[Wonder Warthog]

"The problem with this argument is that you then have to come up with a recent natural mechanism that's sequestering the CO2 we know human activity has produced in the last few hundred years."

Nope. There's no way in hell that we even REMOTELY know all the sources and sinks for CO2. To assume so is the height of arrogance.

Not a month goes by that there isn't some reportage on yet another "previously unidentified" driver (positive OR negative) for "global warming". To name just two that have been discovered recently, and which are NOT included in any GW models---the emission by phytoplankton of organic sulfur compounds, and cosmic rays--and the concomitant effect of both on cloud cover (and hence on global warming).

To the best of my knowledge, NO GW math model treats cloud cover AT ALL, and yet the clouds and their effect on albedo are a major factor in real-world warming and/or cooling.

[M. Dodge Thomas]

"There's no way in hell that we even REMOTELY know all the sources and sinks for CO2. To assume so is the height of arrogance."

Sinks. perhaps not. Recent sources, yes: CO2 from fossil fuels has a different distribution of carbon 13/14 than CO2 from other proposed sources, the he observed isotopic trends are very good fit to CO2 from fossil fuel.