea is that the relationship between temperature levels and CO2 levels is well explained by the solubility curve of CO2 in water, which is a function of temperature.

The author addresses criticisms of his thesis by Gavin Schmidt here. http://www.rocketscientistsjournal.com/2006/11/gavin_schmidt_on_the_acquittal.html, and is worth reading if you are of a scientific bent of mind.

Regards,

I don't know what the evidence is, if any, that measured CO2 increases are primarily due to human activity. Information about recent history is probably very good (we know how much fossil and other fuels are burnt and we know what atmospheric CO2 concentrations are) so we can reliably say today what the natural sinks and sources are doing in the aggregate, even if we can't identify and understand them in detail.

What is known is that there are measured increased of CO2 concentrations (at certain locations). It is also known that human emissions of CO2 have increased.

You are correct, it is easy to draw a correlation, but that does not prove causation. Doesn't rule it out either of course.

So there is an aggregate answer, which appears to be increasing, and there is information regarding one input to that aggregate. Not being able to clearly identify and understand the complete picture of natural sources and sinks of the carbon cycle is as you note a chancy proposition for extrapolating to the future.