From "A declaration of orthodoxy", 3 April, 2015:

Apr 3, 2015 at 11:27 AM | ...and Then There's Physics

"Given conservation of mass, this is virtually impossible."

Oh, God. Not the long discredited "mass balance" argument rearing its ugly head again.

The argument is based on static system analysis, which assumes implicitly that the Earth's CO2 regulatory system does not respond to the additional human input. No greening of the planet. No change in downwelling in the oceans. No change.

The argument goes thusly.

A = N + H - S

A = atmospheric rate of change
N = natural inputs
H = human inputs
S = sink activity

A is about 1/2 of H, so N - S is less than zero, hence nature is a net sink, hence H is responsible for the rise.

Tres facile. And, very stupid.

S is a dynamic response. It is a feedback response to A, and is thereby a function of N and H, S = S(N,H). But

N - S(N,H) less than zero does not mean that N - S(N,0) is less than zero. N - S(N,H) still has a dependence on human inputs. Only by showing N - S(N,0) is less than zero could you make the claim that nature, on its own, is a net sink.