It is possible that the CO2 is coming from natural sources, but those natural sources (e.g volcanoes) would have to be outputting at a steady rate just like manmade CO2 is. The simplest explanation by far is man made.
The real question is if warming is caused by these levels of CO2.
Proponents of AGW maintain that there is a linear relationship at these concentrations between increasing levels of CO2 and the trapping of heat.
In their model, the analogy is a pane of glass you paint with whitewash. When one paints the window, some light is blocked. Add another layer more light is blocked, until after so many layers, no light gets through.
That is a flawed premise at these concentrations. Not only does science not support their premise, the empirical data does not either.
If the data supported their hypothesis, they would not have to manipulate their data to the point of falsification, which they have done and continue to do,
I have read such numbers in the other places. I would like to know background of this value.
Concerning the 5-10ppm:
How was it determined and is it variable over time/area?
Have worldwide measurements over the decades across the globe verified this number?
Does this number originate with "consensus" scientists, or is someone of the character of Professor Linzden of MIT part of this determination?
Throughout the year, is this value linear across all points over water?
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I have great distrust and anger at many of the so called Climate Scientists that are truly corrupting "real" science by knowingly fudging data and unscientifically attacking so called deniers.
What Faux Professor Mann did is so unprofessional he should have resigned in disgrace. But no, he is still lauded. It's sad how this issue is so politicized -- another example of how the left is destroying everything, thus revealing themselves as evil to their core.